Elsevier

Lithos

Volumes 208–209, November 2014, Pages 339-360
Lithos

The role of external fluid in the Shanggusi dynamic granitic magma system, East Qinling, China: Quantitative integration of textural and chemical data

https://doi.org/10.1016/j.lithos.2014.09.023Get rights and content

Highlights

  • Quantitative integration of textural and chemical data in igneous petrology

  • External mantle-derived fluid was involved in the Shanggusi granitic system.

  • Fluid-induced chemical and textural diversity

Abstract

It is well recognized that various degrees of mantle-derived materials are incorporated in the formation of granite, and mantle-derived mafic melts are generally considered to mix with crustal felsic melt. Here, however, we provide an example of the Shanggusi leucocratic granite where external mantle-derived hydrous fluid, rather than mafic melt, might be incorporated into a nearly pure crustal granitic melt system. Field observations suggest that the Shanggusi granite consists of granite porphyry, granite dyke and granitic pegmatite and they have consistent zircon U–Pb ages and molybdenite Re–Os ages. The marginal pegmatite, interconnected miarolitic cavities, heterogeneous molybdenite mineralization and significant variation of micro-texture of the Shanggusi granite physically indicate that strong fluid activities occurred in the granitic system. Accumulation of quartz and K-feldspar and bulk-rock major element data imply that fractional crystallization played an important role in the evolution of the granitic system which, however, cannot reasonably explain the significant trace elements fractionation, non-CHARAC trace elements behavior and simultaneous concave and convex REE tetrad effect of the Shanggusi granite, but which can be best explained by the influence of external fluorine-rich hydrous fluid. Importantly, the chemical fractionation, including bulk-rock trace elements and isotopes, is closely correlated with quantitative textural parameter Lmax (the average length of the four largest quartz crystals in each sample), indicating that the vast majority of physical and chemical characteristics of the granitic system were most likely controlled by the wholesale fluid flow. The Shanggusi granite is highly siliceous (SiO2 = 74.91–79.50 wt.%, except granitic pegmatite with SiO2 = 67.41 wt.%), extremely poor in mafic minerals, and with relative homogeneous bulk-rock major element chemistry and mineralogy, which approximate experimentally pure crustal melt that generated by dehydration melting of sedimentary rocks. The mantle-derived Nd–Pb–O isotopic features, however, require a great deal of contributions from mantle materials. Therefore, it is proposed that external mantle-derived fluid was infiltrated into the Shanggusi granitic system at least before emplacement, and fluid–melt mixing may be an important but not easily recognized part in granite origin.

Introduction

Magma, as its original definition, in most cases is composed of liquid (melt), solid (crystals/rocks), and fluid (volatiles). The complex interactions between the three components during either magma generation or evolution processes or both give rise to the diversity of igneous rocks both physically and chemically. The fluid–melt and fluid–solid interactions typically received much more attention in economic geology (e.g., Hedenquist and Lowenstern, 1994, Richards, 2003, Williams-Jones and Heinrich, 2005) and subduction-related magmatism (e.g., Annen et al., 2006, Ayers, 1998, Kamber et al., 2002). It is the melt–melt and melt–solid interaction processes that are generally the dominant focus in igneous petrology especially for granite, such as magma mixing/mingling (e.g., Hibbard, 1981, Huppert and Sparks, 1988, Castro et al., 1991, Bonin, 2004), MASH (Hildreth and Moorbath, 1988) and AFC processes (DePaolo, 1981). For granites water has a critical role (e.g., Clemens, 1984, Johannes and Holtz, 1996, Thomas and Davidson, 2012, Tuttle and Bowen, 1958), and which received much more attention in recent years for an increasing number of petrologists and geochemists (e.g., Berger et al., 2008, Luo et al., 2007, Martin, 2006, Monecke et al., 2011, Sawyer, 2010). Several lines of evidence from many disciplines indicate that fluid, in a magma system that may have various sources, can work independently as a single system to influence either magmas or rocks (e.g., Berger et al., 2008, Liu and Zhang, 2005, Luo et al., 2007, Monecke et al., 2002, Sawyer, 2010, Slaby et al., 2012). In addition, crystals, the major components of solid materials of magmas or igneous rocks, are in recent years found to have multiple crystal populations, and which typically involve open system products such as antecrysts and xenocrysts which are not equilibrium with or foreign to the magmatic host and the magma system (e.g., Charlier et al., 2005, Davidson et al., 2007, Jerram and Martin, 2008). These observations and new understanding of the magma system indicate that magma should be regarded as, at least under certain circumstances, a disequilibrium system of complex mixtures of fluid, melt and solid, rather than a simple near-equilibrium system dominated by cogenetic materials that are commonly observed.

