藏南错那淡色花岗岩LA-MC-ICP-MS锆石U-Pb年龄、岩石地球化学及其地质意义

Age and geochemistry of the Cuona leucogranite in southern Tibet and its geological implications

藏南错那淡色花岗岩位于喜马拉雅造山带的东部。对其进行LA-MC-ICP-MS锆石U-Pb定年,结果显示,结晶年龄为17.7±0.3Ma,代表中新世的地壳深熔作用。淡色花岗岩样品具有高的Si O2(74.46%~75.57%)、Al2O3(14.07%~14.64%)和K2O(4.19%~4.85%)含量,高的K2O/Na2O值(1.09~1.31)和A/CNK值(1.15~1.25),富集Rb、Th和U,亏损Ba、Nb、Sr、Zr等元素,显示高的Rb/Sr值(17.75~29.50)和强烈的负Eu异常(δEu=0.18~0.26),属于壳源成因的高钾钙碱性过铝质S型花岗岩。样品具有高的Isr值(0.78982~0.79276)和低的εNd(t)值(-19.5^-18.2),可与大喜马拉雅结晶杂岩(GHC)中的变泥质岩对比,暗示其来自变泥质岩的部分熔融。样品的Isr值较高,而Sr浓度较低,且随着Ba浓度的增加,Rb/Sr值逐渐降低,表明淡色花岗岩是无水条件下白云母部分熔融的产物,部分熔融可能与藏南拆离系(STDS)伸展拆离导致的构造减压有关。错那淡色花岗岩的形成反映了地壳伸展减薄背景下,构造减压导致的中下地壳中含水矿物脱水熔融,并沿STDS上升侵位的动力学过程。

The Cuona leucogranite pluton is situated in the east of Himalayan orogen. LA-MC-ICP-MS zircon U-Pb dating re- veals that leucogranites were crystallized at 17.7±0.3Ma, representing the Miocene crustal anataxis. Geochemical studies show that the samples are characterized by high SiO2 （74.46%-75.57%）, Al2O3 （14.07%-14.64%）, K20 （4.19%-4.85%）, K2O/Na2O ratios （1.09-1.31） and A/CNK values （1.15-1.25）, enrichment of Rb, Th, U and depletion ofBa, Nb, Zr, Sr, and high ratios of Rb/Sr （17.75-29.50） with strong negative Eu anomalies （δ Eu=0.18-0.26）. These features suggest that they are crust-derived high potassium calcalkaline and peraluminous S-type granite. The relatively high Lr （0.78982-0.79276） and low ε Na（t） （-19.5--18.2） are well comparable with data of the metapelite from Greater Himalayan Crystalline complex （GHC）, indicating that the leucogranites were generated from their partial melting. The features of high Ist and low Sr concentration as well as the decreasing Rb/Sr values with increasing Ba concentration demonstrate that the Cuona leucogranites were derived from muscovite dehydration melting under the water-absent condition, possibly triggered by structural decompression responding to the activity of South Tibetan Detachment system （STDS）. It is held that the Cuona leucogranites reflect dynamics of structural decompression, dehydration melting and emplacement of the melt along STDS under the background of crustal extension and thinning.