A characteristic association of crustal and mantle rocks is commonly used to decipher processes at the mantle–crust interface of HP–UHP collisional orogenic systems. Also, in the Variscan orogenic root of the Bohemian Massif (the Moldanubian Zone), high-pressure felsic granulites are often accompanied by spinel or garnet peridotites. This association was investigated using petrography, zircon geochronology and whole-rock geochemical data from the Náměšť Granulite Massif. The geochemical signature of the granulite is the same as for other Moldanubian occurrences, suggesting nearly isochemically metamorphosed felsic metaigneous rocks of Saxothuringian provenance. SHRIMP zircon dating yielded two main age maxima, at 395.2 ± 4.4 and 337.2 ± 1.7 Ma, reflecting an Early Devonian protolith and Visean HP metamorphism. As shown by Sr–Nd isotopic data, the variably refertilized harzburgite or depleted lherzolite was variously contaminated by mature crustal material resembling the studied granulites. To account for the origin of these HT–HP rock associations we suggest a new geotectonic model. An eastward continental subduction of Early Palaeozoic felsic metaigneous material of Saxothuringian origin was followed by its relamination at the bottom of the autochthonous lower crust. Ascending felsic granulites derived from the relaminated lower plate material sampled refertilized harzburgites originally formed in a back-arc. The complete assemblage was subsequently exhumed, forming large, diapir-like bodies.
We use structural, petrographic, and geochronological data to examine processes of exhumation of partially molten crust in the Late Devonian–early Carboniferous Chandman dome in the Mongolian tract of the Central Asian Orogenic Belt. The dome is composed of a magmatite-migmatite core and a low-grade metamorphic envelope of early Paleozoic metasediments and Carboniferous clastics. Its tectonic evolution can be divided into three main stages. The oldest fabric is a subhorizontal foliation, S1, in migmatites that is subparallel to the magmatic foliation in granitoids and to the greenschist facies schistosity in the enveloping metasediments. This event is interpreted as a result of horizontal deep crustal flow at depths of 20–25 km. The S1 layering was subsequently transposed into a new foliation, S2, or affected by open to close upright F2 folds that are locally truncated by steep walls of diatexites, suggesting influx of partially molten crust into fold cores. The shallow-dipping magmatic foliation in granitoids is locally reworked by vertical magmatic to gneissic S2 fabrics. Syn-S2 metamorphic assemblages and synkinematic to postkinematic cordierite point to exhumation of the migmatites and granitoids from 20–25 km to ∼10 km, and concomitant isobaric heating of the surrounding upper crust. New 40Ar/39Ar ages of 350–340 Ma from both the high-grade core and the metamorphic mantle overlap with previously published crystallization ages of 360–340 Ma, suggesting that magmatism and cooling in the upper crust are partly synchronous. Late syn-D2, S2-parallel leucogranite sheets crosscutting both the magmatic core and the mantling migmatites either exploit S2 or crosscut horizontal S1 fabrics; they are interpreted as brittle expulsion of magma during ongoing syn-D2 exhumation. We suggest that the partially molten crust and magmas rose vertically into the upper crust, along steep planar fabrics that are parallel to the axial fold plane of a crustal detachment folding, without contribution of buoyancy forces. In order to test that crustal-scale detachment folding can exhume partially molten crust, we apply an analogue model with temperature dependent rheology of the lower crust represented by a partially molten wax layer overlain by an upper crustal sand layer. It is shown that the fold core initially filled by low-viscosity partially molten wax rapidly migrates upward during fold lock-up, enhancing upward extrusion of magma and migmatites along the fold axial plane. The exhumation of the lower crust wax is facilitated by erosional unroofing of the upper crustal sand above the hinge of the antiform. In Chandman, localized siliciclastic lower Carboniferous basins rimming the dome attest to this erosional phenomenon. Using a simple geometrical analysis we show that detachment folding can explain magma collection in an orientation perpendicular to the main shortening direction, and episodic emplacement of magmas during amplification of the antiform. In our view, the detachment folding model provides a new model for the exhumation of a weak zone above a rigid floor (basement from which the fold is detached) and its vertical extrusion related to locking of the fold and post-buckle flattening. This model helps elucidate steep retrograde pressure-temperature-time paths along steep fabrics, overlapping ages from different geochronometers, and emplacement of voluminous syntectonic magmas.
Abstract During orogenic processes continental crust experiences significant partial melting. Repeated thermal pulses or fluctuation in fluid content can even cause multiple anatectic events that result in complex intrusion suits. In the Vosges mountains, France, two main generations of magmatic rocks are recorded. The first magmatic event occurred at ca. 340 Ma, and is represented by extensive K-Mg granitoids magmatism. The second magmatic event occurred at ca. 325 Ma and produced large quantity of felsic anatectic melts which further pervasively intruded and compositionally and texturally reworked previously formed granitoids. Detailed field and microstructural observations revealed continuous transitions from porphyritic granite with large euhedral Kfs and Pl phenocrysts (Type I granite) via intermediate granite (Type II) to fine-grained apparently isotropic granite (Type III) dominated by the neo-crystallized melt. The Type I granite preserves the original magmatic assemblage and has only incipient amount of the newly crystallized melt. The new melt-crystallized material forms narrow, fine-grained pathways along grain boundaries or cuts across pre-existing magmatic grains and forms an interlinked network. With increasing amount of the newly crystallized material the original magmatic grains are resorbed and show highly corroded shapes. The early formed feldspars grains have strong compositional zoning, with oscillatory zoned cores reflecting range of original magmatic compositions and rims showing later melt overgrowths. Original magmatic feldspars have different composition from the new phases crystallizing in the partially molten granite. We interpret the fine-grained microscopic corridors as melt pathways that were exploited by the new magma. We suggest that this melt pervasively migrated through the older granitoids resulting in mixture of inherited “xenocrysts” and of new melt-derived crystals. The interaction between the new melt and previously crystallized granitoids results in variety of granite textures and fabrics. These reflect different degrees of equilibration between the bulk rock and the passing melt. Finally, Type III granite carries mixed isotopic signature intermediate between the type I granite and the surrounding metasediments and granulites, suggesting mixing of the original granite with new later magma with source in these rocks.
