藏南定结地区早中新世淡色花岗岩的形成机制及其构造动力学意义

Early Miocene leucogranites in Dinggye area, southern Tibet: Formation mechanism and tectonic implications

定结日玛那穹窿位于高喜马拉雅带中段,由花岗片麻岩、变泥质岩、变基性岩及大量淡色花岗岩等组成,经历了角闪岩-麻粒岩相变质作用。为厘定淡色花岗岩的形成机制以及与高级变质岩的关系,我们对淡色花岗岩和高级变质岩进行了全岩元素和Sr和Nd同位素组成和SHRIMP锆石U-Pb地质年代学测试。全岩元素和Sr-Nd同位素测试结果揭示淡色花岗岩具有以下特征:(1)高SiO2(>72%),高Al2O3(>12%)和高A/CNK比值(>1·0);(2)高Rb,低Sr,高Rb/Sr比值(>1·0);(3)高∑REE和明显的负Eu异常;(4)高Sr同位素初始比值(0·7621~0·8846)和低εNd(t)值(-13·0^-20·2)。淡色花岗岩的高Rb/Sr比值和Sr-Nd同位素系统特征表明其形成机制为主要为白云母脱水部分熔融作用,源区为由花岗片麻岩和变泥质岩组成的混合源区。SHRIMP锆石U-Pb年代学研究揭示出定结地区淡色花岗岩具有21·0±0·7Ma和15·8±0·1Ma2期年龄,花岗片麻岩的锆石变质增生边年龄为22·2±1·4Ma,与该区的榴辉岩退变质年龄一致。这些数据共同表明,花岗片麻岩和变泥质岩在22~21Ma发生高级变质和深熔作用,形成早期淡色花岗岩岩浆,在~16Ma进一步深熔,形成晚期淡色花岗岩岩浆。

The Ama Drime Massif, Dinggye area, is located in the central Greater Himalaya and consists of granitic gneisses, metapelites, metabasites and massive leucogranites, and has experienced amphibolite- to granulite-facies metamorphism. Whole-rock major- and trace-element, radiogenic isotope （Sr and Nd）, and SHRIMP zircon analyses were undertaken to investigate the petrogenic relationship between the leucogranites and the metamorphic rocks, as well as the formation mechanism of the leucogranites. Bulk element and Sr-Nd isotope analyses show that the leucogranites have （1） high SiO2 （72%）, Al2O3 （12%）, and A/CNK （1.0）; （2） high Rb and low Sr, thus high Rb/Sr ratios （1.0）; （3） high contents of total REE, and substantial negative Eu anomalies; and （4） high initial Sr （87Sr/86Sr（t）=0.7621~0.8845） and low Nd （εNd（t）=-13.0^-20.2） isotopic compositions. High Rb/Sr ratios and Sr-Nd isotope systematics indicate that the leucogranites were mainly derived from muscovite dehydration melting of metapelite with a substantial contribution from granitic gneiss. SHRIMP zircon U-Pb dating indicates that the leucogranites have preserved a record of two episodes of partial melting at 21.0±0.7Ma and 15.8±0.1Ma, respectively; and the granitic gneisses experienced high-grade metamorphism at 22.2±1.4Ma, similar to the timing of retrograde metamorphism in this area. In conclusion, both granitic and pelitic gneisses have experienced high-grade metamorphism and anatexis. The Miocene anatectic event could be episodic and at least produced leucogranitic melts at 22~21Ma and ~16Ma, respectively.