mechanics and mechanisms of magmatic underplating: inferences
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ELSEVIER Earth and Planetary Science Letters 165 (1999) 271286
Mechanics and mechanisms of magmatic underplating: inferencesfrom mafic veins in deep crustal mylonite
M.R. Handy a,, J.E. Streit b
a Institut fur Geowissenschaften, Justus-Liebig Universitat Giessen, Senckenbergstrasse 3, D-35390 Giessen, Germanyb Department of Geology, Australian National University, Canberra ACT 0200, Australia
Received 24 June 1998; revised version received 23 November 1998; accepted 3 December 1998
Dioritic to gabbrodioritic veins with extreme length to width ratios (>1000 : 1) are localized along an amphibolitefacies shear zone (the CMB Line) between exposed segments of originally middle and lower continental crust (StronaCeneri and IvreaVerbano Zones, northern Italy). The geometry of these veins and their mutual cross-cutting relationshipswith the mylonitic foliation indicate that veining was coeval with noncoaxial flattening of the lower crust in Early Permiantime. The veins formed as closely spaced extensional shear fractures and propagated parallel to the originally gently tomoderately dipping (30) mylonitic foliation. Vein opening at high angles (6090) to the inferred 1 direction andsubparallel to the pre-existing planar fabric requires that melt pressure slightly exceeded the lithostatic pressure and thatdifferential stress was low (1020 MPa) in the vicinity of the veins. The interaction of regions of tensile stress concen-tration at vein tips caused the concordant veins to curve and link up across the mylonitic foliation. Once interconnected,the veins served as conduits for the rapid movement of mafic melt along the shear zone. Thermal modelling constrainsthe mafic melt in the narrowest, 1 mm wide veins to have crystallized almost instantaneously. Such veins extend no morethan a meter from host veins into the country rock, indicating that the minimum rate of vein tip propagation and meltflow was at least several m=s. Maximum crystallization times of only hundreds to thousands of years for even the thickestmafic veins (10100 m) in the IVZ are short compared to the 1520 Ma duration of Early Permian crustal attenuationand magmatism in the southern Alps. This suggests that veining in the lower crust occurred episodically during extendedperiods of mylonitic creep. Concordant vein networks within deep crustal shear zones that are inclined (as the CMB Linemay have been) can also channel overpressurized mafic melt from deeper sources, e.g. lower crustal magma chambers, intocooler, intermediate crustal rock. This locally widens the depth interval of combined viscous and brittle deformation withinthe crust and can trigger partial melting of the country rock. 1999 Elsevier Science B.V. All rights reserved.
Keywords: magmas; underplating; shear zones; brittle materials; viscous materials; mylonites
Magmatic underplating plays an important rolein forming and transforming the continental crust
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, yet the mechanisms by which large volumes ofmantle-derived melt are emplaced within the lowercrust remain enigmatic. Debate centers on two in-terrelated questions: (1) Was the melt originally em-placed as dikes or as sills? and (2) Do melt layerspre-, syn- or post-date deformation of the lowercrust? Laterally continuous zones of enhanced con-
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272 M.R. Handy, J.E. Streit / Earth and Planetary Science Letters 165 (1999) 271286
ductivity and seismic reflectivity within the lowercrust of intracontinental rifts , passive continentalmargins  and thinned orogenic continental crust are often attributed to subhorizontal laminae ofdense mafic rock or melt within felsic granulites[5,6]. Some workers  have speculated that thesubhorizontal attitude of these laminae is not pri-mary, but resulted from pervasive shearing and rota-tion of dikes during lateral crustal attenuation. Morerecently, others have proposed that overpressurizedmafic magmas can intrude the crust as dikes or sillsdepending on the melt pressure and viscous strengthof the rheologically stratified lithosphere . Numer-ical fracture propagation models support the notionthat dikes serve as conduits for the rapid ascent oflarge volumes of granitic melt [9,10]. Melt migra-tion without fracturing of the rock has been invokedfor some migmatites where leucosome is observedto collect in boudin necks . However, layer-par-allel leucosome or magma intrusions in migmatiticterranes are often associated with fracturing at highmelt pressures [11,12].
