mélanges in southern mexico: geochemistry and metamorphism...

Mélanges in southern Mexico: geochemistry andmetamorphism of Las Ollas complex (Guerreroterrane)

Oscar Talavera Mendoza

Abstract: Las Ollas complex (LOC) is a subduction complex spatially associated with the early CretaceousZihuatanejo–Huetamo subterrane (Guerrero terrane) in southern Mexico. LOC tectonic mélanges compose of a stack ofeast-dipping, west-vergent tectonic sheets containing blocks of metabasalt, metadolerite, metagabbro, ultramafics,volcaniclastics, quartz-rich sandstone, and chert enveloped in a highly sheared clastic or serpentinitic matrix. Most ig-neous and igneous-derived metamorphic blocks show geochemical and isotopic features typical of island-arc tholeiiticsuites: (i) low TiO2 (0.13 to 0.91%) and Zr (5 to 57 ppm) contents; (ii ) high (LFSE/HFSE)N ratios; low LaN/YbN (0.5to 4) values; and, highεNd

(T) (+7.9 to +8.0) ratios. Petrographical and mineral chemistry evidence indicates that blocksunderwent early recrystallization under high pressure and low temperature (HP–LT), blueschist facies conditions duringsubduction. Typical assemblages include blue (sodic through calco-sodic to Na-rich calcic) amphibole + lawsonite ±tremolite ± Mg-chlorite ± white mica ± albite ± quartz. Phase relations and chlorite thermometry suggest temperaturesof about 200°–330o C and pressures of 5–7 kbar. It is proposed that sedimentary blocks were generated by in situremobilization and mixing, whereas igneous blocks most probably derived from the chemically and isotopically identi-cal Zihuatanejo island-arc suite. Our data suggest that LOC represents part of a subduction complex formed by east-ward-directed subduction related with the evolution of the early Cretaceous Zihuatanejo island arc.

Résumé: Le complexe de Las Ollas (LOC) est un complexe de subduction associé dans l’espace au sous-terraneZihuatanejo-Huetamo, datant du Crétacé, et localisé dans le sud du Mexique. Les mélanges tectoniques du LOC com-prennent un empilement de feuillets tectoniques à vergence ouest et à pendage est qui contiennent des blocs de méta-basalte, métadolérite, métagabbro, roches ultramafiques, roches volcanoclastiques, grès riches en quartz et du chertenveloppés dans une matrice clastique ou serpentineuse hautement cisaillée. La plupart des blocs ignés et métamorphi-ques provenant de roches ignées présentent des caractéristiques géochimiques et isotopiques typiques de suites tholéiiti-ques d’arc insulaire : (i) faibles concentrations de TiO2 (0,13 à 0,91 %) et de Zr (5 à 57 ppm); (ii) rapports élevés(LFSE/HFSE)N; des valeurs LaN/YbN faibles (0,5 à 4) et des rapportsεNd

(T)(+7,9 à +8,0) élevés. Selon les évidencespétrographiques et de chimie minérale, les blocs auraient subi une recristallisation précoce à des pressions élevées/basses températures au faciès des schistes bleus au cours de la subduction. Des assemblages typiques comprennent del’amphibole bleue (sodique, à claco-sodique, à calcique riche en Na) + lawsonite ± trémolite ± chlorite Mg ± micablanc ± albite ± quartz. Les relations de phases et la thermométrie de la chlorite suggèrent des températures d’environ200–300°C et des pressions de 5 à 7kbar. On propose que les blocs sédimentaires ont été générés par de la remobili-sation et des mélanges in situ, alors que les blocs ignés ont probablement été dérivés de la suite d’arcs insulaires Zi-huatanejo qui est chimiquement et isotopiquement identique. Nos données suggèrent que le LOC représente une partied’un complexe de subduction formé par de la subduction vers l’est reliée à l’évolution de l’arc insulaire Zihuatanejodatant du Crétacé précoce.

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IntroductionIntraoceanic plate convergence throughout much of the

late Mesozoic resulted in widespread volcanic arc activityalong the western margin of Mexico (Campa and Coney1983). As a consequence, the Late Jurassic–Cretaceous arc-related sequences of the Guerrero terrane (GT) of westernMexico were formed (Fig. 1). It has been demonstrated that

this terrane represents large juvenile contributions to theEarth’s crust and that terrane is certainly of composite na-ture. Recent studies south of the present Mexican volcanicbelt have resulted in a subdivision of the GT into three dis-tinctive subterranes (for a detailed description of the geologyand geochemistry of the entire southern Guerrero terrane,see Talavera 1993): (i) The Teloloapan subterrane representsan evolved island-arc suite of Hauterivian–Aptian age);(ii ) The Arcelia – Palmar Chico subterrane is interpreted asan island arc – back-arc basin system of Albian–Cenomanian age; and (iii ) The Zihuatanejo–Huetamosubterrane comprises a subduction complex – island arc –marginal basin system of Late Jurassic(?) – Albian age.Complex tectonics related to accretion during Laramide

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Received April 30, 1999. Accepted March 28, 2000.Published on the NRC Research Press Web site onSeptember 1, 2000.

