the lower lesser himalayan sequence: a...

ABSTRACT

The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, how-ever, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intru-sive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of meta-

selahs lacipyt morf reffid skcor yratnem idesand suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and meta basalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleo-proterozoic arc may have formed the strati-graphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses (“Ulleri”) likely repre-sent shallow intrusions, not Indian basement.

INTRODUCTION

Understanding the origins and predeformed geometry of the northern exposed edge of the Indian plate is crucial for unraveling the defor-mation history attending collision of India with Asia, and hence for reconstructing India’s po-sition in former supercontinents (e.g., see re-

views of Gansser, 1964; Le Fort, 1975, 1996; Yin, 2006). The Lesser Himalayan sequence plays a central role in both endeavors. It is directly involved in several major Himalayan thrusts, most signifi cantly the Main Central and Munsiari (or Ramgarh) thrusts. Interpreta-tion of the genesis of Lesser Himalayan rocks also fi gures prominently in the placement of India in the hypothesized, ca. 1800 Ma super-continent Columbia. Columbia, in turn, is im-portant for understanding the supercontinent cycle: did supercontinents form in the Paleo-proterozoic and Archean, and if so what infl u-ence did they play in surface processes (e.g., Reddy et al., 2009)?

Metasedimentary rocks are reported to con-stitute the lower portion (≥~4 km) of the Lesser Himalayan sequence (total thickness ≥~8 km; e.g., Stöcklin, 1980; Valdiya, 1980; Schelling, 1992; DeCelles et al., 2001; McQuarrie et al., 2008), and are generally interpreted as the pas-sive margin sedimentary cover to the Indian craton (e.g., Brookfi eld, 1993; Upreti, 1999; Myrow et al., 2003; Gehrels et al., 2006). Yet, in contrast to the passive margin para-digm, igneous events are also recorded within the lower Lesser Himalayan sequence. Well-constrained radiometric ages are sparse, but zircon U-Pb and Pb-Pb ages for intercalated orthogneisses and metabasalts, and detrital zir-con grains from the entire breadth of the Hima-laya indicate a common Paleoproterozoic age of 1800–1900 Ma (Fig. 1, Table 1). This age span is not consistent with zircon ages from the Indian shield (Parrish and Hodges, 1996; this study), so a fresh interpretation is warranted. In NW India, plume or rift magmatism is com-monly invoked (e.g., Bhat et al., 1998; Ahmad et al., 1999; Ahmad, 2008).

In this paper, we present and discuss sev-eral lines of evidence to argue that the Paleo-proterozoic assemblage at the base of Lesser Himalayan sequence represents a continental arc, rather than a passive margin, a collisional belt, or a plume- or rift-related environment.

We interpret these rocks to consist predomi-nantly of reworked volcanogenic sediments interspersed with intrusive, volcanic, and volcaniclastic rocks. Supportive data include new fi eld observations in NW India and cen-tral Nepal, previously published fi eld descrip-tions across the Himalaya, new and previously published chronologic results (Table 1), and new and previously published whole-rock, major- and trace-element chemistries (Tables A1 and A2). This active margin sequence has not been identifi ed previously as such but is laterally traceable as a ~2500 km long persis-tent hori zon, albeit in detached outcrops, right from the NW Himalayan sector, through Nepal and Bhutan, into NE India (Fig. 1). We discuss implications of our interpretation for correlating Himalayan stratigraphy, and also for interpret-ing possible geodynamic scenarios related to the ca. 1800 Ma Columbia supercontinent.

GEOLOGIC SETTING AND STRATIGRAPHY

Richards et al. (2005), Robinson et al. (2006), Upreti (1999), and McQuarrie et al. (2008) pro-vide recent accounts and reviews of the Lesser Himalayan lithologies and radiometric ages from northwestern India, western Nepal, central Nepal, and Bhutan, respectively (Fig. 2). The following stratigraphic descriptions are based on their discussion and references therein (es-pecially Stöcklin, 1980; Valdiya, 1980; Gansser, 1983; Bhargava, 1995; Colchen et al., 1986; Schelling, 1992; DeCelles et al., 2001). In India , lower Lesser Himalayan rocks are variously re-ferred to as the Jutogh metasediments, Munsiari Formation or Group, Jaunsar and Damtha Group, and Rampur Formation or Group (within the Rampur window). In this paper , we correlate all these rocks based on lithol-ogy and age, and generically refer to them as “Munsiari.” In Nepal, this interval corresponds with the lower Nawakot Group, which is further subdivided, most notably into the Kushma and

For permission to copy, contact [email protected]© 2009 Geological Society of America

323

GSA Bulletin; March/April 2010; v. 122; no. 3/4; p. 323–335; doi: 10.1130/B26587.1; 9 fi gures; 3 tables; Data Repository item 2009191.

†E-mail: [email protected]

The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate

Matthew J. Kohn1, Sudip K. Paul2, and Stacey L. Corrie1

1Department of Geosciences, Boise State University, Boise, Idaho 83725, USA2Wadia Institute of Himalayan Geology, Dehra Dun, India

http://gsabulletin.gsapubs.org/

Kohn et al.

324 Geological Society of America Bulletin, March/April 2010

Ranimata Formations in the west, the Kuncha Formation in central Nepal, and the Tuml ingtar Group in the east. The Kushma Formation is a nearly pure quartzite, stratigraphically below the Rani mata Formation, and not obviously asso ciated with igneous rocks. The Kuncha and Ranimata Formations are dominated by clastic material but contain minor amphibolites and a felsic orthogneiss that is usually correlated with the Ulleri augen gneiss, although neither the Ulleri nor other felsic orthogneisses ubiqui-tously bear augen structure. In Bhutan, basal quartzite (Shumar Formation) is overlain by chloritic phyllite and quartzite (Daling For-mation), which additionally contains sheared orthogneiss. The Shumar, Daling, Kuncha, Kushma, Ranimata, Tumlingtar, and Munsiari units are all part of the lower Lesser Himalayan sequence. The boundary between “upper” and “lower” Lesser Himalayan rocks is not agreed upon but is usually placed below units that ex-hibit signifi cant carbonate components (Fig. 2).

