Early Palaeozoic alkaline trachytes in the North Daba Mountains, South Qinling Belt: petrogenesis and geological implications

Early Palaeozoic alkaline magmatism is widely preserved in the North Daba Mountains, South Qinling Belt (SQB), predominately composed of basalt–diabase and trachyte–syenite. The petrogenesis of the felsic rocks and their genetic connections with the mafic members remain controversial. Here, an integrated investigation, combining geochronology and whole-rock and Sr–Nd isotopic geochemistry, is conducted to further constrain the origin and tectono-magmatic evolution of the trachytes in Pingli–Zhuxi area, North Daba Mountains. Zircon U–Pb dating for the Pingli–Zhuxi trachytes yield ages of 406.0 to 427.9 Ma, which are close to those of the mafic rocks (420–455 Ma) in research area. The trachytes show relatively high SiO2 (60.88–63.87 wt.%) and total alkali (Na2O + K2O = 10.19–12.24 wt.%) contents, and are characterized by pronounced enrichment in LREEs and HFSEs, with insignificantly negative Eu anomalies and significantly negative Sr anomalies. All of the samples display low and variable initial 87Sr/86Sr ratios of 0.6959 to 0.7083 and narrow range initial 143Nd/144Nd ratios of 0.5123 to 0.5125 with positive εNd(t) values of +3.0 to +7.3, suggesting a depleted source. Together with the published data from coeval SQB diabases, our geochemistry evidences show regular and linear variations between mafic and felsic end-members, suggesting their closely genetic link. A plausible petrogenetic hypothesis for the genesis of the Pingli–Zhuxi trachytes implies a protracted process of fractional crystallization driven by separation of K-feldspar, plagioclase, biotite, apatite, and Ti-magnetite from a basaltic melt. Enrichment in REEs and HFSEs and similar geochemical characteristics point to that the Pingli–Zhuxi trachytes and the coeval mafic rocks originate from a cogenetic metasomatized lithosphere source in rift setting. Asthenospheric upwelling is a key factor for continental break up and lithosphere metasomatism. Collectively, melting of the metasomatized source, followed by protracted K-feldspar-dominated fractional crystallization, leading to the occurrence of the Pingli–Zhuxi trachytes in the North Daba Mountains.

南秦岭造山带（South Qinling Belt, SQB）北大巴山地区广泛出露早古生代碱性岩浆作用产物，岩石组合以玄武岩-辉绿岩及粗面岩-正长岩为主。长英质岩石的岩石成因及其与镁铁质组分的成因联系迄今尚存争议。本文对北大巴山平利-竹溪地区的粗面岩开展了地质年代学（geochronology）、全岩地球化学（whole-rock geochemistry）及Sr-Nd同位素地球化学（Sr–Nd isotopic geochemistry）的综合研究，以进一步约束其成因及构造-岩浆演化过程。对平利-竹溪粗面岩的锆石U-Pb定年（zircon U–Pb dating）结果显示，其形成时代为406.0~427.9 Ma，与研究区内镁铁质岩石的形成时代（420~455 Ma）较为接近。该套粗面岩具有较高的二氧化硅（SiO₂）含量（60.88~63.87 wt.%）和总碱含量（Na₂O+K₂O=10.19~12.24 wt.%），表现为轻稀土元素（Light Rare Earth Elements, LREEs）和高场强元素（High Field Strength Elements, HFSEs）显著富集，铕异常（Eu anomaly）不明显但锶异常（Sr anomaly）显著负异常。所有样品的初始⁸⁷Sr/⁸⁶Sr比值（initial 87Sr/86Sr ratio）较低且变化范围较大（0.6959~0.7083），初始¹⁴³Nd/¹⁴⁴Nd比值（initial 143Nd/144Nd ratio）变化范围较窄（0.5123~0.5125），且εNd(t)值（εNd(t)）为+3.0~+7.3，指示其源区为亏损地幔源区（depleted source）。结合已发表的同期南秦岭造山带辉绿岩数据，本次地球化学证据显示镁铁质端元与长英质端元之间呈现规律的线性变化趋势，表明二者存在密切的成因联系。针对平利-竹溪粗面岩的合理岩石成因假说认为，其成因源于玄武质熔体（basaltic melt）经历了由钾长石（K-feldspar）、斜长石（plagioclase）、黑云母（biotite）、磷灰石（apatite）及钛磁铁矿（Ti-magnetite）分离所驱动的长期分离结晶作用（fractional crystallization）过程。轻稀土元素和高场强元素的富集特征，以及相似的地球化学属性，指示平利-竹溪粗面岩与同期镁铁质岩石均起源于裂谷环境（rift setting）下的同源交代岩石圈源区（metasomatized lithosphere source）。软流圈上涌（Asthenospheric upwelling）是大陆裂解（continental break up）及岩石圈交代作用的关键控制因素。综上，交代源区的熔融，加之以钾长石为主导的长期分离结晶作用，共同造就了北大巴山平利-竹溪地区粗面岩的形成。