西藏南木林县格张地区早白垩世火山岩岩石成因

Petrogenesis of Early Cretaceous volcanic rocks in the Gezhang area, Namling County, Tibet, China

西藏南木林县格张地区在构造上位于中冈底斯带南部(即中拉萨地块南部)的革吉—申扎弧后盆地。以格张地区出露的火山岩为研究对象,进行岩石学、LA-ICP-MS锆石U-Pb定年、元素地球化学和Lu-Hf同位素地球化学分析研究。结果显示,格张地区火山岩主要为一套酸性火山碎屑岩夹少量的火山熔岩,岩石组合为流纹质含角砾晶屑凝灰岩和流纹质(含角砾)晶屑熔结凝灰岩夹少量流纹岩和玄武岩。火山岩中发育典型的岩浆锆石。对1件流纹岩和1件流纹质晶屑熔结凝灰岩样品中的锆石进行U-Pb LA-ICP-MS定年分析,分别获得的年龄为(133±1.7) Ma和(128±1.5) Ma,表明火山岩形成于早白垩世(K1)时期。火山岩主量元素在质量分数上表现为高SiO2(69.4%-75.8%)、高K2O(2.93%-5.58%)、富Al2O3(12.7%-14.6%)、低TiO2(0.20%-0.34%)、低Na2O(0.05%-2.5%)、弱过铝质过铝质(A/NK为0.92-1.83,A/CNK为1.06-3.59)、高钾钙碱性系列的钾质火山岩特征。稀土元素配分模式呈现为轻稀土元素强烈富集的右陡倾斜型,其(La/Yb)N为12.3-22.0,具明显的负铕异常(Eu/Eu^*=0.47-0.62)。微量元素普遍表现出Rb、Th、K等元素的正异常和Ba、Ta、Nb、Sr、P、Ti等元素的负异常。火山岩的εHf(t)值主要集中在-12.4--7.2,两阶段地壳Hf模式年龄tDM2主要为1.5-1.7 Ga,相当于念青唐古拉群(AnZNq)的地质时代。格张地区早白垩世火山岩应为中部拉萨地块基底的念青唐古拉群(AnZNq)部分熔融的结果,并伴有少量幔源物质的加入,形成于因班怒洋板片南向俯冲所产生的陆缘弧构造环境。

The Gezhang area of Namling County,Tibet is structurally located in the southern part of the middle Gangdese belt(i.e.,south-middle Lhasa block)in the Geji-Shenzha back-arc basin.In this paper,we studied the volcanic rocks exposed in the Gezhang area through petrology,LA-ICP-MS zircon U-Pb dating and elemental geochemistry researches and Lu-Hf isotope analysis.The results show that the volcanic rocks in the Gezhang area comprises mainly a set of acidic volcanic clastic rocks and minor volcanic lava.The rock assemblage consists of primarily rhyolitic tuff and ignimbrite(both contain breccia crystal fragments)and small amounts of rhyolite and basalt.Typical magmatic zircon developed in volcanic rocks.Through LA-ICPMS U-Pb dating of zircons from rhyolite and rhyolitic ignimbrite samples,we obtained rock ages of 133±1.7 Ma and 128±1.5 Ma,confirming the volcanic rocks are formed in the Early Cretaceous(K1).Major elemental analysis shows high SiO2(69.4%-75.8%),K2O(2.93%-5.58%)and Al2O3(12.7%-14.6%)and low TiO2(0.20%-0.34%)and Na2O(0.05%-2.5%)contents.The volcanic rock is weakly peraluminous to peraluminous(A/NK=0.92-1.83,A/CNK=1.06-3.59)and of high potassium calc-alkaline series.The rare earth element(REE)partition mode shows strong light REE-enriched right-sloping type.The(La/Yb)N ratio is 12.3 to 22.0,with obvious Eu negative anomalies(Eu/Eu*=0.47-0.62).Trace elements generally exhibit positive anomalies of Rb,Th and K,and negative anomalies of Ba,Ta,Nb,Sr,P and Ti.Volcanic rocks have εHf(t) values concentrated between-12.4 and-7.2 and two-stage crust Hf model ages(tDM2)concentrated between 1.5 and 1.7 Ga,equivalent to the geological age of the Nyainqentanglha Group (AnZNq).The Early Cretaceous volcanic rocks in the Gezhang area,therefore,should result from partial melting of the Nyainqentanglha Group(AnZNq)at the base of middle Lhasa block with minor mantle participation,and formed in the continental-margin magmatic arc environment created by the southward subduction of the Bangong-Nujiang plate.