吉尔吉斯天山造山带晚白垩世-古新世小规模大陆溢流玄武岩的成因及深部地球动力学过程

Petrogenesis and deep dynamics processes of the Late Cretaceous-Paleogene small-scale flood basalts in the Kyrgyz Tianshan orogenic belt

以短时间内巨量喷发为特征的大火成岩省一般被解释为是地幔柱作用的产物,但是关于小规模大陆溢流玄武岩的源区和形成机制上仍然存在较大的争论。本文选择吉尔吉斯北天山造山带中小规模大陆溢流玄武岩为研究对象,通过40Ar/39Ar同位素定年精确厘定这些玄武岩的形成时代,并利用全岩地球化学以及单矿物成分分析揭示其岩石成因和深部地球动力学过程。吉尔吉斯北天山中Uchkuduk小规模大陆溢流玄武岩的喷发年龄为81.5~84.0 Ma,明显早于吉尔吉斯天山造山带中其它地区玄武岩(58~76 Ma)。其主要斑晶矿物为橄榄石(Fo72.5~91.4)、铬尖晶石(Cr#=13.2~50.1,58.0%~76.2%FeOT和12.5%~20.4%TiO2)和单斜辉石(Wo47.3-51.3En35.5-43.2Fs9.37-14.2)。Uchkuduk玄武岩微量元素特征与OIB极其相似,富集大离子亲石元素和高场强元素,显示Pb、Sr、Nb-Ta的正异常以及K的负异常,并且具有正的εNd(t)值(+5.28^+5.42)、适度高的(87Sr/86Sr)i(0.7038~0.7047)和(206Pb/204Pb)i(18.231~18.339),暗示其来源于一个富集的软流圈地幔源区。橄榄石斑晶中高的NiO含量和Fe/Mn比值、低的CaO含量,与典型的来源于辉石岩熔体所结晶的橄榄石特征一致。此外,计算的原始岩浆成分落在L+Ol+Cpx+Grt共结线附近,同时具有高的FeOT(11.2%~11.7%),TiO2(2.01%~2.12%)含量,表明Uchkuduk玄武岩来源于石榴石辉石岩为主的地幔源区。该石榴石辉石岩源区可能与早期的地幔交代事件有关,通过俯冲再循环洋壳物质与周围橄榄岩的固相相互作用形成。Uchkuduk玄武岩原始岩浆形成所对应地幔潜能温度1574~1598℃,与地幔柱边部的热异常相当。因此,我们提出吉尔吉斯天山造山带及其邻区的晚白垩世-古新世小规模大陆溢流玄武岩可能是地幔柱活动的产物,地幔柱在上升过程中发生低程度(<10%)的部分熔融产生-系列的碱性玄武质岩浆。

Large igneous provinces, which are characterized by prodigious volumes of basalts erupted over a short duration,are commonly attributed to hot mantle plumes. However,the source and mechanism for small-scale flood basalts are subjected to considerable debate.In this study,we chose a case study from the Kyrgyz Tianshan orogenic belt to precisely determine their eruption ages,constrain the petrogenesis and discuss the deep dynamic processes using 40Ar/39Ar dating,mineral and whole-rock geochemistry. 40Ar/39Ar dating results show that the Uchkuduk lavas in the Kyrgyz Northern Tianshan were emplaced from 81.5 to 84.0 Ma,which formed earlier than the basalts from the other localities in the Kyrgyz Tianshan orogenic belt. Phenocryst minerals of olivine,Cr-spinel and clinopyroxene range from Fo72.5~91.4,Cr#=13.2~50.1,Wo47.3~51.3 En35.5~43.2 Fs9.37~14.2,respectively. The Uchkuduk basalts are characterized by oceanic island basalt(OIB)-like trace element distribution patterns with enrichment in LILE and HFSE and positive Pb,Sr,Nb-Ta and negative K anomalies in the primitive mantle-normalized trace element diagram,positive εNd(t)(+5.2+5.42) and moderately high(87Sr/86Sr)i(0.7038~0.7047) and(206Pb/204Pb)i(18.231~18.339)i sotopic compositions, indicating an enriched asthenospheric mantle source.The olivine phenocrysts have high NiO and Fe/Mn,and low CaO contents,which are typical of the olivines crystallized from melts derived from pyroxenitic mantle source. Besides,calculated primary compositions of the Uchkuduk basalts have compositions close to L+Ol+Cpx+Grt cotectic,and they also have high FeOT(11.2%~11.7%),TiO2(2.01%~2.12%)contents, suggesting a mainly garnet pyroxenite source,which may have been formed by the solid-state reaction between recycled oceanic crustal materials and their surrounding peridotite during earlier subduction events.The high mantle potential temperature calculated from the primary magma of the Uchkuduk basalts is comparable to the thermal anomalies of the peripheral part of a mantle plume.We proposed that low degrees of partial melting of upwelling mantle plume could be a possible mechanism to explain the Late Cretaceous-Paleogene small-scale flood basalts in the Kyrgyz Tianshan orogenic belt and neighboring areas.