Petrogenesis of mafic enclaves from the north Taihang Yanshanian intermediate to felsic plutons: Evidence from petrological, geochemical, and zircon Hf-O isotopic data

Abstract

We report the zircon Hf-O isotopic data for mafic enclaves from the Taihang Yanshanian intermediate to felsic plutons, and use them, along with the petrological, whole-rock chemical and Nd-Sr isotopic data, to reveal the petrogenesis of mafic enclaves. Mafic enclaves show magmatic textures and are finer-grained than host rocks. In places they are highly elongated due to stretching within the partially crystallized, convective felsic magma, but show no solid-state deformation. These data suggest that mafic enclaves and host rocks were co-existing, but compositionally distinct magmas. The mafic enclaves contain abundant hydrous minerals such as hornblende and biotite, with pyroxene relict being surrounded by hornblende reaction rim. Plagioclase xenocrysts from mafic enclaves show a complicated compositional and textural disequilibrium. Comparison between mafic enclaves and the immediate host rocks suggests that the two rock units are compositionally correlated. The ε Nd values of mafic enclaves are generally higher than the host rocks, though the Sr isotopic ratios of the two rock units are indistinguishable. Zircons from a single enclave sample show a significant variation in Hf isotopic compositions, with ε Hf = −10-−22, suggesting an origin through magma mixing between mafic and felsic magmas. This is supported by the relatively large variation of zircon O isotopic ratios (δ 18 O = 5.5‰-7.8‰) of the mafic enclaves. The petrogenesis of mafic enclaves could be described as below. Evolved basaltic magma (via fractionation of olivine and pyroxene) first mixed with crustally derived granitic melts in depths, forming a hybrid magma; then the hybrid magma broke up into discrete lumps upon entering the above felsic magma. Subsequently, the enclave-forming magma experienced a double mechanical transfer of plagioclase, and inward chemical transfer of fluid and Na, P, Y, Nb and Pb at the contacts with host felsic magmas. mafic enclaves, compositional zoning, Nd-Sr isotopes, zircon Hf-O isotopes, magma mixingThe petrogenesis of mafic enclaves has been an issue of controversy for long. Models for the origin of mafic enclaves include: (1) mafic enclaves are formed as cognate fragments of cumulate minerals from a mafic magma [1,2] , (2) mafic enclaves represent recrystallized, refractory and residual phase assemblage derived from a granite source [2,3] , and (3) mafic enclaves are formed through hybridization between mafic and felsic magmas [4][5][6][7][8][9][10] .More and more studies suggest that most enclaves are formed by magma mixing between mantle-and crustle-derived melts. However, the mechanisms of magma

Syn-collisional adakitic granodiorites formed by fractional crystallization: Insights from their enclosed mafic magmatic enclaves (MMEs) in the Qumushan pluton, North Qilian Orogen at the northern margin of the Tibetan Plateau

“…xenoliths (usually country rocks; e.g., , refractory and residual phase assemblages derived from granitoid sources (e.g., the restite model; Chappell et al, 1987;Chappell and White, 1991), chilled material or cumulate of early-formed co-genetic crystals (e.g., Dodge and Kistler, 1990;Dahlquist, 2002;Donaire et al, 2005;Chen et al, 2015), and basaltic melt material incompletely digested and homogenized during a magma mixing process (e.g., Didier, 1987;Castro et al, 1990;Dorais et al, 1990;Chappell and White, 1991;Chen et al, 2009aChen et al, , 2013b.…”

“…Actually, there are many compelling lines of evidence for magma mixing in many granitoids, especially (1) where a clear isotopic contrast exists between the MMEs and the hosts (e.g., Chen et al, 2009b;; and (or) (2) where disequilibrium features occur in the MMEs, e.g., complex zoning of clinopyroxene crystals that have distinctly low-MgO cores surrounded by high-MgO rims (e.g., Chen et al, 2013a;, or resorption textures or reversed zoning of plagioclase Chen et al, 2009aChen et al, , 2009b). In the case of our study, however, none of the above has been observed.…”

Genesis of Mafic Microgranular Enclaves in the Shaocunwu Granodiorite, Southern China, and their Implications: Evidence from Zircon and Whole‐rock Geochemistry

“…MMEs are commonly observed in the granitoid intrusions, and have been used to determine the genesis of the granitoid intrusions and their tectonic setting (Chen et al, 2009;Duan et al, 2015;Sun et al, 2016;Zheng et al, 2016;Sun et al, 2017). Previous research proposed that MMEs could be possible products of several magmatic processes: (1) restite after partial melting in the source region (White et al, 1999;Sun et al, 2016); (2) the xenoliths of the earlier magmatic cumulates (Clements et al, 1986); (3) wall rocks xenoliths from the emplacement of the granodiorite (Zhou et al, 1994); and (4) magma mixing (Janousek et al, 2004;Kumar et al, 2004;Kemp et al, 2004;Chen B et al, 2013).…”