滇西腾-梁地块印支造山事件——花岗岩的锆石U-Pb年代学和岩石学证据

Indosinian orogenesis of the Tengchong-Lianghe block,Western Yunnan:evidence from zircon U-Pb dating and petrogenesis of granitoids

滇西腾冲-梁和地区分布大量的花岗岩,对其中的二云母花岗岩进行了LA-ICP-MS锆石U-Pb定年、锆石Hf同位素组成和地球化学的测定。结果表明,岩石属于强过铝质花岗岩,A/CNK=1.11～1.20,K2O/Na2O为1.87～2.20。岩石富Rb、Th和U等元素,Eu/Eu*=0.25～0.47,(La/Yb)N=6.53～34.06。锆石LA-ICP-MSU-Pb同位素测年结果表明:二云母花岗岩体存在256.0±5.6Ma(2σ,MSWD=4.1)和234.1±3.1Ma(2σ,MSWD=5.6)两个岩浆作用幕。根据花岗岩的地球化学特征和Hf同位素组成,二云母花岗岩岩浆来源于古老地壳中含粘土质变质硬砂岩的部分熔融。腾-梁地块晚二叠世-中三叠世碰撞型强过铝花岗岩的确定表明,三江古特提斯洋在印支早期曾发生碰撞造山,三江印支早期造山事件与古特提斯洋的闭合与随后的造山作用存在密切联系。

Lots of granitoids are distributed in Tengchong-Lianghe area,western Yunnan Province.According to data available,the ages of these granitoids are mostly concentrates in the Late Mesozoic-Cenozoic period.In this paper,ages of Late Permian-Middle Triassic peraluminous granites in this area were obtained for the first time.Granitoid bodies are distributed near southeastern Lianghe County.The major rock types include mainly grayish medium-grained two-mica granite and subordinately gray fine-grained two-mica granite in the form of bosses or dikes one to several meters in width scattered in medium-grained two-mica granite.The medium-grained two-mica granite exhibits hypidiomorphic granular texture and is mainly composed of K-feldspar （perthite,±40%）,quartz （±30%）,plagioclase （oligoclase,±15%）,biotite and muscovite（±15%）,with accessory minerals comprising zircon and iron oxides.The fine-grained two-mica granite exhibits hypidiomorphic granular texture too and is mainly composed of K-feldspars （perthite and microline,±30%）,quartz （±25%）,plagioclase （oligoclase,±25%）,biotite and muscovite（±20%）,with accessory minerals being zircon and magnetite.The two-mica granites have the following geochemical characteristics：SiO2=69.74%～74.44%,Al2O3=13.91%～14.52%,MgO=0.26%～0.87%,CaO=0.72%～1.36%,K2O=5.24%～5.78%,Na2O=2.63%～2.82%,K2O/Na2O=1.87～2.20,and Al index （A/CNK）=1.11～1.20.Moreover,corundum can be found in the CIPW norm minerals,which possesses some features of strongly peraluminous granites.The two-mica granites composed of medium-grained and fine-grained granite are characterized by the enrichment of Rb（315×10-6～452×10-6）,U（7.68×10-6～11.2×10-6）and Th（15.2×10-6～111×10-6）.Primitive mantle-normalized element concentration patterns of both granites show remarkable negative anomalies of Ba,Nb,Sr,P,Ti.Likewise,both of the granites display similar REE patterns：Eu/Eu=0.25～0.47 and （La/Yb）N =6.53～34.06.Most of the zircons from the sample show oscillatory zoning in CL images with Th/U ratios from 0.1 to 1.8,indicative of magmatic zircon in genesis.Zircon LA-ICP-MS U-Pb dating from the same sample yielded concordant ages of 256.0 ±5.6 Ma （MSWD=4.1） and 234.2 ±3.1 Ma （MSWD=5.6）,which respectively indicate the early （i.e.Late Permian） and late （i.e.Middle Triassic） magmatic episodes during the early Indosinian period in this area.An inherited zircon age（ca.596 Ma）was also obtained.In-situ Hf isotopic analyses yielded εHf（t） values of-8.5 to ＋1.3,corresponding to the Hf isotopic crustal modal ages （tDMC ） of 1 195～1 815 Ma.The geochemical signatures and zircon Hf isotopic compositions suggest that the magma of the two-mica granites was dominantly derived from partial melting of clay-rich psammite in the older crust.These two-mica granites were formed in a continental collision environment.Combined with available data,it is held that the generation of the collisional strongly-peraluminous granite in the Late Permian-Middle Triassic period suggests an orogenic event during early Indosinian in the Sanjiang Paleo-Tethys Sea.The Sanjiang early Indosinian orogenesis was intimately related to the closure of the Paleo-Tethys Sea and the subsequent orogenic events.