Magma Source of the A-Type Granite and Slab Break-Off in the Middle Segment of the Bangonghu-Nujiang Suture, Tibet Plateau
Abstract:During the geological investigation of last several years, a group of A type granitic plutons has been found in the middle segment of the Bangonghu Nujiang suture. These granitic plutons occur as stocks with an exposed area of less than1 km2 and lithologically include two types of biotite granite and granodiorite porphyry, which intrude into Cretaceous sedimentary rocks. A systematic analysis of whole rock element geochemistry and Nd, Sr, Pb isotope for these A type granites indicates that they contain high SiO2 content varying from 68.62%～75.36%. Moreover they have relatively high K2O+Na2O(8.03%～9.37%）and Fe2O3T(0.86%～5.39%) and low Al2O3(12.76%～15.54%) content, appearing weak peraluminous and metaluminous features. Trace elements N MORB normalized spider diagram displays apparent enrichment in large ion lithophile element(LILE) Rb, Th, U, K, Pb with Ba and Sr of marked depletion. High field strenth element(HFSE) Nb, Ta and Ti are sharply depleted with Zr and Hf of relative enrichment. All of these are characteristic of A type granite and different from arc granite. These granites have high REE contents(∑REE=122.37×10-6～291.19×10-6，with an average of 201.31×10-6). Light REE are rich relative to heavy REE(LREE/HREE=4.89～9.58, with an average of 5.93)and negative Eu anormalies are sharp(〖QX(Y12#〗δ〖QX)〗Eu=0.14～0.54，with mean value of 0.34), making their chondrite normalized rare earth element patterns appear slightly right dipping V type. Nd, Sr and Pb isotope results indicate that the A type granites from the middle segment of the Bangonghu Nujiang suture are rich in radioactive isotope. Their 87Sr/86Sr and ISr ratio values vary from 0.719842～0.786395 and 0.706900～0.710378, respectively. 143Nd/144Nd ratio values are from 0.512123～0.512392, with large negativeεNd values(-3.37～-10.34), reflecting important involvement of crustal components. Pb isotope composition 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb display ranges of 18.703～19.070, 15.680～15.732 and 39.121～39.576, respectively, which plot on the upper crustal line and above the orogenic evolution line in the plumtotectonic framework diagram of Zartman and Doe (1981) and show anormaly high enrichment components. The study points out that the A type granites were derived from partial melting of enriched lithospheric mantle (approximating EMⅡ), which were metasomalized by sediment melt from slab during the oceanic crust subduction and later heat by upwelling asthenospheric materials through slab windows formed as a result of slab break off during the postcollisional extension of the orogenic zone.