冈底斯印支期造山事件:花岗岩类锆石U-Pb年代学和岩石成因证据

对冈底斯中部地区二云母花岗岩和花岗闪长岩进行了LA-ICP-MS锆石U-Pb定年、主量元素、微量元素和锆石Hf同位素组成的测定.结果表明, 二云母花岗岩的岩浆结晶年龄为(205± 1)Ma, 岩石属于强过铝质花岗岩, A/CNK= 1.16~ 1.20, K₂O/Na₂O= 1.67~ 1.95.岩石富Rb、Th和U等元素, Eu/Eu* = 0.29~ 0.41, (La/Yb)_N= 22.62~ 35.08.锆石ε_Hf(t)= -12.4~ -1.8.二云母花岗岩的岩浆产生于地壳中泥质岩类在无外来流体加入的情况下云母类矿物脱水反应所诱发的部分熔融作用, 其岩石形成机制类似于喜马拉雅新生代淡色花岗岩.花岗闪长岩的岩浆结晶年龄为(202± 1)Ma, 岩石属于准铝质(A/CNK= 0.96~ 0.98), K₂O/Na₂O= 1.42~ 1.77, Eu/Eu* = 0.54~ 0.65, (La/Yb)_N= 6.76~ 13.35.锆石ε_Hf(t)= -8.2~ -5.5.根据花岗闪长岩的地球化学特征和锆石Hf同位素组成, 花岗闪长岩的岩浆来自于地壳中基性岩类的部分熔融.冈底斯印支晚期强过铝质花岗岩的确定, 表明了冈底斯在印支晚期以前曾发生地壳的缩短与加厚作用, 从而进一步明确了冈底斯印支早期的造山事件及冈底斯经历了多期造山作用的演化. 

Indosinian Orogenesis of the Gangdise Terrane: Evidences from Zircon U-Pb Dating and Petrogenesis of Granitoids

This paper reports LA-ICP-MS zircon U-Pb ages, whole-rock major and trace element and zircon Hf isotopic com- positions from two-mica granite and granodiorite plutons occurring in the middle part of the Gangdise terrane, Tibet. Magma zircons from the two-mica granite yielded a weighted 206Pb/238 U mean age of(205± 1)Ma(MSWD= 0.47), which is inter- preted as its magma crystallization age(Late Indosinian). The two-mica granite is strongly peraluminous, with Al index(A/ CNK)= 1.16 -1.20 and K₂O/Na₂O= 1.67 -1.95. The two-mica granite is characterized by enrichments of Rb, Th and U etc. Rare earth element(REE)data display Eu/Eu*= 0.29 -0.41 and(La/Yb)_N= 22.62 -35.08.ε_Hf(t)(205 Ma)values from the dated zircons range from -12.4 to -1.8. It is suggested that the magma for the two-mica granite was dominately derived from patial melting of argillaceous rocks in crust, induced by dehydration of mica minerals. The petrogenesis of the two-mica granite is similar to that of the Himalayan Tertiary leucogranites. Magma zircons from the granodiorite yielded a weighted 206Pb/238 U age of(202± 1)Ma, representing its magma crystallization age. The granodiorite is metaluminous, with Al index(A/CNK)= 0.96 -0.98, K₂O/Na₂O= 1.42 -1.77. REE data show Eu/Eu* = 0.54 -0.65 and(La/Yb)_N= 6.76 -13.35. Dated zircon Hf isotopic compositions exhibitε_Hf(202 Ma)values ranging from -8.2 to -5.5. The geo- chemical signatures and zircon Hf isotopic compositions suggest that the magma of granodiorite formed by partial melting of basaltic rocks in crust. The Late Indosinian stronlgly peralumineous granite is the first report in the Gangdise terrane. The occurring of the stronlgly peralumineous granite reveals Gangdise crustal thickening prior to Late Indosinian, and gives an impelling evidence that the Gangdise terrane took place an Early Indosinian orogenic event.