The Munsiari has been described as con-taining garnet-staurolite-mica schist, quartzite , marble, calc-silicate, mafi c amphibolite and graphitic schist, with occasional quartzo-feldspathic gneiss (Richards et al., 2005). The Rani mata, Kuncha, and Daling Formations are generally described as chloritic phyllite with scattered quartzite, and either sparse dioritic in-trusions (Ranimata Formation: Robinson et al., 2006; Kuncha Formation: Stöcklin, 1980), or mylonitized orthogneiss (Daling Formation: McQuarrie et al., 2008). We found at least one felsic metavolcanic rock in the Kuncha Forma-tion (Fig. 3), and Richards et al. (2006) inter-preted a rock from the Daling Formation as a metarhyolite. Because many sections are domi-nated by clastic material, previous work has emphasized sedimentary, not igneous origins, essentially describing the units as dominated by metashale, metasandstone, or metacarbonates, with intercalated but uncommon igneous bod-ies of unspecifi ed origins. For example, rocks in NW India are sometimes referred to as the Jutogh metasediments, and clastic sedimentary protoliths are always listed fi rst among rock types in formation descriptions. Whereas previ-ous workers did faithfully record the occurrence of igneous rocks, and many Indian geoscientists proposed various tectonic scenarios based on igneous geochemistry (e.g., Bhat et al., 1998; Ahmad et al., 1999; Ahmad, 2008), such rocks have been largely ignored in interpretations of the confi guration of the northern Indian margin.

Although not a focus of this study, carbonate- and graphite-rich units of the upper Lesser Himalayan sequence are worth discussing for stratigraphic and tectonic context. In Nepal, these rocks are commonly considered early

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A Lesser Himalayan arc

Geological Society of America Bulletin, March/April 2010 325

to middle Proterozoic in age, and unconform-ably overlain by upper Paleozoic to Cenozoic rocks of Gondwanan affi nity (e.g., Upreti, 1999; DeCelles et al., 2001). In contrast, upper Lesser Himalayan rocks including carbonates and associated quartzites are reported to con-tain ~500–600 Ma fossils in NW and NE India (Tewari, 2001; Azmi and Paul, 2004; Hughes et al., 2005), and lower Paleozoic zircons and isotopic signatures in Bhutan (Long et al., 2008; McQuarrie et al., 2008). When correlated into Nepal, these observations imply that the upper Nawakot is more likely late Proterozoic to lower Paleozoic, and that a major hiatus or highly condensed section occurs somewhere within the Nawakot Group (Azmi and Paul, 2004; Hughes et al., 2005). Such a break would separate early Proterozoic, ca. 1830 Ma rocks (at least Shumar, Daling, Kuncha, Ulleri,

Kushma, Ranimata, and Munsiari units), from ~500–600 Ma rocks that are dominated by gra-phitic slate and carbonate. Defi nitive ages have not been recovered from upper Lesser Hima-layan rocks in Nepal; therefore, the correla-tion remains tentative. Note that Myrow et al. (2003) proposed contemporaneous deposition of Lesser Himalayan, Greater Himalayan, and lower Tethyan rocks, and if correct, their model applies only to the upper section of the Lesser Himalayan sequence (Richards et al., 2005).

The exact location of the inferred ca. 1 Ga unconformity or condensed section remains unknown because several unconformities have been proposed above and below the level of the Galyang, Nourpul, and Blaini Formations (Fig. 2). Azmi and Paul (2004) proposed that the Blaini Formation is of Vendian age (ca. 600 Ma) because it is associated with the overlying

Precambrian–Cambrian Krol Formation in the Tejam Group. The Blaini has been similarly cor-related with Neoproterozoic (ca. 650 Ma) rocks in China (Jiang et al., 2003; Zhang et al., 2008). Azmi and Paul (2004) further correlated the Blaini and Nourpul Formations based on gen-eral lithostratigraphy. But according to DeCelles (2008, personal commun.), the Nourpul Forma-tion contains ca. 1770 Ma mafi c rocks, imply-ing that the Nourpul should be grouped with lower Nawakot rocks, and that any major uncon-formity must occur at higher stratigraphic levels.

EVIDENCE FOR AN ARC ORIGIN

Field and Textural Observations

Field relationships and microscopic textures of many Munsiari rocks in NW India indicate igneous origins, both intrusive and extrusive. For example, granite porphyries crop out in the Pabar region (Figs. 3A and 3B), as well as coarse-grained felsic gneiss that we interpret as deformed granite (Fig. 3C). Rocks that resemble (meta)sandstones in hand-sample have typical igneous whole-rock compositions (Table A1) and reveal textures that are equally consistent

TABLE 1. AGE CONSTRAINTS FROM LOWER LESSER HIMALAYAN ROCKS ecnerefeRkcoR)aM(egAaerA

1 1880 ± 24* Besham gneiss Treloar and Rex (1990)1864 ± 4* Shang orthogneiss DiPietro and Isachsen (2001)1836 ± 1* Kotla Complex DiPietro and Isachsen (2001)

2 1850 ± 14* Iskere gneiss Zeitler et al. (1989)3 1904 ± 70†,# Rampur Complex Frank et al. (1977)

1860 ± 60† Rampur Complex Trivedi et al. (1985)1840 ± 16* Rampur Window orthogneiss Miller et al. (2000)1820 ± 19*,** Rampur Window metabasalts Miller et al. (2000)1866 ± 10* Wangtu orthogneiss Singh et al. (1994)1866 ± 64† Wangtu orthogneiss Rao et al. (1995)1866 ± 6* Wangtu orthogneiss Richards et al. (2005)1797 ± 19* Jutogh leucogranites Chambers et al. (2008)

4 1907 ± 91† Ramgarh basalts Ahmad et al. (1999)1870 ± 7* Ramgarh Complex Celerier et al. (2008)1865 ± 3* Ramgarh Complex Celerier et al. (2008)1856 ± 10* Ramgarh Complex Celerier et al. (2008)

5 1865 ± 60† Amritpur Complex Trivedi et al. (1984)1880 ± 40§ Amritpur Complex Varadarajan (1978)

6 1850–1875* Detrital Zrc from Kuncha Fm. DeCelles et al. (2000)1840 ± 30*,†† “Ulleri” augen gneiss DeCelles et al. (2000)

7 1840 ± 40* “Ulleri” augen gneiss Celerier et al. (2008) 1700–1800 Ulleri augen gneiss Deniel (1985)

1780 ± 23* Ulleri augen gneiss This study8 ca. 1870* Detrital Zrc from Kuncha Fm. Parrish and Hodges (1996)

1878 ± 11* “Ulleri” augen gneiss This study1877 ± 11* Lower LHS metatuff This study

9 1780 ± 23* Granitic orthogneiss This study1791 ± 12* Granitic orthogneiss This study1832 ± 23* Granitic orthogneiss This study1795 ± 8* Detrital zrc from pelitic schist This study1812 ± 3* GHS (?) orthogneiss Liao et al. (2008)

10 >1600 “Ulleri” augen gneiss Upreti et al. (2003)11 1760 ± 70* Felsic gneiss Daniel et al. (2003)

1790 ± 30* Metarhyolite Richards et al. (2006) 1750–1850* Detrital zrc from Daling Fm. Richards et al. (2006)

1896 ± 16* Orthogneiss, Daling Fm. Long et al. (2008)12 1743 ± 4* Bomdila augen gneiss Yin et al. (2009)

1747 ± 7* Augen gneiss Yin et al. (2009)1800–1900† Bomdila augen gneiss Bhalla and Bishui (1989), Dikshitulu et al. (1996),

Rao (1998) *Zircon U-Pb or Pb-Pb age.

