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.

南秦岭带（South Qinling Belt, SQB）北大巴山地区广泛发育早古生代碱性岩浆作用，岩石组合以玄武岩-辉绿岩（basalt–diabase）和粗面岩-正长岩（trachyte–syenite）为主。长英质岩石的成因及其与镁铁质组分的成因联系仍存在争议。本文开展了一项整合地质年代学（geochronology）、全岩地球化学与Sr-Nd同位素地球化学（Sr–Nd isotopic geochemistry）的综合研究，以进一步约束北大巴山平利-竹溪地区粗面岩（trachytes）的成因与构造-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，其形成年龄为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.%）与总碱含量（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，铕异常不明显但锶异常显著负异常。所有样品的初始⁸⁷Sr/⁸⁶Sr比值较低且变化范围较大（0.6959~0.7083），初始¹⁴³Nd/¹⁴⁴Nd比值范围较窄（0.5123~0.5125），εNd(t)值为+3.0~+7.3，指示其源区为亏损源区（depleted source）。结合已发表的同期南秦岭带辉绿岩数据，本文的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，表明二者存在密切的成因联系。针对平利-竹溪粗面岩的合理成因假说认为，其起源于玄武质熔体经长期的分离结晶作用：熔体先后分离出钾长石（K-feldspar）、斜长石（plagioclase）、黑云母（biotite）、磷灰石（apatite）和钛磁铁矿（Ti-magnetite）等矿物相。稀土元素与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（cogenetic metasomatized lithosphere source）。软流圈上涌（Asthenospheric upwelling）是大陆裂解（continental break up）和岩石圈交代作用（lithosphere metasomatism）的关键因素。综上，交代源区的部分熔融，叠加以钾长石为主的长期分离结晶作用，共同造就了北大巴山平利-竹溪地区的粗面岩。