An early Paleozoic accumulation-foundering cycle of ultramafic cumulates in the Harlik arc and its implications for continental crustal growth in the Altaids

Abstract

<p>Lower crustal ultramafic cumulates that are normally denser than the ambient mantle <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/peridotite" title="Learn more about peridotites from ScienceDirect's AI-generated Topic Pages">peridotites</a> are prone to founder back into the convecting mantle, prompting the formation of continental Moho and andesitic crust at <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/subduction-zone" title="Learn more about subduction zones from ScienceDirect's AI-generated Topic Pages">subduction zones</a>. However, foundering events of ultramafic cumulates have not been widely reported in <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/island-arcs" title="Learn more about island arcs from ScienceDirect's AI-generated Topic Pages">island arcs</a>, including the Altaids, one of the largest accretionary orogens. The Altaids involved prominent continental crustal growth by the development of an archipelago in the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/phanerozoic" title="Learn more about Phanerozoic from ScienceDirect's AI-generated Topic Pages">Phanerozoic</a>. But the scarcity of foundering events of ultramafic cumulates hinders a better understanding of the formation mechanism of andesitic crust in the Altaids. In this study, we report two dioritic plutons in the Harlik arc, southern Altaids, trying to identify the accumulation-foundering cycles of dense ultramafic cumulates. LA-ICP-MS dating revealed that the Miaoergou and Banfanggou plutons in the Harlik arc were intruded at ca. 445 ± 6 Ma and 423 ± 4 Ma, respectively. The homogeneous Miaoergou pluton is mainly composed of quartz <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/diorite" title="Learn more about diorites from ScienceDirect's AI-generated Topic Pages">diorites</a>, they have a high- to medium-K calc-alkaline composition, and show enrichments in light <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/rare-earth-element" title="Learn more about rare earth elements from ScienceDirect's AI-generated Topic Pages">rare earth elements</a> (LREE) and large ion <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lithophile-element" title="Learn more about lithophile elements from ScienceDirect's AI-generated Topic Pages">lithophile elements</a> (LILE), but depletions in high field strength elements (HFSE, especially Nb and Ta) and heavy <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/rare-earth-element" title="Learn more about rare earth elements from ScienceDirect's AI-generated Topic Pages">rare earth elements</a> (HREE). They also have juvenile isotopic signatures (εNd(t) = +3.3 − +3.7, initial ⁸⁷Sr/⁸⁶Sr = 0.70399–0.70452), which together suggest that they were derived from <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/fractional-crystallization" title="Learn more about fractional crystallization from ScienceDirect's AI-generated Topic Pages">fractional crystallization</a> of the coeval hydrous basaltic <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magma" title="Learn more about magmas from ScienceDirect's AI-generated Topic Pages">magmas</a> in an arc setting. The heterogenous Banfanggou pluton comprises a spectrum of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/petrography" title="Learn more about lithologies from ScienceDirect's AI-generated Topic Pages">lithologies</a> from hornblende gabbro to diorite. The hornblende gabbros and <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/diorite" title="Learn more about diorites from ScienceDirect's AI-generated Topic Pages">diorites</a> have a high- to medium-K calc-alkaline composition, and show enrichments in LREE and LILE, but depletions in HREE and HFSE. They all have juvenile <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/isotopic-composition" title="Learn more about isotopic compositions from ScienceDirect's AI-generated Topic Pages">isotopic compositions</a> with εNd(t) ranging from +3.5 to +4.2, and (⁸⁷Sr/⁸⁶Sr)_i from 0.70384 to 0.7047, respectively, indicating that their parental <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magma" title="Learn more about magmas from ScienceDirect's AI-generated Topic Pages">magmas</a> were intruded in a subduction-related setting, and underwent <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/fractional-crystallization" title="Learn more about fractional crystallization from ScienceDirect's AI-generated Topic Pages">fractional crystallization</a> of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/clinopyroxene" title="Learn more about clinopyroxene from ScienceDirect's AI-generated Topic Pages">clinopyroxene</a>, plagioclase, <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/amphibole" title="Learn more about amphibole from ScienceDirect's AI-generated Topic Pages">amphibole</a> and quartz. Both pulses of hydrous basaltic <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magmatism" title="Learn more about magmatism from ScienceDirect's AI-generated Topic Pages">magmatism</a> involved open-system conditions, with the development of mafic microgranular enclaves (MMEs) in the diorites and their mingling. In a synthesis of the published data, we found that the Harlik arc was continuously affected by subduction in the early Paleozoic, through extensive arc <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magmatism" title="Learn more about magmatism from ScienceDirect's AI-generated Topic Pages">magmatism</a> from ca. 452 to 440 Ma, the Harlik arc matured with the accumulation of thick ultramafic cumulates in its <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lower-crust" title="Learn more about lower crust from ScienceDirect's AI-generated Topic Pages">lower crust</a>. The foundering of these ultramafic cumulates occurred later, which led to strong extension and related A-type granitic magmatism at ca. 436 Ma. This accumulation-foundering cycle of ultramafic cumulates facilitated the formation of andesitic composition and continental Moho in the Harlik arc, and contributed to the significant continental crustal growth in the Altaids.<br></p>