O. Talavera Mendoza.Escuela Regional de Ciencias de laTierra, Universidad Autónoma de Guerrero. A. P. 197, Taxco(Gro.) Mexico. (e-mail: [email protected])

times juxtaposed arc elements, resulting in a stack of east-vergent fault-bounded slices.

The Zihuatanejo–Huetamo subterrane forms the western-most segment of the southern Guerrero terrane (Fig. 1). Ithas been recently proposed that this subterrane representsthe evolution of a Late Jurassic – Early Cretaceous intra-oceanic island arc installed on previously deformed, (pre-Middle Jurassic) oceanic crust and includes the Arteagacomplex, the Zihuatanejo volcanic arc massif suite, theHuetamo marginal basin assemblage, and the Las Ollassubduction complex (LOC; Ramírez et al. 1991; Talavera etal. 1993; Centeno et al. 1993a, 1993b). The latter is nowconsidered an important piece within the GT and provides adetailed petrotectonic record of convergence of the ZHSTarc system, as well as a fundamental marker of subductionpolarity. Although the presence of this block-in-matrix se-quence has been recognized in the Zihuatanejo region sinceit was presented by Vidal (1984a), and was thought to bepart of a newly discovered subduction complex, its geology,structure, composition, and metamorphism remain essen-tially unknown. This paper summarizes and discusses themain structural, petrological, geochemical, isotopic, andmetamorphic characteristics of the LOC in an attempt tobetter understand the petrotectonic history of this poorlyknown subduction complex. In this paper, the term mélangeis used in its descriptive meaning and refers to a block-in-matrix sequence.

Sampling and analytical techniques

A number of traverses encompassing the LOC and theoverlying Zihuatanejo arc series were carried out in the field.Based on the continuity of outcrops, two cross-sections(Puerto Escondido and Las Ollas) were selected for detaileddescriptions and sampling (Figs. 1, 2). Systematic samplingwas focused on blocks based on their megascopic character-istics, i.e., block size, rock type, phenocryst content, fabric,and colour variations. More than 120 block samples werepetrographically studied, eight of which were selected formicroprobe analyses and six for whole rock determinations.No matrix samples were analyzed.

Mineral compositions were measured in an automatedCAMEBAX microprobe at the ENSEEG, Université JosephFourier, Grenoble, France using Albite (Na), K-Feldspar(K), Corundum (Al), Wollastonite (Si), Forsterite (Fe), Apa-tite (Ca), Chromite (Cr), Rutile (Ti), Rodonite (Mn), andMgO (Mg) as standards. Analytical conditions were a con-stant beam current at 10 na, a 15 kV accelerating potential,and a spot size at 1–3µm. Count times were, in general,10 s, except for Na for which shorter 6 s count times wererequired. Under these conditions, concentrations below 0.1%are not representative. Concentrations of major-, trace-, andrare earth (REE) elements in whole rock were measured us-ing inductively coupled plasma – atomic emission spectrom-etry (ICP-AES) techniques at the Centre de RecherchesPétrographiques et Géochimiques de Nancy, France. Analyt-ical errors are as follows: 0.1 to 0.3% for major elements;0.5 ppm for trace elements and REE lower than 10 ppm; and5% for those higher than 10 ppm.

Structure and age

A generalized geological map of the Zihuatanejo regionafter Vidal (1984b) showing the known outcrops of the LOCis presented in Fig. 1. LOC exposures are generally isolatedand often are found as scattered tectonic windows. The larg-est and most representative outcrops are those of PuertoEscondido, Camalotito, and Las Ollas, the latter being un-doubtedly the best exposure recognized at present. LOC is infault contact and overlain by the Zihuatanejo volcanics, butit is more commonly surrounded by granitic intrusions.Figure 2 shows schematic structural sections of PuertoEscondido and Las Ollas areas. The complex is composed ofa pile of variable-sized, fault-bounded packages with arather variable overriding angle and a very constant west-southwest vergence. Two main lithological associations canbe recognized within the packages: (i) an association con-taining blocks of metabasalt, metadolerite, metagabbro, andvolcaniclastics surrounded by a clastic matrix (Fig. 3a); and(ii ) an association containing blocks of metagabbros andpartially or completely serpentinized ultramafites within amatrix of fibrous serpentine (Fig. 3b). Blocks are elongate,irregularly shaped or rounded and range in size from 10 cmto more than 250 m in the longest dimension. Matrix is over-all not abundant, and in some localities, it is completely ab-sent. In this case, blocks often override one another.

Matrix is highly sheared and folded, developing asubhorizontal, anastomosing schistosity and west-vergentisoclinal to tight folds. Bedding in the clastic matrix is gen-erally disrupted, pulled apart, or boudinaged (Fig. 3c). Ser-pentinite-rich matrix shows no isoclinal folds, butdeformation is evident by the presence of numerous sheared,undulated surfaces. Deformation in blocks is less penetra-tive, but has produced elongation in the blocks of small size(<2 m), with the largest axis following the strike of the re-gional schistosity. Some blocks show mylonitization and de-veloped asymmetrical structures, which systematicallyindicate a top-to-the-west sense of shearing. S-C surfaces indisrupted matrix are consistent with this statement (Fig. 3c).In the larger blocks (>2 m), deformation was less pervasiveand shearing limited to narrow bands (Fig. 3d).