†Rb-Sr whole-rock age. §K-Ar age. #Recalculated for λ(87Rb) = 1.42 × 10– 11 a– 1.

**Recalculated from raw data reported in Miller et al. (2000): age is weighted mean; uncertainty is weighted scatter of data about the mean.

††Recalculated from raw data reported in DeCelles et al. (2000). Abbreviations: Fm.—Formation; GHS—Greater Himalayan sequence; LHS—Lesser Himalayan sequence; Zrc—zircon.

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Figure 2. Simplifi ed stratigraphic column of Lesser Himalayan sequence (LHS; Upreti, 1999; DeCelles et al., 2001; Azmi and Paul, 2004), illustrating proposed correspondence with a continental arc (indicated by patterns) and ca. 1 Ga unconformity between lower and upper Lesser Himalayan rocks. Ques-tion marks indicate that assignment of the Nourpul, Galyang, and Blaini Formations to either ca. 1800 Ma versus 600 Ma ages is un-certain. Note that Stöcklin (1980) places the upper-lower Nawakot boundary stratigraph-ically higher within calcareous rocks.



Kohn et al.


with a tuff protolith, originally containing quartz phenocrysts and stretched and fl attened pumice fragments (Figs. 3D–3F). Some Lesser Hima-layan rocks from central Nepal retain feldspar-rich metaigneous chemistries, mineralogies, and textures, and refl ect both volcanic and intrusive felsic phases (Figs. 3G and 3H).

Mafi c igneous rocks also occur in the lower Lesser Himalayan sequence, commonly as isolated strata that may be intercalated with metasedimentary rocks such as quartz arenites, but they are also associated with felsic meta-igneous rocks. One key observation is that minor but widespread chlorite schist in NW India exhibits textures consistent with a mafi c volcanic or sill protolith that has been hydrated and metamorphosed. In rare instances, chlorite schist is localized at the margins of mafi c am-phibolites (Fig. 4A). We interpret this schist as the tops of fl ows, altered and hydrated either soon after deposition or during metamor-phism as a result of enhanced fl uid fl ow along lithologic boundaries. In other instances, mm-diameter white spheroids are hosted in a mafi c matrix (Fig. 4B). In thin section, the spheroids are dominated by plagioclase with subordinate quartz, carbonates, and chlorite (Figs. 4C and 4D). The matrix contains abundant coarse-grained plagioclase. We interpret the spheroids as metamorphosed amygdules and the matrix as metamorphosed porphyritic basalt. More gener-ally, we fi nd a progression of variably hydrated mafi c assemblages from chlorite-rich through amphibole-rich schist, with variable retention of original porphyritic textures (Figs. 4E and 4F).

The stratigraphic and structural relation-ships of felsic and mafi c rocks with surrounding metasedimentary rocks help constrain possible genetic and tectonic interpretations. Felsic plu-tonic bodies (Ulleri gneisses) in Nepal are inter-calated with “tuffaceous” metasedimentary rocks (Le Fort, 1975; Le Fort and Raï, 1999). The Ulleri and surrounding schists share fabrics, so must have been codeformed, presumably dur-ing the late Cenozoic (Le Fort, 1975). Contacts have been described both as gradational with adjacent schists and quartzites (Le Fort, 1975; Le Fort and Raï, 1999) and as obliterated by later deformation (Yin et al., 2009). So, whereas some have proposed that the lower contacts are thrust faults and that the Ulleri represents In-dian basement (Gansser, 1964; Yin, 2006; Yin et al., 2009), others interpret the felsic gneisses to refl ect either syngenetic porphyritic extrusive rocks (Le Fort, 1975; Le Fort and Raï, 1999), or intrusions into a dominantly sedimentary se-quence (e.g., DeCelles et al., 2000), with trans-formation to gneisses during later deformation.

Along-strike variations are evident in the abundances of igneous components in the lower

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Figure 3. Field and thin-section photographs of felsic rocks from Munsiari rocks, north-western India (A–F) and from Kuncha Formation, central Nepal (G and H). (A–C) Granites and granitic gneiss in Pabar region, India. (D–F) Hand sample and photomicrographs in plane-polarized light and cross-polars of fi ne-grained “sandstone,” here interpreted as a metamorphosed altered tuff that originally contained quartz phenocrysts (knots in inset of D; coarse quartz in E and F), and pumice fragments (light streaks in D; compositionally dis-tinct domains in E and F). (G) Felsic metavolcanic rock associated with chlorite-rich schist, Kuncha Formation, Langtang region, Nepal. Zircon rims from this sample give an age of ca. 1880 Ma. (H) Augen gneiss in Langtang region, Nepal that is correlated with Ulleri augen gneiss. Zircon rims from this sample give an age of ca. 1880 Ma.





Lesser Himalayan sequence. For example, plu-tonic rocks are relatively common in the Mun-siari of NW India but are relatively rare in the Kuncha and Ranimata Formations of Nepal, which are instead dominated by chlorite- and feldspar-rich schist. Thus, any putative arc in Nepal must be either largely buried under over-lying thrusts (e.g., the Main Central Thrust), or dominated instead by volcanic rocks or vol-canogenic sediments whose postdeformational and postmetamorphic physical appearance now masquerades as deformed and metamorphosed passive margin sediments.

