Sr-Nd-Pb isotope systematics of the Permian volcanic rocks in the northern margin of the Alxa block (the Shalazhashan belt) and comparisons with the nearby regions: implication for a Permian rifting setting?
Sr-Nd-Pb isotope systematics of the Permian volcanic rocks in the northern margin of the Alxa block (the Shalazhashan belt) and comparisons with the nearby regions: implication for a Permian rifting setting?
复制标题
阿拉善地块北缘（沙拉扎山带）二叠纪火山岩Sr-Nd-Pb同位素系统分析及与附近地区的比较：对二叠纪裂谷环境的启示？
作者：
Shi Guanzhong;Wang Hua;Liu entao;Huang Chuanyan;Zhao Jianxin;Song guanzeng;Liangchao
阿拉善地区二叠纪大量岩浆岩的成因有助于我们了解中亚造山带的构造演化。对沙拉扎山带(SLB)二叠纪火山岩，包括玄武岩、粗安岩和粗面英安岩进行了全岩主量元素和锶-钕-铅同位素分析。两个玄武岩样品具有负的ɛND(T)值(-5.4~-1.5)和较高的放射性成因Pb值，这与古老的次大陆岩石圈地幔有关。1个玄武岩样品的ɛND(T)值为正(+10)，来自镁铁质年轻源区。粗安岩的年龄为2 84±3 Ma，ɛNd(T)=+2.7~+8.0，ISR=0.7052~0.7057，是镁铁质岩浆与地壳熔体不同程度混合的产物。粗面英安岩具有较高的ɛNd(T)值和略高的ISR值，表明其来自地壳混染的年轻物质。与代表阿拉克斯地块北缘的努如-狼山带(NLB)、代表Solonker构造带的Mandula(STZ)和代表NCC北缘的白云鄂博(代表NCC北缘)的二叠纪岩浆岩的对比表明，二叠纪岩浆岩的放射性同位素组成有从南(白云鄂博和NLB)向北(SLB和Mandula)逐渐演化的趋势。二叠纪岩浆岩的形成涉及三个端元成分：富集型的SCLm、镁铁质年轻地壳或新底侵的镁铁质岩石以及地壳。来自年轻地壳或新底侵的镁铁质岩的岩石主要分布在SLB和Mandula地区(STZ)，而来自古老的富集型SCLm的岩石主要分布在NCC和NLB的北缘。岩浆岩的类型、时空分布与地幔热柱有关的岩浆活动相抵触。
The petrogenesis of the Permian large vast of magmatic rocks in the Alxa is helpful for us to understand the tectonic evolution of the Central Asian Orogenic Belt (CAOB). The Permian volcanic rocks in the Shalazhashan belt (SLB), including basalts, trachyandesites and trachydacites, are carried out for whole-rock major-trace elements and Sr–Nd–Pb isotopic analyses. Two basalt samples show negative ɛNd(t) values (–5.4 to –1.5) and higher radiogenic Pb values, which are linked with the ancient subcontinental lithosphere mantle (SCLM). One basalt sample has positive ɛNd(t) value (+10) and is derived from mafic juvenile sources. The trachyandesites are dated at 284±3 Ma with ɛNd(t)= +2.7 to +8.0; ISr=0.7052 to 0.7057, and they are generated by different degrees of mixing between mafic magmas and crustal melts. The trachydacites have high ɛNd(t) values and slightly higher ISr contents, suggesting derivation from juvenile materials with crustal contamination. The comparisons with the Permian magmatic rocks in the Nuru-Langshan belt (NLB, representing the northern margin of the Alax block), the Mandula (representing the Solonker tectonic zone, STZ) and the Bayan Obo (representing the northern margin of the NCC), indicate that the radiogenic isotopic compositions from the Permian magmatic rocks have an increasingly evolved trend from the south (the Bayan Obo and the NLB) to the north (the SLB and the Mandula). Three end-member components are involved to generate the Permian magmatic rocks: the enriched SCLM, the mafic juvenile crust or newly underplated mafic rocks, and the crust. The rocks derived from the juvenile crust or newly underplated mafic rocks are predominantly distributed along the SLB and the Mandula area (the STZ), and the rocks derived from ancient, enriched SCLM are mainly along the northern margin of the NCC and the NLB. The magmatic rock types, temporal and spacial distributions argue against them originated from mantle plume related magmatism.