This paper presents field evidence that magmaticunderplating involves the intrusion and linkage ofthin, long mafic veins along active mylonitic shearzones. We show how existing failure criteria appliedto the geometry of these veins provide constraintson both the differential stress and melt pressure atthe time of intrusion, and how consideration of veintip stresses allow the evolution of the vein arrayto be reconstructed. Finally, we speculate on howmantle-derived melts interact with large shear zonesand condition both the structure and rheology of thecontinental crust.
2. Geological setting
Mafic veins are localized within an amphibo-lite facies mylonite belt, the so-called CMB Line(CMB D CossatoMergozzoBrissago Line ),that forms the subvertically dipping tectonic con-tact between the lower crustal IvreaVerbano Zone(IVZ) and intermediate crustal StronaCeneri Zone(SCZ) of the southern Alps (Fig. 1). The my-lonites of the CMB Line are derived from highgrade gneisses and schists of these two Zones andcontain the syn-kinematically stable mineral parage-
nesis quartzplagioclasebiotitemuscovitegarnetsillimanite (fibrolite). We refer to the deformedrocks of the CMB Line as mylonites or my-lonitic because they are foliated and, in thin section,contain microstructural evidence that the mineralwhich accommodated most of the bulk strain (in thiscase, quartz) underwent complete dynamic recov-ery and recrystallization. Mylonitization along theCMB Line is actually related to amphibolite to gran-ulite facies mylonitic shear that affected much ofthe IVZ and the northernmost part of the SCZ inEarly Permian time . This deformation coincidedwith the intrusion of large volumes of gabbroic,gabbrodioritic and ultramafic magmas in the orig-inally deepest parts of the exposed crustal section,presently exposed in the northwestern part of theIVZ (Fig. 1;  and references therein). Early Per-mian magmatic underplating is temporally relatedto granitic and rhyolitic magmatism in originallyshallower levels of the southern Alpine crust ,and probably occurred during regional transtensionaltectonics .
At this point, we emphasize two aspects of thelocal geology that bear directly on this study of syn-mylonitic veining: (1) the basement containing theCMB Line north of the Lago Maggiore (Fig. 1) doesnot occupy its original, Early Permian orientation,but was rotated at least 60 into its present subver-tical attitude during Tertiary Insubric faulting andfolding . Paleomagnetic and structural studiesin the area indicate that prior to Alpine orogenesis,the mylonitic foliation within the IVZ and along theCMB Line was inclined 30 or less to the southeast( and references therein). The CMB mylonitesare therefore interpreted to be the deeply eroded partof an oblique-extensional shear zone that, prior toAlpine differential rotation and erosion, was linkedto steep faults bounding Early Permian basins in theupper crust of the southern Alps . The originallyshallow dip of the mylonitic foliation along the CMBLine becomes important below when discussing theEarly Permian kinematic regime and stress field inthe vicinity of the mafic veins. (2) The central seg-ment of the CMB Line as well as the narrow, north-eastern part of the IVZ (north of Lago Maggiorein Fig. 1) is overprinted by Early Mesozoic, retro-grade amphibolite to greenschist facies mylonites ofthe Pogallo Line . We obviously avoided these
M.R. Handy, J.E. Streit / Earth and Planetary Science Letters 165 (1999) 271286 273
gneisses and schistsvolcanics
Fig. 1. General map of the Ivrea crustal cross section showing location of the CMB Line (CMB) forming the contact between theIvreaVerbano and StronaCeneri Zones in the Southern Alps. PL D Pogallo Line, IL D Insubric Line. Details of Val Orasso outcropshown in Figs. 2 and 3.
areas because of the modifying effect this later de-formation had on the primary geometry of the maficveins.
3. Veinmylonite relations
For the purposes of this study, we restricted de-tailed investigations to a well exposed outcrop in thestream bed of the Orasso valley that runs parallel tothe strike of the subvertical mylonitic foliation (Sm)and mafic veins of the CMB Line (Fig. 2). The veins
along the CMB Line are dioritic to locally gabbrodioritic and comprise mostly magmatic plagioclaseand hornblende with rare inclusions of clinopyrox-ene . The hornblende is partly transformed tometamorphic biotite in fully mylonitized veins andat the rims of deformed veins. The primary mineral-ogy of the veins is virtually identical to that of EarlyPermian gabbrodiorites of the Mafic Complex inthe Ivrea Zone [19,20] and their main and traceelement chemical compositions clearly manifest anupper mantle heritage [16,21].