Major- and Trace-Element Geochemistry and Zr-Saturation Temperatures

The overall mineralogy and major-element chemistry of some lower Lesser Himalayan “sediments” in NW India and Nepal is consistent with a volcanogenic or even volcanic origin. In Nepal, lower Lesser Himalayan schist is gener-ally graphite-poor, uniformly feldspar-rich, and contains low-Al assemblages (Catlos et al., 2001; Kohn, 2008). Some samples from the Munsiari in NW India that were identifi ed as metasedimen-tary rocks have compositions similar to felsic volcanic rocks (Table A1), specifi cally exhibiting much lower Fe contents (<5.5 wt% Fe

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K/Na ratios (<1.7) than average pelites (>5.7 wt% Fe

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closely match those of dacite and rhyolite.Trace-element geochemistry further supports

either an arc or collisional setting. Mafi c rocks have been variously interpreted as arc, rift, or fl ood basalt magmas (Bhat et al., 1998; Ahmad et al., 1999; Miller et al., 2000; Ahmad, 2008). However, high large ion lithophile elements (LILE), low TiO

2, and negative Ta and Nb anom-

alies are more consistent with an arc (Miller et al., 2000). Discrimination diagrams (Pearce et al., 1984) were considered for Y, Nb, and Rb in felsic rocks because data for these ele ments are available for many samples. For Nb versus Y, data plot within the volcanic arc and syncol-lisional fi elds, implying that within-plate plume or ridge settings are unlikely (Fig. 5A). For Rb versus Nb + Y, data plot near the triple point of the fi elds for within-plate, vol canic arc, and col-lisional granites (Miller et al., 2000; Fig. 5B). These interpretations should be viewed with cau-tion because Nd-model ages (Miller et al., 2000; Richards et al., 2005) exceed crystallization ages, perhaps indicating crustal con tami nation that would bias trace-element compositions, particularly for Rb, which is generally viewed as more mobile than Y and Nb. Nonetheless, Zr-saturation thermometry (Watson and Harri-son, 1983) for felsic gneiss and some possible volcanic rocks indicates magmatic temperatures

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C D

E F

Figure 4. Field and thin-section photographs of mafi c rocks from Munsiari rocks, north-western India. These observations suggest that widespread chlorite schist that we observed in the fi eld was, in fact, sourced by basaltic material. (A) Multiple amphibolites with chlorite schist at top, interpreted as possible basalt fl ows. Mori region, NW India. (B) Metamor-phosed amygdaloidal basalt. Most white spheroids are plagioclase, but some also contain carbonate, quartz, or chlorite. These are interpreted as metamorphosed zeolitic infi llings. Pabar region, NW India. (C–F) Photomicrographs of greenschist- and amphibolite-facies metabasalt from the Pabar region, NW India, illustrating relict igneous textures and pro-gressive development of chlorite schist from a basaltic precursor. (C and D) Metamorphosed amygdaloidal basalt in plane polarized light and in crossed polars. Amygdules in this rock are now dominated by plagioclase, which probably represents the metamorphosed equiva-lent of original infi lling zeolites. Amygdules in other rocks contain carbonate + plagioclase ± chlorite, or quartz. Matrix silicate assemblage is plagioclase + hornblende + chlorite + bio-tite + epidote + quartz. (E) More deformed and hydrated amygdaloidal metabasalt, showing sheared relict amygdule and relict porphyritic feldspar texture. Matrix silicate assemblage is plagioclase + chlorite + biotite + epidote + titanite + quartz. (F) Chlorite schist, retaining a few relict porphyritic feldspars. Silicate assemblage is plagioclase + chlorite + biotite + titanite + quartz.



Kohn et al.


of 800 ± 50 °C (Table A2; Fig. 6), again consis-tent with relatively wet melting at low tempera-ture in an arc or collisional setting rather than the higher temperatures anticipated for a fl ood basalt or rift setting. Chambers et al. (2008) iden-tifi ed even lower temperatures for ca. 1810 Ma leuco granites in northwestern India, and as-cribed these to wet crustal melting. Correction of Zr contents for any zircon inheritance would lower calculated temperatures, further under-scoring low magmatic temperatures.

Geochronology

Published dates for mafi c rocks, orthogneiss, and crosscutting leucogranite generally fall be-tween 1810 and 1870 Ma (Table 1). These ages are distributed across the length of the Hima-laya, and their relatively limited age range and common geochemical characteristics imply a single coeval origin for these igneous rocks. The curvilinear distribution of these coeval rocks may refl ect their original orientation, al-

though later Cenozoic deformation could have changed their distribution. New data from Arunachal from felsic gneisses appear younger (ca. 1745 Ma; Yin et al., 2009) than most other lower Lesser Himalayan ages. These ages could represent a younger phase of the same magmatic event we propose across the Himalaya. Alterna-tively ages of plutonic rocks in Bangladesh are as old as 1720–1730 Ma (Ameen et al., 2007; Hossain et al., 2007), and a discrete younger magmatic event may be regionally signifi cant. It is important to note that Sharma and Rashid (2001) recognized the common ages for some of these rocks along the Himalaya and argued for formation in some consistent tectonomagmatic setting. However, they did not propose a geo-dynamic setting for these rocks, or otherwise offer any other genetic explanation.

New zircon geochronologic data (Table 1; Fig. 7; see GSA Data Repository1) further sup-port the occurrence of a Paleoproterozoic arc in Nepal but expand the possible age range to 1780–1880 Ma. Sampling and analytical meth-ods are described in the Appendix, and results are shown on standard Concordia diagrams (Fig. 7). These zircons are interpreted as igne-ous, rather than metamorphic, based on high Th/U ratios (see data repository; Hoskin and

1GSA Data Repository item 2009191, Zircon U-Pb isotopic data and XRF whole-rock data from possible igneous rocks from India and Nepal, compi-lation of zircon U-Pb and Pb-Pb ages from the Indian craton, and locations of samples from Nepal, is avail-able at http://www.geosociety.org/pubs/ft2009.htm or by request to [email protected].

10

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Y (ppm)

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)

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A

B

Figure 5. Trace-element data (Rb, Y, Nb) for Lesser Himalayan felsic igneous and metavol-canic (?) rocks plotted on discrimination diagrams (Pearce et al., 1984), showing best agree-ment with either volcanic arc or syncollisional origin. Compositions of Langtang (LT) samples, leucogranites from the Sutlej Valley, and possible metavolcanic rocks are slightly more consistent with an arc. Some specifi c outliers are identifi ed.

600 650 700 750 800 850 9000

3

6

9

12

15

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Zr-saturation temperatures

Leucogranites

Gneisses

Volcanic (?)

Num

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Figure 6. Zirconium saturation tempera-tures from Paleoproterozoic felsic gneisses, showing typical (maximum) temperatures of ~800 °C. Data from Le Fort and Raï (1999), Richards et al. (2005), Miller et al. (2000), Chambers et al. (2008), Sharma and Rashid (2001), and this study.





206Pb/238U

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Kohn et al.