Since no paleontological data are available, the age of thecomplex is poorly constrained. Based on field observations,some authors (e.g., Campa and Ramírez 1979; Vidal 1984b)have considered Las Ollas complex as an assemblage relatedto the evolution of the early Cretaceous Zihuatanejo arc se-quence. The40Ar/39Ar and K/Ar ages obtained by Delgadoet al. (1992) on brown (magmatic) amphibole from severalgabbro dikes intruding LOC at Puerto Vicente Guerrerorange from 112 ± 3 to 96.3 ± 2.5 Ma (Albian – lowerCenomanian), which could be considered as the minimumage of the complex. However, the whole-rock Rb–Srisochron of 311 ± 30 Ma (Carboniferous) reported by DeCserna et al. (1978) from four gabbros and leucomonzonitesat Petatlan, near Puerto Vicente Guerrero, could suggest anolder age for the complex, although whether these gabbrosand leucomonzonites are intruding the LOC or thesubvolcanic sustratum (Arteaga Complex) is unclear. Youn-ger K/Ar and Rb/Sr ages obtained from green (metamorphic)amphibole and on whole rock are mid-Tertiary and rangefrom 33.1 ± 1.5 to 40 ± 1.5 Ma (Delgado and Morales 1983;

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Talavera 1993), which are consistent with radiometric K/Arages, obtained in undeformed plutons (34 to 44 Ma. DamonIn Vidal 1984b; Stein et al. 1994), and thus they might re-flect reequilibration during thermal metamorphism. Thereare no available ages of the HP–LT metamorphic event.

Petrography and magmatic affinity ofblocks

Blocks within LOC comprise a diverse range of rocktypes, which, in decreasing abundance, are metagabbro,metabasalt, ultramafite, volcaniclastics, amphibolite, andmetadolerite. Vidal (1984b) describes, in addition, ribbonchert, limestone, and acidic plutonic rocks in the Camalotitoarea, a locality that is not discussed in this paper. In spite ofdeformation and metamorphism, igneous blocks have somemagmatic structures and textures preserved, although pri-mary phases were nearly completely replaced by metamor-phic assemblages.

Metagabbros show either cumulative or doleritic texturesand consist of amphibole, uralitized clinopyroxene,plagioclase, magnetite cubes (Mag97–100 Chrom0–3), andacicular ilmenite (Ilm50–93 Hem0–14). Porphyritic metabasalts(10–30% volume of phenocrysts) include completelyuralitized clinopyroxene, plagioclase, magnetite (Mag97–100Chrom0–3), and ilmenite (Ilm93–96 Hem3–6). Ultramafites aregenerally highly serpentinized, although in some bodies pri-mary phases were partially preserved. Ultramafites are com-posed of clinopyroxene alone (clinopyroxenite), olivinealone (dunites), or, more commonly, of olivine associated

with orthopyroxene and clinopyroxene (werhlites andlherzolites). Amphibole and plagioclase are common phasesin some ultramafites, and oxides are present in most ofthem. Amphibolite is not uncommon within the complex andhas been only recorded in the Las Ollas area. Amphiboliteshows a well-defined foliated fabric, which sometimes doesnot coincide with the regional schistosity, suggesting exten-sive disruption during deformation. Amphibolite is mediumto coarse grained and is composed essentially of green tobrown amphibole, plagioclase, and oxides.

Geochemically, the analyzed blocks are uniformly basic(45.6 wt.% < SiO2 < 52.7 wt.%), although there is a moder-ate to great variation in most major and trace element abun-dances (Table 1). Metagabbros show the widest elementvariation and certainly reflect variations of their cumulatenature, whereas volcanic and volcanic-related metamorphicrocks show identical compositions. Absolute concentrationsof MgO (5.94–11.35 wt.%), Cr (200–535 ppm), and Ni (46–163 ppm), although variable, are globally moderate. The Mgnumbers (52–54) are also moderate and rather constant, ex-cept for the metabasalt, which has the lowest (39) value, in-dicating that samples represent crystallization of somewhatevolved magmatic melts. TiO2 (0.13–0.91 wt.%) and Zr (5–57 ppm) contents are also variable but rather low, and fall inthe range recorded for subduction-related magmatic suites(Shervais 1982; Pearce 1983). Alkali and related elementssuch as CaO (7.91–16.85 wt.%), Na2O (1.03–5.19 wt.%),K2O (0.01–2.06 wt.%), Rb (5–33 ppm), Ba (45–284 ppm),and Sr (91–258 ppm) also show an extremely large variationeven among samples that have identical SiO2 contents.

