Cordilleran granitic batholiths represent significant episodes of crustal growth and differentiation, and commonly display lateral isotopic and chemical variations. Establishing the tectono-magmatic processes responsible for generating this compositional asymmetry is important for understanding crustal evolutionary processes throughout the Phanerozoic. The Bega Batholith, an example of a ‘Cordilleran style’ granite batholith, is the largest I-type Siluro-Devonian granite complex in the Lachlan Fold Belt (LFB) of southeastern Australia and comprises seven granite supersuites that display systematic lateral isotopic and chemical asymmetry. From west to east towards the present-day continental margin, an increase in the content of Na2O, Sr, Al2O3, and P2O5, with concomitant decreases in CaO, Sc, Rb, and V are observed. In the same direction, whole-rock initial 87Sr/86Sr decreases from 0.7098 to 0.7039, εNd values increase from −8.3 to +4.4, and δ18O decreases from 10.2 ‰ to 7.9 ‰. Depleted-mantle model ages also decrease from ca. 1800 Ma in the west to 600 Ma in the east. Here, we address whether these chemical and isotopic variations were generated by interaction between two distinct components (mantle-derived magmas and supracrustal sources) or were alternatively produced by partial melting of infracrustal source rocks formed sequentially by much earlier episodes of crustal underplating. Combined whole-rock Nd-Sr-O isotopic and geochemical analyses indicate that several I-type supersuites exhibit chemical and isotopic correlations consistent with two-component magma mixing. This new evidence challenges the long-held view that I-type granites derive exclusively from the melting of infracrustal sources, and that granite terranes represent wholesale crustal reworking rather than new crustal growth. Our results show that the compositional zoning within the Bega Batholith is multifaceted. Firstly, the presence of two discrete mantle sources endows chemically and isotopically distinct eastern and western segments in the batholith. Secondly, within these compositionally distinct regions the lateral compositional changes across supersuites derives from mixing between mantle-derived and supracrustal sources. Finally, progressive extension within a developing back-arc environment regulates the ratio of crust-mantle contributions and compositional architecture of each I-type supersuite.

中文翻译：

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澳大利亚东南部 I 型 Bega Batholith 的化学和同位素研究：对增生造山带中浴岩成分分区和地壳演化的影响


科迪勒拉花岗岩浴岩代表了地壳生长和分化的重要事件，通常表现出横向同位素和化学变化。建立负责产生这种成分不对称性的构造岩浆过程对于理解整个显生代的地壳演化过程非常重要。Bega Batholith 是“科迪勒拉风格”花岗岩浴岩的一个例子，是澳大利亚东南部 Lachlan 褶皱带 （LFB） 中最大的 I 型志留纪-泥盆纪花岗岩杂岩体，由七个花岗岩超级套组成，显示出系统的横向同位素和化学不对称性。从西向东到今天的大陆边缘，观察到 Na2O、Sr、Al2O3 和 P2O5 的含量增加，同时 CaO、Sc、Rb 和 V 的含量降低。在同一方向上，全岩初始 87Sr/86Sr 从 0.7098 降低到 0.7039，εNd 值从 −8.3 增加到 +4.4，δ18O 从 10.2 ‰ 降低到 7.9 ‰。枯竭地幔模型的年龄也从西部的约 1800 马 减少到东部的 600 马。在这里，我们讨论了这些化学和同位素变化是由两种不同成分（地幔衍生的岩浆和地壳上源）之间的相互作用产生的，还是由更早期的地壳底板事件依次形成的地壳下烃源岩的部分熔化产生的。结合全岩 Nd-Sr-O 同位素和地球化学分析表明，几个 I 型超套件表现出与双组分岩浆混合一致的化学和同位素相关性。 这一新证据挑战了长期以来的观点，即 I 型花岗岩完全来自地壳下来源的熔融，而花岗岩地层代表了大规模的地壳再加工，而不是新的地壳生长。我们的结果表明，Bega Batholith 内的成分分区是多方面的。首先，两个离散的地幔源的存在赋予了浴岩中化学和同位素不同的东西段。其次，在这些成分不同的区域内，跨超群的横向成分变化来自地幔衍生和地壳上来源之间的混合。最后，在发展中的后弧环境中的渐进延伸调节了每个 I 型超群的地壳-地幔贡献和组成结构的比例。