Schaltegger, 2003) and elongate morphologies (Fig. 7; Corfu et al., 2003). Many zircons show evidence for major recent Pb loss that is either modern or Hima layan (<~40 Ma) or both. Yet considering analytical errors and the antiquity of these rocks, young Pb loss does not signifi cantly obscure crystallization ages. For samples from the Arun Valley in eastern Nepal, zircon ages range between 1780 and 1830 Ma, overlapping but somewhat younger than most published ages of other igneous rocks across the orogen (Table 1). Type Ulleri augen gneiss from Ulleri , Nepal (AS01-5), gives a similarly young, preferred crystallization age of ca. 1780 Ma, but with a likely inherited component of ca. 1880 Ma. Sample LT01-102, from Langtang, Nepal, is correlated with Ulleri augen gneiss, yet gives an age of ca. 1880 Ma—clearly older than type Ulleri , but indistinguishable from the age we infer for a felsic metavolcanic rock inter calated with Kuncha schist (LT01-44a). Note that we focused on analyzing zircon rims, as determined from cathodoluminescence images, but in LT01-44A we also analyzed numerous, presum-ably inherited cores. Core analyses show a radi-cally different pattern compared to rim analy ses: whereas rims strongly cluster at ca. 1880 Ma, cores range widely from 1880 to 3300 Ma.

Putatively detrital zircons in the lower Lesser Himalayan sequence across Nepal show a pre-ponderance of 1800–1900 Ma ages, implying abundant primary magmatic material of that age (e.g., DeCelles et al., 2000, 2004). In at least two instances, metasedimentary rocks yielded a single zircon age peak in that range (DeCelles et al., 2000, 2004). Similarly, we found a pro-nounced age peak in pelitic schist from Arun (AR01-4), although our analyses are strongly biased toward rims. Possibly some of these sedimentary rocks are of volcanic origin or have a major volcanogenic component, with zircons derived from local contemporaneous sources. These zircons probably did not derive from cra-tonal India, which exhibits a distinct age gap be-tween 1750 and 2450 Ma (Parrish and Hodges, 1996; Fig. 8). The only ages from the Indian craton that overlap the age distribution of Lesser Himalayan rocks are for a pluton in the Aravalli-Delhi belt, and for a suite of mafi c dikes in the Bastar craton (Figs. 1 and 8).

DISCUSSION

Most Evidence Points to an Arc

Several lines of evidence support an arc in-terpretation for the origin of many lower Lesser Himalayan rocks. Field observations reveal a widespread felsic igneous component within the Ranimata, Kuncha, and Daling Formations across

the ca. 1000 km breadth of the Nepal (Le Fort and Raï, 1999) and Bhutan sectors (Gansser, 1983). Metamafi c rocks are even more wide-spread (Stöcklin, 1980; Valdiya, 1980; Gansser, 1983; DeCelles et al., 2001; Robinson et al., 2006; McQuarrie et al., 2008), both as amphibo-lite and as mafi c chlorite schist (Fig. 4). These observations and interpretations differ markedly from previous studies in northwest India (e.g., Vannay and Grasemann, 1998; Richards et al., 2005) but agree better with reports from west-ern Nepal (e.g., DeCelles et al., 2001; Robin-son et al., 2006). Although some sections do contain abundant schists of sedimentary origin, we found abundant intrusive and volcanic rocks in the Munsiari of NW India, especially in the Pabar region (Figs. 3 and 4). The occurrence of these geographically widespread igneous rocks along a curvilinear belt further favors an active, rather than passive margin interpretation. Finally, distributed igneous rocks could also form in a rift environment, but the low Zr-saturation tem-peratures in felsic rocks (Chambers et al., 2008; this study), the trace-element geochemistry of mafi c and felsic rocks (Miller et al., 2000; this study), and the protracted (ca. 100 Ma) magma-

tism are more consistent with either an arc or a collisional origin.

An arc may also explain some lithologic changes along strike. In northwest India, fel-sic intrusive and felsic to mafi c volcanic rocks can dominate the Munsiari, for example in the Pabar region (Figs. 1, 3, and 4), whereas in Nepal such obvious igneous rocks are relatively rare at the same stratigraphic level. Instead, we hypothesize that the abundant schist in Nepal and Bhutan has a major volcanic or volcani-clastic component, based on low-Al, feldspar-rich mineral assemblages and mesoscopic and microscopic textures (Figs. 3 and 4). These differences along strike could simply refl ect geographic variations in magmatic intensity, dif-ferences in exposure through the arc sequence, or both. In India and Bhutan, a thick, lower Lesser Hima layan sequence of ca. 1830 Ma arc-related rocks overlain by a thick, upper Lesser Himalayan sequence of post–ca. 600 Ma rocks further underscores the fundamental strati-graphic disparity highlighted by Azmi and Paul (2004) and Hughes et al. (2005). Altogether our model implies that a Paleoproterozoic arc forms the stratigraphic base to the northern edge of the

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Age (Ma)

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Aravalli

LHS

Zircon ages

Indian Craton

BastarCratonmaficdikes

Figure 8. Compilation of zircon U-Pb and Pb-Pb ages of igneous rocks from the Indian cra-ton (black bars), showing pronounced peaks at ca. 2500 and ca. 1700 Ma, but paucity of ages at ca. 1800 Ma. Zircon ages from Table 1 shown as white bars. Age of proposed arc overlaps with ages of only two cratonal rocks: a ca. 1885 Ma mafi c dike complex in the Bastar craton and a ca. 1850 Ma intrusion in the Aravalli belt. Relatively young ages of Lesser Himalayan igneous rocks in Arunachal (ca. 1745 Ma; Yin et al., 2009) overlap with tail of a ca. 1700 Ma peak (e.g., data from Bangladesh and the Aravalli belt). The Arunachal gneisses may repre-sent rocks formed in a different setting, or continuation of arc activity to ca. 1750 Ma.





exposed Indian plate, and provides a stronger basis for correlating rocks along the Himalaya and for inferring structure, particularly along the Main Central Thrust, where Greater and Lesser Himalayan rocks are juxtaposed. The arc is mostly preserved as volcanically derived sediments but with important felsic and mafi c intrusive and volcanic components.

Although the northern edge of the arc, in-cluding any accretionary material, is likely buried beneath the Himalaya and Tibet, some enigmatic rocks on the Indian craton could per-haps be genetically linked. Specifi cally, a suite of mafi c dikes in the Bastar craton has recently been dated at ca. 1885 Ma (French et al., 2008), at the beginning of the time when we propose the arc was active. Possibly this dike swarm refl ects thermal disturbances related to arc ini-tiation or, alternatively, to backarc spreading. Further geochemical analysis of the dikes might help elucidate their origin(s) and links to Lesser Himalayan rocks. Note that the long hiatus be-tween deposition of lower and upper Lesser Hima layan rocks complicates any attempts to infer postsubduction processes.