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Talavera Mendoza 1311

Fig. 1. Geological map of the Zihuatanejo–Huetamo Terrane in the Zihuatanejo region showing the main outcrops of the Las Ollascomplex (After Vidal 1984b). Sample location is shown in the two studied sections.

These variations were very likely produced by metasomaticaddition or leaching during metamorphism.

Variations of elements can be observed in the multi-element patterns normalized to normal mid-ocean ridge bas-alts (N-MORB; Sun and McDonough 1989) shown inFig. 4a. Volcanic and volcanic-related metamorphic rocksshow only little variation, whereas metagabbros display arather wide range of lithophile element compositions. Cumu-late metagabbros show the lowest high field strength element(HFSE) concentrations, whereas doleritic metagabbros, am-phibolites, and metabasalts contain the highest concentra-tions. It suggests that cumulate metagabbros representcrystallization of less-differentiated magmas relative todoleritic metagabbros, amphibolites, and metabasalts. Inspite of this, all samples are significantly enriched in mostlow field strength element (LFSE) and depleted in HFSE rel-ative to N-MORB, and resemble those patterns reported forpresent-day immature island-arc tholeiitic suites (Pearce1983; Sun and McDonough 1989). Ratios of N-MORB-normalized LFSE relative to HFSE, although variable, aresystematically high (e.g., (Rb/Yb)N = 14.7 to 197.5;(Ba/Er)N = 40.6 to 291.9), consistent with arc features. Fur-thermore, most samples show a negative anomaly in the so-called orogenic elements, Zr and Ti, which is considered asdistinctive of arc-related magmatic suites (Sun andMcDonough 1989).

REE abundances are variable and reflect the nature ofrock types. Cumulate metagabbros have the lowest absoluteabundances (LaN = 2.41–10 YbN = 2.67–5.03), which is con-

sistent with their crystallization from less differentiatedmelts, whereas doleritic metagabbros, amphibolites, andmetabasalts have the highest REE abundances (LaN = 7.83–12.54; YbN = 5.52–11.64). REE patterns shown in Fig. 4bwere constructed using the chondrite normalizing values ofEvensen et al. (1978). Two samples of cumulatemetagabbros show different REE patterns. The Mx–211metagabbro is highly depleted in light REE (LREE) relativeto heavy REE (HREE; (La/Yb)N = 0.5), a characteristiccommonly linked to the presence of cumulus clinopyroxene.In contrast, the Mx–206 metagabbro is enriched in LREEand depleted in HREE ((La/Yb)N = 4), typical of amphibole-and plagioclase-rich cumulates (McKay 1989). Doleriticmetagabbros show also different patterns. The M–39 meta-gabbro is characterized by depletion in both LREE andHREE relative to medium REE (MREE; (La/Yb)N = 1.42),whereas the M–29 metagabbro shows a flat pattern((La/Yb)N = 1.07). The metabasalt and amphibolite samplesshow similar patterns, and they are characterized by deple-tion of LREE relative to HREE ((La/Yb)N = 0.73–0.91), typ-ical of tholeiitic suites. The little positive anomaly in Euobserved in some samples probably results from plagioclaseaccumulation.

143Nd/144Nd and 87Sr/86Sr isotopic values of onemetagabbro (Mx–211) and one metabasalt (M–40) blockwere reported by Freydier et al. (1993) and Talavera (1993).Initial ratios of εNd and εSr were calculated at T = 110 Ma(upper Albian) using normalizing values and proceduressuggested by DePaolo (1979). TheεNd and εSr values of

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Fig. 2. Structural sections from Las Ollas and Puerto Escondido areas showing location of studied samples.

LOC blocks are plotted in Fig. 5 together with available dataof the Zihuatanejo and Huetamo volcanics. Typical fields ofprimitive and evolved intraoceanic arc series are also shownfor comparison and discussion.εNd

(T) ratios of blocks varyfrom +7.9 to +8.0 and fall in the range considered typical ofmost primitive intraoceanic island-arc suites.εSr

(T) ratios arerelatively high (–3.0 to –1.7) and reflect secondary processesinvolving fluids and (or) contribution of a crustal componentto magma. As a whole, isotopic features are strikingly com-parable to those reported for the Zihuatanejo and Huetamovolcanics, which could suggest that they all derive from asimilar mantle source.

Metamorphism of blocks

Field observations, together with textural, mineralogical,and chemical evidence clearly suggest that LOC was af-fected by two contrasting metamorphic episodes. Distribu-tion and nature of phase assemblages, textural relationsamong phases, deformation-distribution of phase relation-ships, and phase composition, suggest that the first episodeproduced HP–LT metamorphic assemblages within numer-ous crosscutting shear bands in exotic blocks. Because ofnature of metamorphic assemblages, this metamorphism isthought to be related to subduction processes during earlyCretaceous. The second episode originated low pressure andhigh temperature (LP–HT) metamorphic assemblages and itwas recorded in all lithologies, including those of the overly-ing Zihuatanejo sequence. This episode is interpreted to berelated to thermal metamorphism during mid-Tertiary plutonemplacement. In this study, only the former metamorphismwill be considered.