<h3>High-pressure granitic orthogneiss of the south-eastern Orlica&#8211;&#346;nie&#380;nik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P&#8211;T path deduced from the thermodynamic modelling indicates decompression from ~15&#8722;16 kbar and ~650&#8211;740 &#186;C to ~6 kbar and ~640 &#186;C. Melt was already present at the P&#8211;T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6&#8211;8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse.</h3>
A characteristic association of crustal and mantle rocks is commonly used to decipher processes at the mantle–crust interface of HP–UHP collisional orogenic systems. Also, in the Variscan orogenic root of the Bohemian Massif (the Moldanubian Zone), high-pressure felsic granulites are often accompanied by spinel or garnet peridotites. This association was investigated using petrography, zircon geochronology and whole-rock geochemical data from the Náměšť Granulite Massif. The geochemical signature of the granulite is the same as for other Moldanubian occurrences, suggesting nearly isochemically metamorphosed felsic metaigneous rocks of Saxothuringian provenance. SHRIMP zircon dating yielded two main age maxima, at 395.2 ± 4.4 and 337.2 ± 1.7 Ma, reflecting an Early Devonian protolith and Visean HP metamorphism. As shown by Sr–Nd isotopic data, the variably refertilized harzburgite or depleted lherzolite was variously contaminated by mature crustal material resembling the studied granulites. To account for the origin of these HT–HP rock associations we suggest a new geotectonic model. An eastward continental subduction of Early Palaeozoic felsic metaigneous material of Saxothuringian origin was followed by its relamination at the bottom of the autochthonous lower crust. Ascending felsic granulites derived from the relaminated lower plate material sampled refertilized harzburgites originally formed in a back-arc. The complete assemblage was subsequently exhumed, forming large, diapir-like bodies. Supplementary material: Sample coordinates from the Náměšť Granulite Massif, analytical techniques, SHRIMP age measurements on zircon grains and whole-rock geochemical data are available at http://www.geolsoc.org.uk/SUP18833 .
Research Article| July 01, 2014 Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow Melanie Finch; Melanie Finch † 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia †E-mail: melanie.finch@monash.edu Search for other works by this author on: GSW Google Scholar Pavlína Hasalová; Pavlína Hasalová 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia2Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic §Current address: Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 118 21, Prague 1, Czech Republic. Search for other works by this author on: GSW Google Scholar Roberto F. Weinberg; Roberto F. Weinberg 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar C. Mark Fanning C. Mark Fanning 3Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2014) 126 (7-8): 892–924. https://doi.org/10.1130/B30817.1 Article history received: 07 Nov 2012 rev-recd: 09 Aug 2013 accepted: 22 Nov 2013 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Melanie Finch, Pavlína Hasalová, Roberto F. Weinberg, C. Mark Fanning; Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow. GSA Bulletin 2014;; 126 (7-8): 892–924. doi: https://doi.org/10.1130/B30817.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Zanskar shear zone is a ductile, normal-sense shear zone that exploited the contact between the High Himalayan Crystalline series and the Tethyan sedimentary series. The Zanskar shear zone is an extension of the South Tibetan detachment system with similar timing and nature, and, in Zanskar, it accommodated 24 km of normal movement. Early thrusting is preserved in the footwall and hanging wall and is overprinted by normal shearing. Thrusting and normal shearing were coplanar and codirectional, with SW-directed thrusting overprinted by NE-directed normal shearing—a simple inversion of movement sense. The telescoped isograds related to normal shearing define a broad pattern of colder rocks on top of hotter. However, we found preserved thrust-related metamorphic series, with hotter rocks on top of colder, severely telescoped by normal shearing. Some determinations of the amount of displacement and thinning on the Zanskar shear zone prior to the current work have assumed a steady-state crustal profile and have disregarded preexisting perturbations of isograds such as those indicated here. Miocene leucogranitic intrusions accumulated within and below the normal Zanskar shear zone. Intrusions were sheared during thrusting and normal movement, and magmatism outlasted normal shearing. We have dated monazites by U-Pb sensitive high-resolution ion microprobe (SHRIMP) from leucogranite samples that were sheared by the thrusting event, by the normal movement event, and those that postdate all shearing. Results constrain the timing of the switch from thrusting to normal movement to between 26 and 24 Ma and ca. 22 Ma. At ca. 20 Ma, normal shearing in Zanskar shear zone was no longer active, and magmatism was waning, producing late, undeformed leucogranitic dikes. Taking into account the shear zone thickness of 0.83 km, the maximum duration of normal movement of 6 m.y., and the estimated strain of γ = 28.6, we estimate the lower bound of strain rate for the Zanskar shear zone to be 1.5 × 10–13 s–1. Given the short duration of the normal shearing event and magmatism, we find little support for the hypothesis of channel flow in Zanskar. We propose instead that Miocene anatexis weakened the midcrustal levels and caused the switch from thrusting to normal movement, doming, and cooling of the anatectic core of the High Himalayan Crystalline series. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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