Mineral Compositions of Syn-collisional Granitoids and their Implications for the Formation of Juvenile Continental Crust and Adakitic Magmatism

Abstract

Continentalcollision zones have been proposed as primary sites of net continental crustal growth. Therefore, studies on syn-collisional granitoids with mafic magmatic enclaves (MMEs) are essential for testing this hypothesis. The Baojishan (BJS) and Qumushan (QMS) syn-collisional plutons in the North Qilian Orogen (NQO) on the northern margin of the Tibetan Plateau have abundant MMEs in sharp contact with host granitoids, sharing similar constituent minerals but with higher modal abundances of mafic minerals in MMEs. The QMS host granitoids have high Sr/Y and La/Yb ratios, showing adakitic compositions, which are differentfrom the BJS granitoids. Based on bulk-rock compositions and zircon U-Pb age-dating, recent studies on these two plutons proposed that MMEs represent cumulates crystallized early from the same magmatic system as their host granitoids, and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies. Here, we focus on the trace-element geochemistry of the constituent mineral phases of both MMEs and their host granitoids of the QMS and BJS plutons. Weshow that different mineral phases preferentially host different trace elements; for example, most rare earth elements (REEs and Y) reside in titanite (only found in the QMS pluton), amphibole, apatite, epidote and zircon (mostly heavy-REEs); and high-field-strength elements (HFSEs) reside in biotite, titanite, amphibole and zircon. Based on the mineral chemical data, we show that for these two plutons, MMEs are of similar cumulate origin, crystallized from primitive andesitic melts in the early stage of granitoid magmatism. The primitive andesitic melts for these syn-collisional granitoids are most likely produced by the partial melting of the oceanic crust, supporting the hypothesis of continental crustal growth considering the syn-collisional granitoids represent juvenile continental crust. As evidenced by distinct mineral compositions, the two plutons have different parental magma compositions, for example higher TiO2 content and higher Sr/Y and La/Yb ratios in the QMS parental magmas, a signature best understood as being inherited from the source. The higher TiO2 content of the parental magma for the QMS pluton leads to the common presence of titanite in the QMS pluton (absent in the BJS pluton), crystallization of which in turn controls the trace-element (REE, Y, Nb, Ta and others) systematics in the residual melts towards an adakitic signature. Therefore, parental magmas with high TiO2 content and high Sr/Y and La/Yb ratios, as well as their further fractionation of titanite, are important factors in the development of adakitic compositions, as represented by the QMS host granitoids. This model offers a new perspective on the petrogenesis of adakitic rocks. The present study further demonstrates that, in general, mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks.

Schedule of Mafic to Hybrid Magma Injections Into Crystallizing Felsic Magma Chambers and Resultant Geometry of Enclaves in Granites: New Field and Petrographic Observations From Ladakh Batholith, Trans-Himalaya, India

“…The cognate model states that the enclave is a cumulate rock of early crystallized cumulus phases or rapidly cooled (chilled) border rock series of the host granite magma or more mafic cogenetic fractions with trapped interstitial melts (e.g., Phillips et al, 1981;Dodge and Kistler, 1990;Dahlquist, 2002;Esna-Ashari et al, 2011;Xiao et al, 2020). The cumulate hypothesis appears valid where the evidence of excessive nucleation of FIGURE 7 | Hypothetical model illustrating the likely mechanism of (A) injection of mafic magma into highly crystallized (>65%) felsic granite magma that (B) ruptured and brecciated into angular mafic enclave swarms under the extensional environment of granite magma, which is very well demonstrated (C) at outcrop scale, similar to as shown in Kumar et al (2016).…”

A Test of the Hypothesis That Syn‐Collisional Felsic Magmatism Contributes to Continental Crustal Growth Via Deep Learning Modeling and Principal Component Analysis of Big Geochemical Datasets

“…We therefore argue against IAB for the source of the MBGs according to the unique geochemical and isotopic fingerprints of the MBGs. Although we have not conducted detailed petrological and geochemical studies, we postulated that the basaltic end‐member for the source of the MBGs is most probably supplied by the subducted ocean crust (i.e., MORB; e.g., Kong et al., 2020; Xiao et al., 2020). The MBGs, at least on the GTP, are newly formed juvenile crust that compositionally correlated with hydrous melting of the underthrusting ocean crust (Niu et al., 2013), rather than arc crust (i.e., IAB).…”

“…MORB; e.g., Kong et al, 2020;Xiao et al, 2020). The MBGs, at least on the GTP, are newly formed juvenile crust that compositionally correlated with hydrous melting of the underthrusting ocean crust (Niu et al, 2013), rather than arc crust (i.e., IAB).…”

“…Routinely, to address these questions, geologists have managed to carry out detailed petrological, geochemical, isotopic, and geochronological studies of the syn‐collisional granitoids from other collisional belts (e.g., Chen et al., 2018; Dong et al., 2020; Huang et al., 2016; Xiao et al., 2020). However, individual studies on the MBGs can hardly provide us with a regional to even a global perspective of the three critical questions, not to mention statistically.…”

Origin of magmatic harzburgite as a result of boninite magma evolution – An illustration using layered harzburgite-dunite cumulate from the Troodos ophiolite complex

“…For most trace elements, the accuracy and precision of USGS glasses and GSE-1G are better than 5% (Supplementary Table S3). The analytical details are given in Xiao et al (2020).…”