南海扩张前序岩浆活动:解译华南三水盆地古近纪玄武质岩浆作用过程

Magmatism Prior to the Spreading of the South China Sea:Constraints on Magmatic Processes of the Early Paleogene Basalts in the Sanshui Basin,South China

华南三水盆地古近纪玄武岩(64~43 Ma)记录了南海扩张前大陆裂解过程中的岩浆‒构造活动,但其岩浆演化过程及其动力学机制并不清楚。为此,作者对其开展了详细的岩石学和矿物学研究。三水盆地古近纪玄武岩具有OIB型的微量元素组成特征,由软流圈地幔源区部分熔融形成。该玄武岩中单斜辉石斑晶显示复杂的环带结构,包括正环带、反环带及韵律环带结构,指示了复杂的岩浆作用过程。正环带单斜辉石斑晶为核‒边结构,其核部圆化或呈不规则形状,边部自形,指示了早期晶体与晚期演化岩浆混合后发生熔蚀‒再生长过程。绝大部分反环带单斜辉石斑晶为核‒幔‒边结构,其核部相对低Mg,并被部分熔蚀呈不规则状或圆化,幔部相对高Mg,边部自形,反映了相对原始岩浆的补给作用。个别反环带单斜辉石为核‒边结构,其核部Al_(2)O_(3)、TiO_(2)含量和Mg^(#)值明显偏低,为下地壳捕虏晶,表明岩浆从地幔源区上升汇聚到地壳岩浆房的过程中经历了轻微的地壳同化混染作用。韵律环带结构单斜辉石核部常为不规则状,幔部成分呈韵律状变化,边部自形,记录了岩浆对流及多次岩浆演化‒补给过程。虽然单斜辉石斑晶结构复杂,但具有相近的结晶压力(0.32~0.47 GPa)和结晶温度(1080~1135℃),表明其源于一个相对稳定的浅部岩浆房。南海扩张前夕,北部陆缘岩石圈强烈拉伸、减薄,致使岩石圈‒软流圈界面明显抬升,岩浆主要汇聚于浅部地壳岩浆房发生岩浆分异、补给与对流作用。由于地壳岩浆房发生了持续的幔源岩浆补给作用,持续的热传导促使三水盆地局部区域的地壳熔融形成粗面岩和流纹岩(厚度超过1000 m),为三水盆地双峰式火山岩成因机制提供了新的视角。

The Early Paleogene basalts in the Sanshui Basin(64-43 Ma)recorded the magmatic-tectonic activity of the continental breakup prior to the spreading of the South China Sea.This paper presents detailed petrological and mineralogical constraints on the Early Paleogene basalts in the Sanshui Basin,and investigates their magmatic evolution and related geodynamics.The Early Paleogene basalts in the Sanshui Basin show typical OIB-type trace element compositions,which were derived from the partial melting of asthenosphere mantle source.Clinopyroxene phenocrysts in the rocks show complex zonation,including normal zoning,reverse zoning,oscillatory zoning,sector zoning,etc.,which denotes complex magmatic processes.Normal zoned clinopyroxene phenocrysts are usually composed of the core and the rim.The cores are rounded or irregular,and were wrapped by euhedral rims,which indicates the processes of dissolution and overgrowth of clinopyroxene due to mixing between the early crystals and the later evolved magma.Most of reverse zoned clinopyroxene phenocrysts are composed of core,mantle and rim.The low-Mg cores have been partially dissolved as irregular or rounded,and were wrapped by the high-Mg mantle and euhedral rim,which indicates the replenishment of relatively primitive magma.In addition,a few reverse zoned clinopyroxene phenocrysts show core-rim structure;the cores are crustal xenocryst with distinctly low Al_(2)O_(3),TiO_(2) contents and Mg^(#)values,which suggests slight crustal assimilation and contamination when the magma was pumping into the crustal magma chamber from mantle source.Oscillatory zoned clinopyroxene phenocrysts have irregular cores wrapped by the compositionally oscillatory mantles and euhedral rims,which recorded the processes of magmatic convection and repeated differentiation and replenishment.In spite of the complex zonation,clinopyroxene phenocrysts exhibit relatively constant crystallization pressure(0.32-0.47 GPa)and temperature(1080-1135℃),indicating they were derived from a relatively stable and shallow magma chamber.Therefore,the lithosphere of the northern continental margin should have been strongly stretched and thinned prior to the spreading of the South China Sea.This resulted in significant uplift of the lithosphere-asthenosphere boundary,and subsequent melt migration into the crustal magma chamber at shallow depth,leading to magma differentiation,replenishment and convection.Due to repeated magma replenishment in a stable magma chamber,the crust at the periphery of the chamber was continuously heated and might partially melt to produce trachyte and rhyolite(thickness of>1000 m).This paper offers a new perspective on the petrogenesis of the bimodal volcanic rocks in the Sanshui Basin,South China.