| Carboniferous Nujinshan ophiolite melange is situated from Nujinshan to Mengyong area, where is dominated by Shuangjiang County and Gengma County respectively, southwestern Yunnan Province. The ophiolite melange can be connected with Tongchangjie ophiolite melange further north in Mengyong area Continuous outcrop of the ophiolite melange is more than 100km in length, and range from 0.2 to 2km in width. The ophiolite melange may be buried underground further south in Nujinshan area because tectonism influenced in later. Major petrographic category are amphibolite, schistous amphibolite, layered amphibolite, greenschist. Subordinate are plagioclase granite dike, layered tonalite, tonalitic conglomeration dispersed in amphibolite, ultramafic rock and metabasalt etc.. Plenty of igneous accumulated banding were discovered in amphibolite from Nujinshan and Bujing area Some residual clinopyroxene also were found in a few of sample. Protolith ought to be accumulative gabbroite and has undergone a amphibolite facies metamorphism. Ultramafic rock fragments, which almost is altered to ophite, distributed in structural zone and displayed structural lens. So far, 9 fragments were discovered and mapped in Nujinshan and Bujing areaUltramafic rock can be divided two kinds according to it's geochemical characteristics. One can be correlated with metaperidotite, which represented residual part that primary pyrolite underwent moderate partial melting, from some representative ophiolite melange in the world. It is characterized by high and steady MgO, lower Al2Oj and trace CaO, m/f= 8.0-8.4. According to C.I.P.W. system of rock, it can be named harzburgite, composed of magnesic end-member faioo and enioo. Other kind can be considered as ultramafic cumulate, which wide ranged and continuously varied in MgO, Al2O3, CaO and EREE, m/f= 9.8-3.6, it can be named plagio-lherzolite-plagioclase-bearing olivine websterite-dark olivine norite as above criteriaMajor elements of metamorphic mafite, characterized by K2O=0.09-0.76%, Ti2O=0.21-2.1%, show some geochemical characteristics of typical mid-ocean ridge basalt (MORB) or low kalium tholeiite (LKT) from ocean bottom. Residual magma were inferred as mother magma formed mafite, due to it's [Mg] index (37-72, average 59) is further less [Mg] index (65-72) of primary magma balanced with pyrolite. Two REE distributed pattern, LREE-depleted and flat, which display a specific character of typical mid-ocean ridge tholeiite, indicate that primary magma may be derived from near primary or a typical large ion lithophile element (LILE)-depleted mantle. Strong compatible elements, as Cr,Ni, Co, Sc, from most samples is very low. It also implied that mother magma formed mafite was a residual melt Trace elements distributed pattern is similar to transitional mid-ocean ridge basalt Simultaneously, the pattern, which displayed a clear (K). Rb. Ba, Th, (Th)-enriched, also can be explained as an ocean ridge environment overlapped with mantle plume. Lead isotopic data from neighboring areas, With slight Dupal-abnormity, carried overprints from mantle lithosphere of Gondwanaland. Surface age of zircon from schistous amphibolite is 330.69+27.1~349.05+40.0.According to occurrence and petrographic data from field, the leucocratic rock in the ophiolite melange can be divided into four kinds as follows: (l)medium-grained biotite tonalite, dispersed in amphibolite as conglomeration, (2)fme-grained layered tonalite and plagioclase granite, which is rhythmic interbed scattered in schistous and layered amphibolite, (3)plagioclase granite dike, intruding in amphibolite. (4)gneissosetonalite and plagioclase granite. Respective lithogenesis may be different, notwithstanding their geochemical characteristics are similar to ocean plagiogranite in whole.All in all, in Carboniferous Nujinshan ophiolite melange, petrographic category and their geochemical characteristics can be correlated with representative ophiolite melange in the world. This discovery offered a convincing evidence from petrology, it tes... |
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