Newly discovered eclogites from the Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications

Abstract

2015): Newly discovered eclogites from the Bangong Meso-Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications, International Geology Review To link to this article: http://dx. ABSTRACT We report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong Meso-Tethyan suture zone (Gaize, central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene whereas Type 2 ones are characterized by a peak metamorphic mineral assemblage of garnet + clinopyroxene + rutile. Type 2 clinopyroxene and garnet are relatively more enriched in Mg contents than Type 1 ones. Type 2 eclogites are significantly more enriched in TiO 2 , P 2 O 5 , and light rare-earth elements, but more depleted in Al 2 O 3 contents and Mg numbers. P-T estimates for both types of eclogites are consistent with minimum pressures of 23-25 kbar at 830-920°C, with the retrograde amphibolite facies assemblage of 15.2-17.5 kbar and 500-732°C. Zircon U-Pb dating indicates that the protolith of Type 1 eclogites, with normal mid-ocean ridge basalt affinity, emplaced at~260 Ma, whereas the protolith of Type 2 eclogites, geochemically familiar with oceanic-island basalt (OIB), erupted at~242 Ma. 40 Ar/ 39 Ar analyses of hornblende from the retrograded eclogites indicate that the Gaize eclogite-bearing high-pressure rocks were exhumed at~194 Ma. The occurrence of these high-pressure rocks implies the subduction of oceanic crust to a depth of~85 km along the southern margin of the Qiangtang block during the Late Triassic. ARTICLE HISTORY

High‐Pressure Granulite Facies Overprinting During the Exhumation of Eclogites in the Bangong‐Nujiang Suture Zone, Central Tibet: Link to Flat‐Slab Subduction

“…JSSZ = Jinsha Suture Zone; LSSZ = Longmu Co‐Shuanghu Suture Zone; BNSZ = Bangong‐Nujiang suture zone; IYZSZ = Indus‐Yarlung Zangbo Suture Zone. (b) Geologic map of the Bangong‐Nujiang Suture Zone (modified after Wang, Wang, Chung, et al, ), showing the distribution of the ophiolites, arc‐related magmatic rocks, and HP metamorphic rocks; (c) geologic map of the Dong Co area (modified after Zhang et al, ), showing the outcrops of eclogites and sample locations; 1—Quaternary deposits; 2—Upper Jurassic marine rocks; 3—Jurassic turbidites (Muggargangri Group); 4—Lower Cretaceous shallow marine clastic rocks; 5‐ultramafic rocks of Jurassic ophiolites from BNSZ; 6—gabbros of Jurassic ophiolites from BNSZ; 7—metamorphic greywackes and schists; 8—eclogites; 9—thrust faults; 10—sample locations.…”

“…The eclogite samples from the Dong Co area are distributed in a ~1,500 m by ~400 m metamorphic belt (Figure c), occurring as blocks or lens‐shaped bodies of sizes varying from a few meters to tens of meters in diameter, and are in contact with the surrounding metamorphic greywackes, ultramafic rocks, and schists (Figures a and b). The Dong Co eclogites were first reported as “HP granulite” (Wang, Wang, Xu, et al, ; Zhang et al, ) due to their strong overprinting at HP granulite facies to amphibolite‐facies conditions. They were later identified as retrograded eclogites based on the discovery of minor residual omphacites (Dong et al, ).…”

“…The Dong Co eclogites were recently discovered in the Southern Qiangtang Block, Central Tibet (Dong et al, ), which made it possible to significantly improve our understanding of the Tethys Oceanic evolution during the Mesozoic Era. Previous studies revealed the existence of residual omphacites within the strongly retrogressed Dong Co eclogites (Dong et al, ) and suggested that their protolith could be a fragment of the Late Permian Bangong‐Nujiang Tethys oceanic crust (Wang, Wang, Xu, et al, ; Zhang et al, ). However, so far, their refined P ‐ T paths, metamorphic ages, exhumation mechanisms, and tectonic significance are still enigmatic.…”

Early Permian mafic dikes in the Nagqu area, central Tibet, China, associated with embryonic oceanic crust of the Meso‐Tethys Ocean

“…Moreover, deposition in the region during the Carboniferous to Permian is indicative of a rifting environment based on a transition from predominantly continental to predominately marine sedimentation [ BGTAR , ]. Recently, newly discovered eclogites from the western BNSZ (Gaize) display basalt‐MORB affinities and the U‐Pb zircon dating of eclogites (~260.3 ± 4.8 Ma) indicates that the Meso‐Tethys must have been present at least since the Late Permian [ Zhang et al , ]. Based on field and geochemical evidence, we therefore suggest that the Nagqu mafic rocks (~277.8 Ma) represent a fragment of embryonic oceanic crust that formed during the inception of a spreading ridge system.…”

“…The BNSZ, which extends east‐west across the middle part of the Tibetan Plateau, represents the site of an Early Cretaceous collision between the Qiangtang and Lhasa terranes [ Chen et al , ], which was preceded by northward or southward subduction of Meso‐Tethys oceanic lithosphere beneath central Tibet starting in the Middle Triassic [e.g., Zhu et al , ; Chen et al , , ]. The discontinuous ophiolites that crop out along the BNSZ may represent the remnants of Meso‐Tethys oceanic lithosphere [ Guynn et al , , ; Kapp et al , ; Shi et al , , ; Zhang et al , , ; X. R. Zhang et al , ; Chen et al , ]. However, the timing and nature of formation of the embryonic oceanic crust of the Meso‐Tethys Ocean following the continent breakup remain poorly understood.…”