Indian “Basement” and a Paleoproterozoic Collision?

Several studies have interpreted the ortho-gneisses in the lower Lesser Himalayan se-quence (e.g., “Ulleri”) to represent Indian cratonal basement (e.g., Gansser, 1964; Ray et al., 1989; Richards et al., 2005; Yin, 2006; Yin et al., 2009). In this model lower Lesser Himalayan sediments were either deformed and metamorphosed during the early Protero-zoic, or deposited unconformably on crystalline plutonic rocks and gneisses. In principle, the crystalline rocks could represent new crustal additions at ca. 1830 Ma, i.e., the roots of a Proterozoic arc, or alternatively recrystallized Archean material, i.e., older plutons and sedi-ments that were deformed and metamorphosed during a ca. 1830 Ma collisional event (e.g., see Fig. 9 of Richards et al., 2006). Such a model does have some supporting evidence: relict zircon cores are as old as 3300 Ma, and trace-element geochemistry for many plutonic rocks is as consistent with a collisional origin as with an arc. Two key observations, however, do not favor either a wholly plutonic origin for 1800 Ma igneous rocks or collisional reworking of older materials.

(1) The oldest metamorphic age (for al-lanite) yet recovered from Lesser Himalayan rocks is less than 500 Ma (Catlos et al., 2000). For example, garnet ages are 7–11 Ma (Vannay et al., 2004), and monazite ages are as young as ca. 3 Ma (Catlos et al., 2001, 2007; Kohn et al.,

2004). Thus, there is as yet no direct metamor-phic evidence for a Paleoproterozoic collision.

(2) Felsic volcanic rocks are intercalated with felsic plutonic and mafi c volcanic rocks and yield similar ages as the plutons (Le Fort, 1975; Le Fort and Raï, 1999; Richards et al., 2005, 2006; this study). A shallow origin for these vol-canic rocks is indisputable—in addition to relict volcanic textures (Figs. 3 and 4), many are inter-calated with sedimentary rocks. Thus, igneous rocks of the lower Lesser Himalayan sequence cannot represent only the crystalline roots of a volcanic arc either. The simplest interpretation is that, although some transposition of contacts and shearing must have occurred in the Ceno-zoic, the present intercalation is largely primary. Thus, the lower Lesser Himalayan gneisses can be neither assigned to Indian basement nor as-cribed to Paleoproterozoic collision.

Reconstruction of the Columbia (ca. 1800 Ma) Supercontinent

A 1780–1880 Ma arc along the northern mar-gin of India may help resolve debate about the confi guration of the ca. 1800 Ma supercontinent Columbia. Three basic models have been pro-posed for the relative placements of India, North America, and East Antarctica (Fig. 9). Rogers and Santosh (2002) and Zhao et al. (2004) sandwich East Antarctica between India and North Amer-ica, leaving the northern edge of India as a pas-sive margin, in agreement with many views of the origins of the Lesser Himalayan sequence. In contrast, Hou et al. (2008) place India directly adjacent to North America, with a continuous subduction zone that includes the northern edge of India and parts of East Antarctica. Although some models could perhaps be modifi ed to in-clude subduction along the northern edge of

India , Hou et al.’s model is the only one pro-posed so far that conforms to our interpretation of the Lesser Himalayan sequence. We empha-size that several other competing models have been proposed for the confi guration of Colum-bia that infer quite different positions than Hou et al. (2008) for North America, Baltica, Aus-tralia, etc. (Krapez, 1999; Betts et al., 2008; Bispo-Santos et al., 2008; Payne et al., 2009). Our interpretation for India in no way validates or refutes these other models. It does, however, suggest that ≥2500 Ma Indian cratonal provinces should not be extrapolated to the north onto other continents at ca. 1800 Ma, because this margin appears to have been active at that time.

“Detrital” Zircon Analysis

If correct, our model has additional impli-cations for the collection and interpretation of detrital zircon ages. Many laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) studies of presumed detrital zir-cons focus on analyzing cores. This approach can minimize potential Pb-loss problems that we clearly encountered in several of our analy-ses. It further assumes that the resulting age spectrum is diagnostic of the source material and, for the youngest retrieved ages, limits the maximum depositional age (e.g., DeCelles et al., 2001). While we fully endorse detrital zircon dating in such endeavors, we do note that the data distribution for LT01-44A is highly skewed for cores compared to rims. For rims, 44 of 54 analyses yielded indistinguishable ca. 1880 Ma ages that we interpret as a crystal-lization age. In contrast, only four core analy-ses indicated this age, whereas the remaining 16 core analyses ranged between ~2100 and 3300 Ma. Had we analyzed only cores, as is

NANAEA

NC

EA

NACEA

A B C

NCv v v v

v v v

v

v v

v v

Figure 9. Plate reconstructions for ca. 1800 Ma supercontinent Columbia showing differ-ent positions for India (shaded); v’s indicate proposed volcanic arc. (A and B) Models of Rogers and Santosh (2002) and Zhou et al. (2004) showing passive margin for northern India. (C) Model of Hou et al. (2008) showing subduction zone along northern Indian margin, ex-actly as we propose. NA—North America; EA—East Antarctica; CEA—Coastal East Antarc-tica; NC—North China craton. Note that several other models for Columbia do not attempt to reconstruct India’s position; therefore, our proposed arc does not discriminate among them.



Kohn et al.


commonly done, we would still have identifi ed the youngest, ca. 1880 Ma age, but we would have missed just what a vast preponderance of zircon was formed at that time.

Although we presumed this rock was a metamorphosed tuff based on its mineralogy and physical appearance—an interpretation supported by its bulk chemical composi-tion ( Table A1)—not all volcanic rocks are so distinctive after metamorphism, and could be interpreted and processed as if they were metasandstones. In that regard, we cannot di-rectly evaluate any possible bias inherent in pub-lished zircon age distributions for the Himalaya. Some workers document primary sedimentary features both in the fi eld and petro graphically, indicating substantial reworking of detrital zircons (P. DeCelles, 2009, personal com-mun.), but details are rarely published. Possibly some lower Lesser Himalayan rocks could be metavolcanic with inherited zircon cores, much as we interpret LT01-44A. If so, the common >1900 Ma ages obtained from “detrital” zircon cores represent inheritance, and closer consider-ation of magmatic rim overgrowths will reveal an even more pronounced 1830 ± 50 Ma age peak. Such refi ned ages might help delineate the extent of the Paleo protero zoic rocks as well as the structure of Himalayan thrusts.

