西藏冈底斯南缘冲木达约30Ma埃达克质侵入岩的成因:向北俯冲的印度陆壳的熔融?

Origin of ～30 Ma Chongmuda adakitic intrusive rocks in the southern Gangdese region,southern Tibet:Partial melting of the northward subducted Indian continent crust?

40～25 Ma之间通常被认为是拉萨地块特别是藏南冈底斯带岩浆活动的间歇期,与新特提斯洋板片断离后印度-亚洲大陆的硬碰撞有关。对出露于冈底斯东段南缘的冲木达石英二长岩-花岗闪长岩及相关的闪长质包体进行了锆石LA-ICPMS U-Pb定年和主微量元素、Sr-Nd同位素和锆石原位Hf同位素研究。年代学分析显示,侵入岩及其包体形成时代分别为(30.2±0.7)Ma和(31.0±0.5)Ma,近同时形成。冲木达侵入岩富硅(SiO2=64.56%～68.31%),高Sr(649～881μg/g),低Y(7.82～11.4μg/g)和Yb(0.78～1.04μg/g),Sr正异常,高场强元素亏损等,与典型埃达克岩的特征比较类似,但略微高钾(K2O=3.46%～4.10%)和微弱Eu负异常。其初始87Sr/86Sr比值为0.7057～0.7062,εNd(t)值为?3.34～?2.50,εHf(t)值为+2.2～+6.6。闪长质包体中高硅(SiO2=57.76%)样品显示了高镁(MgO=4.67%)、低Nb(15.1μg/g)和Nb/La(0.18)值的特点,但低硅(SiO2=54.76%)样品则低镁(MgO=3.47%)、高Nb(44.3μg/g)和Nb/La(0.68)值。除εNd(t)值(?4.08～?0.43)变化较大外,包体的初始87Sr/86Sr比值为0.7055～0.7062,εHf(t)变化于+0.7至+6.9之间,与埃达克质侵入岩类似。冲木达埃达克质侵入岩及其包体的形成很可能与俯冲陆壳熔融产生的熔体与地幔的相互作用有关:埃达克质侵入岩主要由早渐新世向北俯冲到拉萨地块下部的印度下地壳的熔融形成,高硅闪长质包体与俯冲陆壳熔体受到了上覆地幔橄榄岩的混染有关,低硅闪长质包体则可能由俯冲陆壳熔体交代的地幔部分熔融形成。另外,约30 Ma埃达克质侵入岩的出现可能表明,在拉萨地块南部,至少在约30 Ma,地壳已经增厚。

It is generally accepted that there is a magmatic gap between -40 Ma and 25 Ma in the Lhasa block, specially the Gangdese region, in respond to the hard collision between India and Asia plates after the breakoff of the Neo-Tethyan slab. In this paper, we report LA-ICPMS zircon U-Pb age and whole-rock major and trace element, Sr-Nd and in situ zircon Hf isotope composition data of the Chongmuda intrusive rocks, which consist of quartz monzonites and minor granodiorites and dioritic enclaves. Zircon U-Pb analyses for the Chongmuda intrusive rocks and enclaves yielded ages of （30.2±0.7） Ma and （31.0±0.5） Ma, respectively, indicating that they were generated synchronously. Except for slightly high K20 （3.46%-4.10%） and weak negative Eu anomalies, the Chongmuda intrusive rocks are geochemically similar to adakites, e.g., high SiQ （64.56%-68.31%） and Sr （649-881 gg/g） contents, low Y （7.82-11.4 gg/g） and Yb （0.78-1.04 gg/g） with positive Sr anomalies. They have relatively homogeneous initial 87Sr/86Sr ratios （0.7057-0.7062） and cNa（t） （ 3.3± 2.50） values as well as slightly variable cif（t） （＋2.2±6.6）. Dioritic enclaves can be classified two types： one type exhibits high SiO2 （57.76%） and MgO （4.67%） contents and low Nb （15.1 ug/g） and Nb/La （0.18） values, but the other type has relatively low SiQ （54.76%） and MgO （3.47%） contents and high Nb （44.3 gg/g） and Nb/La （0.68） values. Except for slightly variable eNa（t） values （-0.43- 4.08）, these two types of enclaves show initial 87Sr/^86Sr ratios （0.7055-0.7062）, and cif（t） （＋0.7-＋6.9） similar to those of the intrusive rocks. We suggested that the Chongmuda adakitic intrusive rocks and enclaves were most probably derived by partial melting of Early Oligocene northward subducted Indian lower crust beneath the Lhasa Block and subsequent interaction between the resultant melts and mantle peridotites. The adakitic intrusive rocks were possibly derived from subducted continental lower crust, but the high-SiO2 dioritic enclaves were generated by the interaction between adakitic melts and mantle peridotites and the low-SiO2 dioritic enclaves were possibly produced from mantle peridotites metasomatized by adakitic melts. In addition, -30 Ma adakitic rocks indicate that the crust of the Lhasa block may have been thickened since -30 Ma or earlier