Early HP–LT metamorphic assemblages reequilibratedpartly or completely during late thermal LP–HT metamor-phism, but they are still observed in many metagabbro andultramafic blocks from Puerto Escondido. In Las Ollas sec-tion, phases related to this episode appear to be completelyreequilibrated during thermal metamorphism. However, it isin Las Ollas section where the block-in-matrix aspect of thesequence is better conserved. Phases related to the HP–LTmetamorphic episode invariably appear within narrow shearbands, where they are arranged parallel to the regional folia-tion planes, strongly suggesting that HP–LT assemblagesformed during a high stress tectonic regime. Because ofreequilibration during subsequent thermal metamorphism,HP–LT phases are limited in number and diagnostic assem-blages consist of a combination of blue amphibole ±lawsonite ± tremolite ± Mg-chlorite ± white mica ± albite

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Fig. 3. Outcrop photographs of Las Ollas complex. (a) Basaltblock enveloped by clastic matrix (Las Ollas section).(b) Ultramafite blocks enveloped by serpentinitic matrix (LasOllas section).(c) Aspect of the general structure of clastic ma-trix showing asymmetrical structures (arrow) and S-C surfaces,which suggest a top to the west sense of shearing (Las Ollassection). (d) Detailed view of a megablock of metagabbro show-ing shear bands (Puerto Escondido section). Bas, Basalt; CMat,Clastic matrix; Ser, Serpentine; Ubas, Ultrabasite. Scale bar isapproximate.

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(Table 2). Representative analyses of HP–LT phases are re-ported in Table 3.

Blue to blue greenish amphibole has sodic (glaucophane)through calco-sodic (katophorite, richterite, and winchite) toNa-rich calcic (pargasite, edenitic hornblende, edenite,tremolitic hornblende, and tremolite) compositions (Fig. 6).Sodic amphiboles show moderate pleochroism from paleblue to lavander, whereas pleochroism in calco-sodic amphi-bole varies from blue greenish to grayish green. Variationsin chemical composition and element correlation suggest theintervention of large substitutions of the tremolite and parga-site molecules (Goodge 1989). Lawsonite is not uncommon,

although the presence of pure anorthite in all samples con-taining relict blue amphibole could represent dehydratedlawsonite. Although some lawsonites contain high contentsof Na2O (<3.5 wt.%), which would suggest partialreequilibration during the subsequent thermal metamorphism,the chemistry of most analyzed lawsonites remembers thatof lawsonites recorded in HP terranes. A mean calculatedstructural formula of pure lawsonites gives Ca0.69–0.85Al1.7–1.8Si2.22–2.26. Colorless tremolite appears to be in textural equi-librium with blue amphibole and chlorite. Chemically, it ishomogeneous and approaches the pure end member. It haslow Na2O (<0.4 wt.%), K2O (<0.9 wt.%), and Al2O3

Sample: M–29 M–39 M–26 M–40 Mx–21 Mx–206

Rock type: Doleriticmetagabbro

Doleriticmetagabbro

Amphibolite Metabasalt Cumulatemetagabbro

Cumulatemetagabbro

SiO2 (wt.%) 45.90 48.65 50.72 52.71 51.71 45.64TiO2 0.91 0.39 0.78 0.69 0.25 0.13Al 2O3 15.33 13.43 15.38 16.53 15.82 20.25Fe2O3 9.49 9.85 7.79 9.16 6.98 6.90MnO 0.17 0.24 0.08 0.16 0.12 0.11MgO 11.35 10.75 8.53 5.94 9.00 7.34CaO 11.55 10.16 12.78 7.91 10.08 16.85Na2O 1.92 2.58 2.58 5.19 4.00 1.03K2O 0.17 2.06 0.01 0.06 0.16 0.32P2O5 0.17 0.14 0.14 0.20 0.05 0.10LOI 2.82 1.44 1.04 1.18 1.64 1.15Total 99.78 99.69 99.82 99.73 99.81 99.82

Ba (ppm) 81 284 50 277 46 45Rb 9 33 5 5 7 11Co 32 36 30 19 34 40Sr 206 191 145 258 227 91Cr 200 535 308 250 401 323Th <5 <5 <5 <5 5 10V 231 182 201 242 189 67Y 20.47 8.45 17.44 19.51 7.76 4.23Nb <5 <5 <5 <5 <5 <5Ni 73 163 104 46 105 149Zr 52 31 33 57 5 14La 3.06 1.91 2.17 2.00 0.59 2.44Ce 8.30 5.76 4.90 5.41 2.17 5.32Nd 6.61 5.03 4.91 5.05 1.72 2.67Sm 2.20 1.76 1.77 2.02 1.03 0.68Eu 0.91 0.64 0.78 0.80 0.35 0.23Gd 2.80 2.19 2.39 2.80 1.38 0.56Dy 3.13 1.50 2.74 2.97 1.33 0.66Er 1.86 1.24 1.57 2.00 0.85 0.42Yb 1.92 0.91 1.68 1.85 0.83 0.44Lu 0.33 0.18 0.26 0.31 0.18 0.07

(Rb/Yb)N 25.53 197.51 16.21 14.72 45.93 136.16(Ba/Er)N 55.51 291.94 40.59 176.54 68.98 136.57(La/Yb)N 1.10 1.49 0.91 0.77 0.50 3.92

εNd(i) — — — (+) 7.9 (+) 8.0 —εSr(i) — — — (–) 1.7 (–) 3.0 —

Note: Isotopic data after Freydier et al. 1993.