CONCLUSIONS

We hypothesize that the basal part of the Lesser Himalayan sequence represents the edge of a 1830 ± 50 Ma continental arc, based on fi eld and textural observations, whole-rock chemistry, and geochronology. This model explains the occurrence of so many coeval igneous rocks distributed widely along the Himalaya, the chemistry of the metasedimen-tary and igneous rocks, and the distribution of zircon ages. Younger, ca. 1745 Ma ages in Arunachal (Yin et al., 2009) may extend the duration of the arc by an additional ca. 30 Ma. These results distinguish among models for the placement of India in the ca. 1800 Ma super-continent Columbia. The model of Hou et al. (2008) is consistent with an active margin for northern India. Thus, although Hou et al. (2008) wrote: “Another subduction zone may have existed…along the northern margin of the Indian Craton, but has been lost beneath the Eurasian Plate,” we fi nd no need to hypoth-esize a “Lost Arc of the Continent”: a Paleo-proterozoic arc is present and can be accounted for. Felsic orthogneisses (“Ulleri”) within the Lesser Himalayan sequence probably do not represent Indian basement but rather relatively shallow intrusions in an arc edifi ce and associ-ated sedimentary pile.

APPENDIX: SAMPLES AND ANALYTICAL METHODS

Samples that we analyzed for U-Pb zircon ages were collected from the Annapurna (sample prefi x AS01), Langtang (LT01), and Arun (AR01) regions of central and eastern Nepal. See the Data Repository (footnote 1) for maps showing sample locations. Arun samples were all collected from a granitic orthogneiss unit sometimes referred to as the Num orthogneiss, but they are also correlated with the Ulleri augen gneiss (e.g. Goscombe and Hand, 2000; Goscombe et al., 2006). Three Arun samples are metagranite, and one sample (AR01-4) is a pelitic schist dominated by quartz, muscovite, and biotite. Sample AS01-5 is from Ulleri augen gneiss from its type locality in the town of Ulleri, Nepal. Sample LT01-102 is granitic orthogneiss from the Munsiari thrust sheet, and is cor-related with Ulleri augen gneiss based on stratigraphic position and appearance (e.g. Kohn, 2008). Because some felsic orthogneisses correlated with Ulleri augen gneiss give different ages, we refer to them as “Ulleri .” Sample LT01-44a appeared texturally to be a metamorphosed felsic tuff (Fig. 3G).

Zircons were separated using standard separation techniques at Boise State University, and mounted with age standard FC-1, which has a U-Pb age of 1099 Ma (Paces and Miller, 1993). Zircons were im-aged using cathodoluminescence at the Center for Electron Microscopy and Microanalysis at the Uni-versity of Idaho, and analyzed for U-Pb and Pb-Pb ages in the Geoanalytical Laboratory, Washington State University, using methods described by Chang

et al. (2006). In brief, each sample was ablated using a 213 nm laser operating with a 30 µm diameter spot and ~10 J/cm2 fl uence. Each spot was accurately plot-ted on a cathodoluminescence (CL) image, particu-larly noting whether a core versus a rim was analyzed, as determined either chemically, spatially, or both. Ablated material was carried on a He stream to the source of an Element2 ICP-MS, a single-collector, magnetic sector instrument. Lead, thorium, and ura-nium isotopes were measured in low-resolution mode, ratioed, and corrected for in-run drift. Mass fraction-ation corrections were based on bracketing analyses of FC-1. No correction for 204Pb was made because, within uncertainty, there was no 204Pb after correct-ing for 204Hg interference (based on measurements of 202Hg), and because even the raw ratio of mass 206 to mass 204 typically exceeded 5000, implying a neg-ligible common Pb correction. Ratios of Th/U were estimated for analyses at least 95% concordant for 235U-207Pb and 238U-206Pb by assuming concordance between U-Pb and Th-Pb ages.

Corrected ratios, uncertainties, and ages include a ~1%–2% standardization error based on the scatter of analyses of FC-1, i.e. the standard deviation, not standard error. As discussed elsewhere (Chang et al., 2006; Kohn and Vervoort, 2008), this error assignment accurately accounts for errors for a single analysis of an unknown, but it overestimates errors for pooled data. Consequently, the mean square of the weighted deviates (MSWD) for age regressions will be er-roneously small. This is evident in our data (Fig. 6), where MSWDs are routinely less than ~0.5—mainly a result of how we choose to propagate standardiza-

TABLE A1. CHEMISTRY OF METAVOLCANIC (?) ROCKS COMPARED TO GRANITE, AVERAGE SHALE, AND CA. 1830 MA HIMALAYAN FELSIC ORTHOGNEISSES

Possible metavolcanic rocksSource R05 R05 C08 C08 C08 This study This studySample W50 W54 63ii 71ii 81 SV798 LT01-44aSiO2 75.89 68.99 68.98 67.84 70.70 74.05 78.67TiO2 0.20 0.63 0.61 0.72 0.49 0.63 0.35Al2O3 13.77 14.97 15.91 15.39 14.47 11.32 11.35Fe2O3 1.44 4.25 4.86 5.47 4.74 4.35 2.05MnO 0.01 0.03 0.04 0.08 0.25 0.08 0.23MgO 0.94 2.80 1.51 1.52 1.32 1.70 0.47CaO 1.25 1.08 1.95 2.62 2.92 1.48 3.09Na2O 2.32 1.85 1.85 3.12 2.74 1.49 1.96K2O 3.36 4.71 3.73 2.54 2.24 2.99 1.21P2O5 0.09 0.11 0.15 0.17 0.11 0.10 0.10LOI 1.00 1.03 1.41 0.96 0.82 N.D. N.D.Sum 100.27 100.46 101.00 100.42 100.80 98.18 99.48K/Na 0.95 1.68 1.33 0.54 0.54 1.32 0.41Zr (ppm) 102 167 176 213 149 250 159

Reference compositions of granite, Lesser Himalayan felsic orthogneisses (M00, R05), and shales (PAAS, NASC, Archean)

