Mg-Sr-Nd isotopic insights into petrogenesis of the Xiarihamu mafic-ultramafic intrusion, northern Tibetan plateau, China
Mg-Sr-Nd isotopic insights into petrogenesis of the Xiarihamu mafic-ultramafic intrusion, northern Tibetan plateau, China
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Mg-Sr-Nd 同位素对中国青藏高原北部夏日哈木镁铁质-超镁铁质岩体岩石成因的见解
DOI：
10.1093/petrology/egaa113
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发表时间：
2021
影响因子：
3.9
通讯作者：
Huang Kang-Jun
中科院分区：
文献类型：
--
作者：
Chen Lie-Meng;Song Xie-Yan;Hu Rui-Zhong;Yu Song-Yue;Yi Jun-Nian;Kang Jian;Huang Kang-Jun
为了表征从地幔源头到岩浆室的岩浆管道系统的综合岩浆过程，我们提供了中国青藏高原北部夏日哈木代表性镁铁质-超镁铁质侵入体的 Mg-Sr-Nd 同位素数据。该侵入岩体蕴藏着世界上造山环境中最大的岩浆镍铜硫化物矿床，主要由方辉锰矿、斜方辉石、韦氏辉长岩和辉长岩组成。堆积方辉石具有相对较低的斜方辉石 δ26Mg（–0·49 至 –0·34‰）和中等较高的 (87Sr/86Sr)i 和正 εNd(t) 值（分别为 0·7067–0·7080 和 +0·4 至 +1·8）。大多数斜方辉石、韦氏辉石和辉长岩的斜方辉石 δ26Mg 略高于方辉石（–0·34 至 –0·21‰），较高的 (87Sr/86Sr)i 和较低的 εNd(t) 值（分别为 0·7087–0·7118 和 –4·0 至 –1·4）。另一方面，边缘相（即距离侵入体与片岩、片麻岩和大理石围岩接触面几厘米到几米），包括网沸石和辉长岩，其特征是斜方辉石δ26Mg较低（–0·44至–0·33‰）和极高的（87Sr/86Sr）和负εNd（t）值（分别为 0·7127–0·7172 和 –5·4 至 –4·8）。方辉橄榄岩相对较低的 δ26Mg、中等较高的 (87Sr/86Sr)i 和正 εNd(t) 最好是通过来自地幔源的继承来解释的，地幔源先前被俯冲的富镁碳酸盐交代。鉴于碳酸盐化地幔的固相线明显低于无挥发分的地幔固相线，碳酸盐岩的地幔交代作用可能对于生成大量镁铁质岩浆以及在全球汇聚环境中形成巨大的夏日哈木镍铜矿床和其他岩浆硫化物矿床至关重要。大多数斜方辉石岩、韦氏辉长岩和辉长岩具有相对较高的 δ26Mg、较高的 (87Sr/86Sr)i 和较低的 εNd(t)，是由深部岩浆房中重同位素地壳岩石的广泛污染产生的。与其他网沸石和辉长岩相比，边缘相δ26Mg略低，(87Sr/86Sr)i偏高，εNd(t)为负值，这是由夏日哈木岩浆房在侵位过程中局部低δ26Mg围岩（如片麻岩和大理岩）的同化控制的。这些观察结果表明，远离边缘相的镁铁质-超镁铁质堆积物可以保留其从深部岩浆房甚至地幔源继承的原始Mg-Sr-Nd同位素特征，在岩浆上升和就位过程中地壳污染可以忽略不计。据我们所知，这些发现首次提供了观测证据来表征从深部地幔源到岩浆室的单一岩浆管道系统的综合岩浆过程，这为研究镁铁质-超镁铁质侵入体的岩石成因和矿化提供了线索。
To characterize the integrated magmatic processes for magma plumbing systems from mantle sources to magma chambers, we present Mg–Sr–Nd isotopic data for a representative mafic–ultramafic intrusion in Xiarihamu, northern Tibetan Plateau, China. The intrusion hosts the largest magmatic Ni–Cu sulfide deposit in an orogenic setting in the world and is composed predominantly of harzburgite, orthopyroxenite, websterite, and gabbronorite. The cumulate harzburgites have relatively low δ26Mg of orthopyroxene (–0·49 to –0·34‰) and moderately high (87Sr/86Sr)iand positive εNd(t) values (0·7067–0·7080 and +0·4 to +1·8, respectively). Most of the orthopyroxenites, websterites, and gabbronorites have slightly higher δ26Mg of orthopyroxene (–0·34 to –0·21‰) and higher (87Sr/86Sr)iand lower εNd(t) values (0·7087–0·7118 and –4·0 to –1·4, respectively) than the harzburgites. On the other hand, marginal facies (i.e. a few centimeters to meters from the contact between the intrusion and country rocks of schist, gneiss, and marble) including websterite and gabbronorite are marked by low δ26Mg of orthopyroxene (–0·44 to –0·33‰) and extremely high (87Sr/86Sr)iand negative εNd(t) values (0·7127–0·7172 and –5·4 to –4·8, respectively). The relatively low δ26Mg, moderately high (87Sr/86Sr)i, and positive εNd(t) of the harzburgites are best interpreted by inheritance from the mantle source, which was previously metasomatized by subducted Mg-rich carbonates. Given the fact that the solidus of carbonated mantle is significantly lower than that of volatile-free mantle, mantle metasomatism by carbonates may be critical to generate voluminous mafic magmas and to form the giant Xiarihamu Ni–Cu deposit and other magmatic sulfide deposits in convergent settings worldwide. The relatively high δ26Mg, high (87Sr/86Sr)i, and low εNd(t) of most orthopyroxenites, websterites, and gabbronorites were produced by extensive contamination of isotopically heavy crustal rocks in a deep-seated magma chamber. The slightly low δ26Mg, extremely high (87Sr/86Sr)iand negative εNd(t) of the marginal facies, compared with other websterites and gabbronorites, were governed by assimilation of local low-δ26Mg country rocks (e.g. gneiss and marble) at the Xiarihamu magma chamber during emplacement. These observations indicate that mafic–ultramafic cumulates away from marginal facies can preserve their primitive Mg–Sr–Nd isotope signatures inherited from deep magma chambers, even from mantle sources, with negligible crustal contamination during magma ascent and emplacement. These findings for the first time, to our knowledge, provide observational evidence to characterize the integrated magmatic processes of a single magma plumbing system from deep mantle source to magma chambers, which sheds light on the petrogenesis and mineralization of mafic–ultramafic intrusions.