Table 1. Major, trace, and rare earth element chemistry and isotopic measurements of blocks from Las Ollas complex.

(<0.8 wt.% with the exception of one analysis containing2.35 wt.% of Al2O3). Greenish to bluish green chlorite iswidespread in most samples and commonly associated withblue amphibole and tremolite. Compositionally, it rangesfrom clinochlore through peninne to pycnochlorite andsherandite (Fig. 7). Calculated chlorite/smectite ratios, fol-lowing procedure suggested by Bettison and Shiffman(1988), indicate high (0.9–1.0 ± 0.05) ratios typical ofstoichiometric chlorites. White mica only appears in a fewsamples associated to blue amphibole. It shows little or nochemical variation with rather high Si/Al (~1.6) ratios, typi-cal of phengite-rich white micas and comparable to thosefound in other white micas associated with HP–LT phases inmany terrains. Plagioclase appears in most samples associ-ated with all phases. Actual composition of plagioclase re-

flects total reequilibration during thermal metamorphism,but it may be suspect that primary plagioclase was albite.

Because of a limited number of phases, the pressure–tem-perature (P–T) conditions are somewhat difficult to define.According to the petrogenetic grid summarized by Kienastand Rangin (1982), the assemblages blue amphibole +lawsonite on one hand, and tremolite + chlorite + albite onthe other, are stable at temperatures between 230° and460°C at pressures of about 5–11 kbar (1 kbar = 100 MPa;Fig. 8). In some rocks these assemblages coexist and showno evidence of instability such as reaction coronas, whichsuggest that assemblages were in equilibrium. If so, P–Tconditions could be near to the stability field of the reactionblue amphibole + lawsonite = tremolite + chlorite + albite.In other way, it is increasingly accepted that partitioning ofAl IV in chlorite reflects crystallization temperature in hydro-thermal systems (e.g., Cathelineau and Nieva 1985,Cathelineau 1988; Schiffman and Friedleifsson 1991) andlow-grade metamorphic terrains (Bevins et al. 1991).Chlorite geothermometry calculated by this method indicatestemperatures in the range of 152–333°C, which, althoughslightly lower than those deduced from phase stability, areconsistent with the absence of jadeitic clinopyroxene. Al-though, the presence of calco-sodic amphibole associated

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Fig. 4. (a) Multi-element patterns for igneous and igneous-derived metamorphic blocks from Las Ollas complex normalizedrelative to the N-MORB values of Sun and McDonough (1989).(b) Rare earth element patterns for igneous and igneous-derivedmetamorphic blocks from Las Ollas complex normalized relativeto the chondrite values of Evensen et al. (1978).

Fig. 5. Isotopic scheme showingεNd and εSr ratios for blocks ofLas Ollas complex. Available data from the Zihuatanejo(Freydier et al. 1993) and Huetamo (Talavera 1993) suites arealso ploted. Typical fields of MORB and primitive and matureisland arcs are also shown for comparison (Faure 1986).

Sample Assemblages

Mx–205 Blue Amphibole + Tremolite + AlbiteBlue Amphibole + Tremolite + Chlorite + AlbiteTremolite + Chlorite + Albite

Mx–206 Blue Amphibole + Lawsonite + White MicaBlue Amphibole + Lawsonite

Mx–207 Blue Amphibole + White Mica + Chlorite + AlbiteBlue Amphibole + Chlorite + Albite

Mx–211 Blue Amphibole + Chlorite + Albite

Table 2. HP–LT assemblages recorded in blocks from Las Ollascomplex at Puerto Escondido.

with lawsonite would suggest pressures near the blueschist – green schist boundary, phase stability and chloritethermometry indicate that peak metamorphic conditionsreach, at least locally, the blue schist facies, and that temper-atures of about 200°–330°C and pressures of 5–7 kbarscould be reasonably suggested for the recorded metamorphicassemblages. Suggested P–T conditions are in accordancewith conditions deduced by Goodge (1989) from similar as-semblages in the Permian–Triassic Stuark Fork complex inNorthern California.

Discussion and conclusions

Mélanges within Las Ollas complex include two mainlithological associations, which differ in the nature and sizeof the blocks as well as in the nature of the surrounding ma-trix. However, both associations share the same structural el-ements. The absence of ductile deformation in the overlyingZihuatanejo volcanic rocks suggests that deformation re-

corded in LOC might be related to high stress produced dur-ing incorporation of material to the accretionary prism. LOCmélanges share many characteristics with Cedros Island andVizcaino Peninsula mélanges: (i) both are related to the de-velopment of an island arc, (ii ) both include basaltic blockswith arc affinity, and (iii ) deduced P–T conditions are quitesimilar. However, in LOC HP–LT, minerals are largely lessdeveloped, and the rocks underwent widespread reequil-ibration during subsequent thermal metamorphism. LOCmélanges could be also compared with the Type IIImélanges described by Cowan (1985) in other subductioncomplexes along Cordilleran North America.