Granite M00 M00 R05 R05 PAAS NASC ArcheanSample HF49/90 HF35/92 W60 W65SiO2 73.30 68.20 74.60 72.37 68.55 62.80 64.80 60.95TiO2 0.28 0.60 0.20 0.37 0.63 1.00 0.70 0.62Al2O3 13.50 13.70 13.40 14.50 15.20 18.90 16.90 17.50Fe2O3 2.30 4.90 1.70 3.12 4.36 6.50 5.66 7.53MnO 0.10 0.00 0.03 0.05 0.11 0.06 n.d.MgO 0.42 0.50 0.30 0.89 0.91 2.20 2.86 3.88CaO 1.30 2.10 1.00 0.96 1.26 1.30 3.63 0.64Na2O 3.20 2.40 2.70 1.87 2.84 1.20 1.14 0.68K2O 4.80 5.50 5.10 4.76 4.63 3.70 3.97 3.07P2O5 0.20 0.10 0.15 0.18 0.16 0.13 0.61LOI 0.08 0.50 0. 01.013.120.103Sum 99.91 98.70 99.40 100.04 99.92 97.87 99.85 95.58K/Na 0.99 1.51 1.24 1.67 1.07 2.03 2.29 2.97Zr (ppm) 240 372 150 148 223 210 200 151 Note: PAAS from Taylor and McLennan (1985); NASC from Gromet et al. (1984); granite and Archean from Condie (1993); M00—Miller et al. (2000); R05—Richards et al. (2005); C08—Chambers et al. (2008).





tion errors. Interstandard errors were not included in our age assignments, and some studies suggest these may contribute an additional 1%–2% systematic error (e.g., Chang et al., 2006). Thus, for intercomparisons of sample ages, an additional ±20–35 Ma systematic uncertainty should be considered.

Two possible metavolcanic rocks that were ana-lyzed for whole-rock chemistry (Table A1) were collected in the Pabar, India (SV798) and Langtang, Nepal (LT01-44a) regions. Whole-rock, major- and trace-element compositions were measured by X-ray fl uorescence (XRF) at the GeoAnalytical Laboratory, Washington State University, using standard tech-niques. The supplemental fi le (see footnote 1) contains additional whole-rock compositions, including sam-ples from the Annupurna, Langtang, and Arun areas.

ACKNOWLEDGMENTS

This material is based upon work supported by the National Science Foundation under grants EAR-0337050, EAR-0439733, and EAR-0803549 to MJK, and by Boise State University. We thank Dr. B.R. Arora, Director, Wadia Institute of Hima-layan Geology, Dehradun, India, for providing logistical support and for granting permission to under take this joint American-Indian fi eld study, and Dr. S. Sensarma (Lucknow) for reviewing the manuscript. Comments from T. Argles, P. DeCelles, C. Dehler , D. Evans, N. McQuarrie, and associate editor A. Yin helped improve the presentation and clarify the authors’ understanding of Himalayan stratigraphy.

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TABLE A2. ZIRCONIUM-SATURATION TEMPERATURES AND TRACE-ELEMENT CONTENTS FOR LOWER LESSER HIMALAYAN INTRUSIVE AND VOLCANIC (?) ROCKS

Sample number

Zirconium temperature

(°C)

Rock Rb, Y, Nb contents (ppm)

Reference

)1(.D.NssiengneguairellU458202OLShang 848 Shang orthogneiss 177, 56, 31 (2)Kotla 859 Kotla Complex 336, 17, 15 (2)W50 765 Jutogh “paragneiss” 150, 27.4, 12.8 (3)W54 805 Jutogh “paragneiss” 279, 23.4, 18.1 (3)W60 796 Jutogh gneiss 234, 32.1, 10.2 (3)W65 818 Jutogh augen gneiss 272, 35.9, 18.2 (3)HF49/90 840 Metarhyolite 259, 35, 19 (4)HF39/90 770 Granite 241, 11, 11 (4)HF46/90 779 Granite 350, 13, 24 (4)

)4(41,3,943etinarG68709/201FHHF106/90 770 Granite 282, 18, 10 (4)HF35/92 780 Wangtu granite 298, 25, 16 (4)63i 665 Jutogh leucogranite 111, 15.3, 12.7 (5)70i 680 Jutogh leucogranite 84, 26.1, 10.0 (5)70ii 662 Jutogh leucogranite 77, 25.3, 5.4 (5)63ii 810 Jutogh “pelite” 163, 30.6, 12.9 (5)71iiiy 812 Jutogh “pelite” 248, 39.7, 16.4 (5)71ii 809 Jutogh “pelite” 131, 36.1, 14.9 (5)

)6(82,192ssiengdeniarg-esraoC08734-JMMJ-44 787 Coarse-grained gneiss 285, 29, 21 (6)

)6(11,42,ssiengdeniarg-esraoC87764-JMMJ-114 770 Coarse-grained gneiss 335, 19, 15 (6)MJ-153 846 Coarse-grained gneiss 246, 18, 17 (6)MJ-154 841 Coarse-grained gneiss 282, 15, 18 (6)MJ-155 827 Coarse-grained gneiss 246, 19, 19 (6)MJ-158 751 Coarse-grained gneiss 225, 17, 13 (6)MJ-160 766 Coarse-grained gneiss 383, 21, 14 (6)MJC-66 748 Coarse-grained gneiss 404, 15, 14 (6)MJC-67 820 Coarse-grained gneiss 383, 21, 17 (6)MJ-11 750 Fine-grained gneiss 511, 13, 27 (6)MJ-16 788 Fine-grained gneiss 205, 20, 22 (6)MJ-21 783 Fine-grained gneiss 436, 26, 20 (6)MJ-22 785 Fine-grained gneiss 329, 25, 17 (6)MJ-23 804 Fine-grained gneiss 318, 22, 20 (6)

)6(31,22,ssiengdeniarg-eniF37762-JMMJ-40 751 Fine-grained gneiss 321, 21, 17 (6)MJ-41 754 Fine-grained gneiss 300, 24, 14 (6)MJ-42 758 Fine-grained gneiss 255, 30, 17 (6)MJ-149 769 Fine-grained gneiss 288, 27, 11 (6)MJ-151 748 Fine-grained gneiss 332, 29, 14 (6)MJ-152 785 Fine-grained gneiss 267, 30, 18 (6)MJ-156 824 Fine-grained gneiss 644, 144, 21 (6)MJ-157 763 Fine-grained gneiss 383, 32, 22 (6)SV798 840 Metatuff 122, 33, 11.5 (7)AS01-5 759 Ulleri gneiss 319, 30, 17.9 (7)

)7(6.9,92,68ffutateM887a44-10TLLT01-102 895 “Ulleri” gneiss )7(8.7,22,29AR01-10c 686 Metagranite 615, 19, 19.3 (7)AR01-15 780 Metagranite 379, 24, 15.9 (7)

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