Petrology and geochemistry of mafic-ultramafic rocks in the North Dabie Complex, central-eastern China.

摘要

Mineral compositions and microstructures of a newly identified eclogite in the North Dabie Complex (NDC), record early very high pressure metamorphism in a dominant amphibolite-facies host, where no eclogite has been positively identified before. The eclogite-facies relics were strongly overprinted by granulite-to-amphibolite-facies assemblages. A spectacular microstructure of oriented quartz needles ({dollar}{lcub}sim{rcub}2{lcub}-{rcub}20 mu{dollar}m thick, {dollar}{lcub}sim{rcub}5{lcub}-{rcub}200 mu{dollar}m long) in matrix clinopyroxene implies the prior existence of a supersilicic clinopyroxene stabilized at ultrahigh-pressure (UHP {dollar}>{dollar} 25 kbar) conditions. It is suggested that part of the NDC was subjected to UHP metamorphism.; The Raobazhai ultramafic body, consisting of spinel-bearing metaharzburgite, metadunite, and metalherzolite, is a tectonic slice enclosed in granulite-to-amphibolite-facies gneisses. In the metalherzolite, fine-grained intergrowths of spinel, orthopyroxene, and clinopyroxene occur, implying decomposition products from a garnet precursor. The meta-ultramafics consist of forsterite, enstatite, magnesio-hornblende to pargasite, and spinel, with or without clinopyroxene, indicating amphibolite-facies and granulite-facies metamorphism. Geothermometric estimations show that the meta-ultramafics reequilibrated at 700-820{dollar}spcirc{dollar}C (6-12 kbar), although an early, higher pressure stage ({dollar}ge{dollar}18-20 kbar) is possible, based on the implied existence of precursor garnet. Petrochemical and geothermobarometric data imply that the ultramafic slice may represent a piece of former mantle wedge, which was tectonically incorporated into a subducted crustal slab and subsequently reequilibrated at granulite-to-amphibolite-facies conditions.; The Jiaoziyan Gabbro is the largest mafic intrusion in the NDC. {dollar}rmsp{lcub}40{rcub}Ar/sp{lcub}39{rcub}Ar{dollar} isotopic analyses of biotite separates demonstrate that cooling ({dollar}{lcub}<{rcub}300spcirc{dollar}C) of the intrusion occurred at 124-125 Ma. The Cretaceous age is about 80-100 Ma younger than the timing of Dabie UHP metamorphism; thus the intrusion cannot be attributed to having been triggered by "slab breakoff" during the Triassic collision between the Sino-Korean and Yangtze cratons. The lack of syncollisional magmatism in the NDC indicates that its protoliths could be parts of a subducted slab. The Cretaceous thermal overprint was so strong that the majority of the formerly subducted NDC were reequilibrated or partially melted after early-stage exhumation.