Although the lithology of the blocks includesvolcaniclastics, quartz-rich sandstones, and chert, whichcould reflect in situ remobilization and mixing, igneous andigneous-derived metamorphic clasts have an obviousextraformational origin and are therefore exotic. In spite ofpartial P–T and chemical reequilibration produced by mid-Tertiary thermal metamorphism, most igneous blocks con-

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Blue Amphibole and Tremolite

Sample: M–206 Mx–207 Mx–211 Mx–205

Analysis: 1 2 3 4 5 6 7 8

SiO2 (wt.%) 49.63 40.84 56.01 55.23 46.58 53.20 57.88 58.48TiO2 0.77 0.29 0.00 0.02 0.12 0.01 0.07 0.04Al 2O3 7.58 13.76 2.65 4.10 11.67 5.94 0.56 0.18Cr2O3 0.02 0.00 0.01 0.00 0.01 0.00 0.10 0.00FeO 12.53 12.20 22.54 18.24 16.52 15.57 2.37 1.00MnO 0.28 0.09 0.36 0.26 0.48 0.26 0.03 0.03MgO 12.68 12.02 7.56 10.87 8.94 11.63 22.78 23.98CaO 6.59 12.76 2.35 4.61 9.81 2.32 13.67 13.93Na2O 5.23 2.87 5.69 4.86 2.99 6.29 0.08 0.08K2O 0.31 0.23 0.26 0.16 0.03 0.16 0.01 0.00

Total 95.61 95.06 97.43 98.35 97.15 95.38 97.55 97.72

23 Oxygens

Si 7.328 6.211 8.293 7.981 6.913 7.834 7.940 7.961AlIV 0.672 1.789 0.000 0.019 1.087 0.166 0.060 0.029Total 8.000 8.000 8.293 8.000 8.000 8.000 8.000 7.990

AlVI 0.646 0.677 0.462 0.679 0.955 0.865 0.030 0.000Ti 0.085 0.033 0.000 0.002 0.013 0.001 0.007 0.004Cr 0.002 0.000 0.001 0.000 0.001 0.000 0.011 0.000Mg 2.791 2.725 1.669 2.341 1.978 2.553 4.658 4.866Fe2+ 1.476 1.552 2.791 1.978 2.051 1.581 0.272 0.114Mn 0.000 0.012 0.045 0.000 0.002 0.000 0.003 0.003Total 5.000 4.999 4.968 5.000 5.000 5.000 4.981 4.987

Fe2+ 0.072 0.000 0.000 0.226 0.000 0.336 0.000 0.000Mn 0.035 0.000 0.000 0.032 0.059 0.032 0.000 0.000Ca 1.043 2.079 0.341 0.714 1.560 0.366 2.009 2.032Na 0.851 0.000 1.634 1.028 0.381 1.264 0.009 0.000Total 2.001 2.079 1.975 2.000 2.000 1.998 2.018 2.032

Na 0.646 0.846 0.000 0.334 0.479 0.532 0.012 0.021K 0.059 0.045 0.049 0.030 0.006 0.030 0.002 0.000Total 0.705 0.891 0.049 0.364 0.485 0.562 0.014 0.021

X Mg 0.64 0.64 0.37 0.52 0.49 0.57 0.95 0.98

Note: Total FeO as Fe2+.

Table 3. Representative analyses of HP–LT phases recorded in blocks from Las Ollas complex.

tain relicts of sodic to calco–sodic amphibole, lawsonite,tremolite, chlorite, white mica, and quartz, which clearlydocument that at least some blocks were previously subjectto a HT (about 5–7 kbar) and LT (about 200–300°C) regime.Metaigneous bodies in LOC have been previously consid-ered to derive from oceanic crust either as remnants of theundergoing oceanic plate incorporated to the subductioncomplex or as an ophiolitic suite obducted onto continentalMexico (e.g., Vidal 1984a). However, geochemical charac-teristics (e.g., TiO2 < 0.91 wt.%; Zr < 57 ppm;LFSEN/HFSEN > 14.7; and, LaN/YbN < 3.9) and isotopicdata (εNd

(T) = +7.9 to +8.0) clearly indicates that igneous andigneous-derived metamorphic blocks were formed in a prim-itive intraoceanic island-arc setting. Furthermore, analyzedblocks feature distinctively an important depletion in HFSErelative to N-MORB and highεNd

(T) ratios. Within the entireGT, such features have been only recorded in clasts withinthe Huetamo marginal basin suite and in lavas from theZihuatanejo arc suite (Talavera 1993). The geochemical and

isotopic similarities found among rocks of the Huetamo,Zihuatanejo, and Las Ollas complex would suggest, accord-ing to DePaolo (1979) and Gill (1981), that they originatedby similar magmatic processes including similar degrees ofpartial melting of a geochemically and isotopically identicalmantle source.

