北大巴山紫阳-岚皋地区碱性粗面岩地球化学特征:与辉绿岩的成因联系

南秦岭北大巴山地区发育大量早古生代碱性粗面?正长岩和碱性玄武?辉绿岩组合,构成了一套双峰式火山岩。前人的研究主要集中在这套岩石的镁铁质端元上,目前关于长英质端元的成因机制及构造环境仍存在争议。本次研究对北大巴山紫阳?岚皋地区早古生代粗面岩进行了主微量元素分析,结果显示样品高硅(SiO₂>58.7%)、富碱(K₂O>3.84%,Na₂O>4.18%),低镁(MgO=0.06%~1.45%)和钛(TiO₂=0.80%~1.08%),表明粗面岩经历了较高程度的演化。样品富集轻稀土元素(LREE)和大离子亲石元素(LILE),具有轻微Eu负异常到正异常的变化(δEu=0.76~1.77),Sr强烈亏损,指示了长石的分离结晶作用。地球化学特征和MELTS软件模拟结果显示粗面岩并非地壳物质的部分熔融成因,而是来自地幔岩浆的分离结晶作用。综合区域上年代学、同位素地球化学和构造地质学资料,粗面岩与同区辉绿岩在时空分布、同位素组成方面具有一致性,在元素变化与矿物组合方面具有连续性。因此本文认为北大巴山紫阳?岚皋地区早古生代粗面岩和辉绿岩都来源于上地幔初始玄武质岩浆的分异,是大陆裂谷背景下同源岩浆经历不同程度演化的结果。软流圈中广泛存在的热对流事件可能是导致板块裂解的重要原因。

Geochemical Characteristics of Alkali Trachytes in Ziyang-Langao Area,North Daba Mountains and Petrogenetic Relation with the Diabases

The voluminous Early Paleozoic alkaline basalt-diabase and trachyte-syenite volcanic rocks in the North Daba Mountains constitute a set of bimodal volcanic rocks.Previous works focused predominantly on the mafic end-members of the bimodal volcanism,but the petrogenesis and tectonic implication of the felsic end-members remain ambiguous.Here,we report the geochemical features of the trachytes in the Ziyang-Langao area of the North Daba Mountains.The Ziyang-Langao trachytes show relatively high SiO₂(>58.7%)and enriched alkali(K₂O>3.84%,Na₂O>4.18%),with low and variable MgO(0.06%–1.45%)and TiO₂(0.80%–1.08%),showing that the trachytes are highly evolved.They are characterized by enrichment of LREEs and depletion of HREEs,slightly negative to positive Eu anomalies,and significantly negative Sr anomalies.Combined with the microscopic features,K-feldspar is the main crystal phase in trachyte samples.The geochemical features and modeling results from MELTS suggest that the Ziyang-Langao trachytes share a common source region with the coeval diabases in the North Daba Mountains,and were results of fractional crystallization of the primary alkaline basaltic magma.Taking into account the previous studies in the North Daba Mountains,we argue that the Ziyang-Langao trachytes were formed in a rift setting at low pressure and underwent high degree evolution.The asthenospheric thermal convection may have important effects on the continental cracking and expansion of the Mianlue ocean basin in the North Daba Mountains.