Fluids typically can easily migrate and interact with melts, crystals and rocks due to low viscosity, and experimentally showing complicated behaviors in chemical fractionation processes (e.g., Borchert et al., 2010, Mayanovic et al., 2009, Veksler et al., 2005). In addition, either external or internal fluid that plays a critical role in a granitic magma system, the resultant granitic rocks can be physically and chemically differed from the granite that is commonly observed, such as REE tetrad effect, non-CHARAC (charge-and-radius-controlled) trace elements behavior, miarolitic cavities and pegmatite which typically occur in hydrous fluid-rich granitic systems (e.g., Bau, 1996, Candela, 1997, Hedenquist and Lowenstern, 1994, Liu and Zhang, 2005, London, 2005, Mayanovic et al., 2009, Monecke et al., 2002, Thomas and Davidson, 2013, Wu et al., 2004). Furthermore the textural and chemical diversity of a granitic magma system may sometimes be coherently controlled by pervasive fluid flow (Yang, 2012). Therefore it becomes difficult to acquire precise information about petrogenesis of those granites that have undergone fluid–melt interactions using traditional perspectives that deal with the melt–melt and melt–solid interactions. The difficulties, however, may be resolved by integrative studies of physico-chemical characteristics of the granitic magma systems, can yield rich rewards to gain insight into the role of the fluid in a specific magma system, and formulate relatively comprehensive understanding of the petrogenesis of granite.

Section snippets

Geology of the Qinling–Dabie orogen

The Qinling–Dabie orogen extends more than 1500 km across central China (Fig. 1a), which comprises two sutures and three blocks (Meng and Zhang, 2000) (Fig. 1b). The southern margin of the North China Craton is bounded by Maochaoyin fault and Sanmenxia–Lushan fault which comprises a late Archaean crystalline basement of gneiss, granulite and migmatite represented by Taihua group (Zhang et al., 2001), discordantly overlain by the Mesoproterozoic Xiong'er group volcanic rocks (Peng et al., 2008).

Analytical techniques

Using the same method of Yang (2012), five new bulk-rock major and trace elements data and Lmax (the average length of four largest crystals in every thin section for each sample) of four granite porphyry and one granitic pegmatite samples are determined, and the results, combined with the reported data (Yang, 2012), are presented in Supplementary data, Table S1 and S2.

Re–Os ages of molybdenite

The five molybdenite samples have a relatively narrow range of ReOs mineral ages varying from 122.5 ± 2.1 Ma to 124.8 ± 1.8 Ma, with an average of 123.6 ± 0.7 Ma (Table 1), showing excellent reproducibility. The samples yield a well-constrained 187Re–187Os isochron, which corresponds to an isochron age of 123.4 ± 2.4 Ma (MSWD = 1.07) and an intercept of 0.02 ± 0.74 (Fig. 6). A nearly zero intercept confirms that all the 187Os in the molybdenites are mainly radiogenic. This indicates that the model age of the

Discussion

One of the fundamental issues in igneous petrology is to understand what process and to what extent of each potential factor influence the physico-chemical diversity of igneous rocks. Geochemistry, compared with texture and other petrological perspectives, is commonly more widely used to gain insight into the magma evolution processes including magma differentiation, magma mixing/mingling, and wall-rock assimilation. However, this only provides the chemical possibilities or alternatives, which

Concluding remarks

As Clemens and Steven (2012) recently pointed out, the physical chemistry of crustal melting is well reasonably recognized, and the chemical characteristics of granite are primarily controlled by source or called peritectic assemblage entrainment; however, there is little information about how these chemical heterogeneities are preserved during magma emplacement and solidification processes, and what are their relationships with rock texture especially from a quantitative perspective. This

Acknowledgements

We thank E. Slaby and R. Thomas for their constructive and thorough reviews. We also thank G.N. Eby for his comments and editorial work. This work is partially supported by the key scientific and technological projects of geology and mineral resources of Henan Province (26417), an open research grant from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (GMPR2013) and the Fundamental Research Funds for the Central Universities (2652014006).

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