Transition from tholeiitic to alkali basalts via interaction between decarbonated eclogite-derived melts and peridotite.
Transition from tholeiitic to alkali basalts via interaction between decarbonated eclogite-derived melts and peridotite.
复制标题
通过脱碳榴辉岩熔体与橄榄岩之间的相互作用，从拉斑玄武岩转变为碱性玄武岩。
DOI：
10.1016/j.chemgeo.2023.121354
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发表时间：
2023
期刊：
影响因子：
3.9
通讯作者：
Xiangkun Zhu
中科院分区：
文献类型：
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作者：
Qiuhong Xie;Zhaochong Zhang;Stephen F. Foley;Chunfei Chen;Zhiguo Cheng;Yu Wang;Weiliang Kong;Yiwen Lv;M. Santosh;Qizhen Jin;Lukáš Krmíček;Xiangkun Zhu
板内玄武岩一般表现出从碱性玄武岩到拉斑玄武岩的地球化学连续性。然而，这两种岩石之间的遗传联系仍然存在争议。中亚造山带南部西准噶尔早侏罗世克拉玛依玄武岩在稳定的大陆板内区域喷发形成小体积露头。玄武岩以无斑晶结构为特征，无明显斑晶。因此，它们不同于普遍存在的斑状结构板内玄武岩，具有接近于幔源原始熔体的成分。与化学和岩石学上定义明确的碱性玄武岩和拉斑玄武岩不同，克拉玛依玄武岩具有从碱性（标准橄榄石+霞石和标准橄榄石+紫苏辉石）到拉斑玄武岩（标准石英+紫苏辉石）的过渡成分，为解决板内玄武岩的地球化学连续体提供了重要案例。与中国东部碱性玄武岩相似，克拉玛依玄武岩具有沉积碳酸盐的同位素印记，Mg同位素（δ 26 Mg =-0.54 ‰ ~-0.34 ‰）和Zn同位素（δ 66 Zn = 0.36 ‰ ~ 0.46‰）明显轻于正常地幔。但它们的初始（（87 Sr/86 Sr）比值为0.7047-0.7051，εNd（t）值为正（3.3-4.2），Nb-Ta-Ti-Zr-Hf正异常，这在回收碳酸盐的情况下是不可能的。这种矛盾可以通过考虑碳酸盐之间的脱碳酸反应（即，在压力> 5GPa的俯冲洋壳板片中，榴辉岩（白云石和菱镁矿）与榴辉岩共存，在地幔深部的停滞榴辉岩残留物中留下轻Mg和重Zn同位素特征。结合样品的地球化学成分和地质证据，并结合前人熔体-橄榄岩反应实验结果，我们认为克拉玛依玄武岩可能是由回收的脱碳酸盐榴辉岩中的富硅拉斑玄武岩熔体在上升过程中与富硅橄榄岩相互作用而形成的。我们的研究强调，板内碱性玄武岩，特别是富硅玄武岩（如，SiO2>45重量%），可以由拉斑玄武岩熔体通过与橄榄岩地幔反应转化而成，并证明了停滞在地幔中的深循环洋壳可以作为碱性熔岩的主要来源。
Intraplate basalts generally show a geochemical continuum from alkali to tholeiitic basalts. However, the genetic link between these two types of rocks has remained controversial. The Early Jurassic Karamay basalts in the West Junggar terrane, southern Central Asian Orogenic Belt (CAOB), erupted to form a small-volume outcrop in the stable continental intraplate region. The basalts are characterized by aphyric textures without any visible phenocrysts. Thus, they are different from the ubiquitous porphyritic-textured intraplate basalts and have a composition close to that of the mantle-derived primary melt. In contrast to chemically and petrographically well-defined alkali and tholeiitic basalts, the Karamay basalts exhibit transitional compositions spanning from alkali (with normative olivine+nepheline and normative olivine+hypersthene) to tholeiitic (with normative quartz+hypersthene), providing an important case to address the geochemical continuum of intraplate basalts. Similar to the alkali basalts in eastern China, the Karamay basalts have isotopic imprints of sedimentary carbonates, i.e., significantly lighter Mg (δ26Mg = −0.54‰ to −0.34‰) and heavier Zn (δ66Zn = 0.36–0.46‰) isotopes than the normal mantle. However, they display initial ((87Sr/86Sr)tratios of 0.7047–0.7051, positive εNd(t) values (3.3–4.2) and positive anomalies of Nb-Ta-Ti-Zr-Hf, which are not expected in the case of incorporation of recycled carbonates. This contradiction can be reconciled by considering a decarbonation reaction between carbonates (i.e., dolomite and magnesite) and co-existing eclogite in the subducted oceanic slab at pressure >5 GPa, leaving light Mg and heavy Zn isotope signatures in the stagnant eclogite residue in the deep mantle. Combining the geochemical compositions of our samples with the geologic evidence, and considering the previous results of melt-peridotite reaction experiments, we conclude that the Karamay basalts might have originated from the interaction of silica-rich tholeiitic melt derived from the recycled decarbonated eclogite with fertile peridotite during its ascent. Our study highlights that intraplate alkali basalts, especially silica-rich ones (e.g., with SiO2>45 wt.%), can be transformed from tholeiitic melts through reaction with peridotite mantle, and demonstrates that deeply recycled oceanic crust stagnated in mantle can serve as a main source for alkaline lavas.