藏南错那地区空布岗中新世淡色花岗岩的成因机制

Genetic mechanism of the Kongbugang Miocene leucogranite in the Cona area,southern Tibet.

空布岗位于特提斯喜马拉雅带东部、桑日-错那裂谷系内,发育二云母花岗岩、白云母淡色花岗岩、含石榴子石淡色花岗岩以及混合岩淡色体。LA-MC-ICP-MS锆石U-Pb定年显示,空布岗白云母淡色花岗岩结晶年龄为16.9±0.1Ma。全岩元素地球化学分析表明:二云母花岗岩为过铝质富钠花岗岩,具有较高的Sr(107×10^(-6)~141×10^(-6))和Ba(230×10^(-6)~311×10^(-6)),较低的Rb(108×10^(-6)~221×10^(-6))和Rb/Sr比值(0.78~2.07),Ba与Rb/Sr比值没有相关性,指示其为白云母含水部分熔融作用的产物。白云母淡色花岗岩为过铝质富钾花岗岩,具有较高的Rb(>270×10^(-6))和Rb/Sr比值(5.2~9.5),但是其Sr(<58.5×10^(-6))和Ba(<167×10^(-6))含量较低,Ba与Rb/Sr比值呈现明显的负相关关系,表明它是白云母脱水部分熔融作用的产物。含石榴子石淡色花岗岩具有显著的Ba、Th、Nb、Ta、P、Sr、Eu、Zr、Ti负异常和高度相关的微量元素变化,是花岗质熔体经历斜长石、锆石、云母等分离结晶作用的产物。混合岩淡色体具有较高的SiO_(2)、K 2O、TiO_(2)、Ba、Nb、Ta、Zr、Hf,部分样品富集轻稀土或重稀土,存在显著的正Eu异常,指示了淡色体形成过程中捕获了源岩中的锆石、独居石、石榴子石等副矿物以及转熔钾长石。结合错那地区已发表的数据,可以推断:空布岗中新世花岗岩与桑日-错那裂谷的开启相关,低喜马拉雅岩系变质脱水产生流体,流体通过裂谷系上升促使了高喜马拉雅结晶岩系发生白云母含水熔融,桑日-错那裂谷的开启时间应该不晚于22.5Ma。

The Kongbugang granite pluton is located within the Sangri-Cuona rift system in the eastern part of the Tethys Himalayan belt.It consists of various types of leucogranites,including two-mica granite,muscovite leucogranite,garnet-bearing leucogranite and leucosome.LA-MC-ICP-MS zircon U-Pb dating results show that the Kongbugang muscovite leucogranite was formed at 16.9±0.1Ma.Geochemical analysis data indicate that the two-mica granites are Na-rich and peraluminous granites with high Sr(107×10^(-6)~141×10^(-6))and Ba(230×10^(-6)~311×10^(-6)),but low Rb(108×10^(-6)~221×10^(-6))and Rb/Sr ratio(0.78~2.07).They are defined by nearly constant Rb/Sr ratio despite large variations in Ba concentrations,which is a distinctive feature of fluid-fluxed melting of muscovite.The muscovite leucogranites are K-rich and peraluminous granites with high Rb(>270×10^(-6))and high Rb/Sr ratio(5.2~9.5),but low Sr(<58.5×10^(-6))and Ba(<167×10^(-6)).The Ba contents are negatively correlated with Rb/Sr ratios,which is considered to be a typical characteristic of fluid-absent melting of muscovite.The garnet-bearing leucogranites have significant Ba,Th,Nb,Ta,P,Sr,Eu,Zr and Ti negative anomalies and highly correlated relationships between different trace element contents,which indicates that they are the product from granitic melt by fractional crystallization of zircon,plagioclase and mica.The leucosomes have high SiO_(2),K 2O,TiO_(2),Ba,Nb,Ta,Zr,Hf contents,and enriched in light rare earth elements or heavy rare earth elements with significant positive Eu anomalies.During the formation of the leucosomes,inherited accessory minerals(such as zircon,monazite,garnet and Ti-rich minerals)and peritectic feldspar were captured from the source rocks.Combined with the published data in the Cuona area,we can infer that the Miocene leucogranite in Kongbugang region is related to the initiation of the Sangri-Cuona rift.The fluid released from the Lesser Himalayan Sequence rose through the rift system and promoted fluid-fluxed melting of muscovite within the High Himalayan Crystalline Series.The initiated time of the Sangri-Cuona rift should be no later than 22.5Ma.