Since HP–LT phases have not been dated, the exact age ofLOC remains uncertain. Available radiometric data indicatean Albian–Cenomanian minimum age for the complex. Thesimilarity of geochemical and isotopic features of LOCblocks with the well-dated rocks from the spatially associ-ated Zihuatanejo and Huetamo volcanics, strongly suggest acommon magmatic evolution, and thus, an early Cretaceousage for LOC is reasonably possible.

In this context, incorporation, recrystallization, and exhu-mation of exotic blocks must have occurred through a com-plex evolution involving several processes. Quartz-richsandstones and chert blocks could be added to thesubduction complex by submarine sliding as coherent strata

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Lawsonite Chlorite White Mica

Mx–206 Mx–211 Mx–206 Mx–207

9 10 11 12 13 14 15

43.17 42.87 29.47 26.97 26.74 53.31 52.820.01 0.01 0.04 0.00 0.05

28.73 29.79 18.93 18.61 20.74 27.96 26.950.01 0.13 0.03

0.21 0.12 14.54 14.77 12.89 1.78 1.420.20 0.40 0.15 0.04 0.05

21.27 21.40 22.68 3.17 2.9614.08 14.89 0.50 0.22 0.07 0.00 0.00

0.13 0.13 0.00 0.00 0.00 0.00 0.060.00 0.00 0.02 0.00 0.00 10.61 9.81

86.31 87.80 84.95 82.52 83.33 96.87 94.11

8 Oxygens 28 Oxygens 22 Oxygens

Si 2.264 2.219 Si 6.022 5.728 5.545 Si 6.936 7.029Al 1.777 1.817 Al 4.560 4.658 5.069 Al 4.288 4.227Fe2+ 0.009 0.005 Ti 0.001 0.002 0.005 Ti 0.000 0.005Ca 0.791 0.826 Fe 2.485 2.624 2.235 Fe 0.194 0.158Na 0.013 0.013 Mg 6.480 6.774 7.010 Mg 0.615 0.588K 0.000 0.000 Ca 0.108 0.051 0.015 Ca 0.000 0.000Total 4.85 4.880 Mn 0.035 0.072 0.026 Mn 0.004 0.006

Cr 0.001 0.022 0.006 Na 0.000 0.014Na 0.000 0.001 0.000 K 1.765 1.669K 0.006 0.000 0.000 Total 13.802 13.695Total 19.699 19.931 19.912

XFe 0.28 0.29 0.24AlIV 1.978 2.272 2.455AlVI 2.582 2.386 2.614

% Chl 0.90 1.03 1.03

T (°C) 256 304 333

and later mobilized and mixed by tectonic processes. Volca-nic and volcaniclastic blocks, originally forming part of themassif arc, could be brought down by debris flows ava-lanches, whereas metagabbros and ultramafics could be car-ried off from deep levels of the arc structure by tectonicerosion. Blocks were subsequently transported to enoughdepths by the undergoing plate to produce high pressurerecrystallization and were finally exhumed.

Finally, our data allow us to better understand the evolu-tion of the Guerrero Terrane, and of southern North Amer-ica. Some authors (e.g., Ortiz et al. 1991; Tardy et al. 1994)have proposed that the entire GT, and hence theZihuatanejo–Huetamo suites, were generated in anintraoceanic island arc produced by the westward-directedsubduction of a marginal basin, which separated the arcfrom nuclear Mexico. The intraoceanic origin theory formost suites forming the GT is becoming generally accepted,and our data are compatible with this interpretation. How-ever, if one assumes an early Cretaceous age for LOC, thepresent location of the LOC in the westermost margin of theGT is easier to explain if a movement of the undergoingplate to the east is considered. Furthermore, recorded struc-tural features, such as the westward vergence of the shearzones and thrust sheets and the top-to-the-west relativemovement deduced from the asymmetrical structures and S-C surfaces are more compatible with the accretion andshearing of material by an eastward subduction.

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Fig. 6. Leake et al. (1997) classification scheme for blue amphi-bole from Las Ollas complex blocks.

Fig. 7. Compositions for Las Ollas blocks chlorite in the classifi-cation diagram of Hey (1954).

Since the study of Vidal (1984a), the LOC has beenthought to be part of a subduction complex related to the evo-lution of the GT. However, no previous study had proven thepresence of this important tectonic element in southern Mex-ico. Our data present evidence to suggest that LOC probablyrepresents part of a subduction complex related to the evolu-tion of the early Cretaceous Zihuatanejo volcanic arc.

Acknowledgments

Author is grateful to J. Ramírez-Espinosa for discussionsthroughout the project. The manuscript was improved by re-views by E. Centeno-García and F. Ortega-Gutiérrez. Sup-port for this project was provided in part by grants ofSecretaría de Educación Pública (SEP) and ConsejoNacional de Ciencia y Technología (CONACYT) of Mexico.

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