Origin, ascent and oblique emplacement of magmas in a thickened crust: An example from the Cretaceous Fangshan adakitic pluton, Beijing
Graphical Abstract
Research Highlights
► The Cretaceous Fangshan pluton has adakitic affinities. ► The pluton was formed by partial melting of lower thickened crust in North China. ► Magmatic and solid-state fabrics with strain of ellipsoidal dioritic enclaves were measured. ► Magma emplacement was involved diapirism and fault-generated space.
Introduction
The so-called “room problem” of magmatic emplacement has been a matter of debate since the intrusive nature of plutons was established (Johnson et al., 1999, Johnson et al., 2003, Johnson et al., 2004, Vernon et al., 2004). To solve this problem requires better understanding of the magma origin, transport and emplacement (e.g. Antolin-Tomas et al., 2009, Petford et al., 2000, Weinberg and Regenauer-Lieb, 2010). End-member mechanisms for magma ascent and emplacement have been proposed but are hotly debated. The classic model suggests that the magma created its own space through diapirism or ballooning (Buddington, 1959, He et al., 2009, Holder, 1979, Ramsay, 1989, Weinberg and Podladchikov, 1994). Castro (1987) considered tectonic factors as primary control on the shape, emplacement model and geometric concordance of a pluton. This tectonic model is supported by the emplacement of magmas along pre-existing faults (Daly, 1903, Hutton, 1982, Hutton, 1988, Hutton et al., 1990, Petford and Atherton, 1992, Petronis et al., 2009, Pitcher, 1979, Weinberg and Mark, 2008, Weinberg and Regenauer-Lieb, 2010, Weinberg et al., 2004, Weinberg et al., 2009). A third model proposed by Cruden (1998) involves an end-member cantilever and piston sinking mechanism. This model explains tubular elliptical plutons with horizontal dimensions much larger than vertical dimensions (McCaffrey and Petford, 1997, Vigneresse, 1995), and considers that magma ascent was partitioned into a low-viscosity center flowing vertically and high-viscosity outer margin flowing helically (Trubac et al., 2009). Thus, magma emplacement is probably a complex process with numerous variations between the three main end members (Antolin-Tomas et al., 2009, Barbey et al., 2008, Paterson and Vernon, 1995, Petronis et al., 2009, Polteau et al., 2008, Tobisch and Williams, 1998).
The depth at which magma forms is thought to be important for determining the transport and emplacement mechanism (Petford et al., 2000), but is not well understood. In particular, how adakitic magmas formed at depth and emplaced in an extensional environment is not explained. A representative example is the formation of adakitic magmas in the Yanshan belt of China, which were probably produced at the base of thickened crust (~ 50 km) by delamination of the lower crust at ~ 134 Ma (Gao et al., 2004, Rapp et al., 1999). The subsequent extensional thinning of the continental crust in this belt occurred between 130 and 120 Ma leading to the termination of high pressure (> 1500 MPa) formation of adakitic melts (Davis, 2003).
The Mesozoic Fangshan monzodioritic pluton in the Western Hills of Beijing is well known for its diversified and well-preserved magmatic fabrics (Li, 1987, Shan et al., 1991), providing an excellent opportunity to study magma emplacement mechanisms. Recent investigations have revealed a complex history of magma emplacement (Shan et al., 2004, Yan et al., 2006). However, it is still unclear whether this pluton was emplaced along a pre-existing, crustal-scale shear zone (extension and overthrust detachment fault system; Yan et al., 2006) or by ballooning and/or diapirism (Shan et al., 2004, He et al., 2005, He et al., 2009, Shan et al., 1991). The Fangshan pluton is an ideal body for examining the “room problem” related to the origin of granitic magmas in the lower continental crust and their subsequent emplacement in the upper crust. In this paper, we utilize strain analysis of enclaves and primary magmatic and solid-state fabrics, which were produced from sub-liquidus to sub-solidus state, to re-examine the emplacement mechanism of the Fangshan pluton. On the basis of new geochemical data, we document that the pluton has adakitic affinities and was derived from melting of the lower part of a thickened crust. We further discuss the origin, ascent and emplacement after the magma was generated and show that the space for the intruding magma was provided by a combination of diapirism and faulting.
Section snippets
North China Block
The North China Block is bounded by the Qinling–Dabie orogenic belt to the south, and by the Mongol–Okhotsk accretionary belt to the north (Fig. 1A). The basement of the North China Block consists dominantly of Late Archean to Paleo-Proterozoic gneisses, granulites, and migmatites, overlain by a variety of Mesoproterozoic to Permian cover rocks (e.g. Beijing Bureau of Geology and mineral Resources (BBGMR), 1991, Chen et al., 2009, Hebei Bureau of Geology (HBG), 1989, Kusky et al., 2004, Zhao et
Ar–Ar dating
Two samples (Fg-1 and Fg-2, collected at 39°42′19″N, 115°57′41″E and 39°45′31″N, 115°58′27″E, respectively) from the monzodiorite phase were selected for hornblende separates. Both samples are fresh and contain magmatic hornblende suitable for 40Ar/39Ar dating. The samples were crushed, washed in distilled water in an ultrasonic bath for 1 h, and then dried. Hornblende separates (1–2 mm long) were handpicked under a binocular microscope. Mineral separates were irradiated along with monitors of
Primary magmatic structures and solid-state fabrics of the enclaves and host rocks
The orientation of long crystal planar of biotite and amphibole crystals, K-feldspar laths, plagioclase megacrysts, and flattened surface of elongate enclaves defines magmatic foliation (Smag), whereas their maximum-crystal or -strain axis defines the lineation (Lmag) on the Smag (Fig. 3B and E). Both Smag and Lmag are subtly developed but common in the Fangshan pluton (Figs. 3A–C and 7A). In general, the primary structures are more evident in the monzodiorite than in the quartz monzodiorite.
Temporal relationships of the Fangshan plutonic rocks
The quartz monzodiorite was previously dated at 130.7 ± 4.0 Ma using the SHRIMP zircon U–Pb method (Cai et al., 2005). Previous studies of the monzodiorite yielded K–Ar biotite ages ranging from 131.0 ± 5.4 to 132.8 ± 0.12 Ma (BBGMR and CUG, 1988), and 40Ar/39Ar biotite ages between 133.0 ± 0.9 and 132.7 ± 1.4 Ma (Liu and Wu 1987), suggesting that the quartz monzodiorite postdated the monzodiorite, consistent with the observed field relations. These dates are essentially consistent with our monzodiorite 40
Conclusions
The Fangshan pluton was emplaced along a pre-existing detachment fault at 136.0–131.0 Ma. This Cretaceous pluton has adakitic affinities and was formed by partial melting of the lower part of a thickened continental crust in the East China Plateau. Magmatic fabrics, the strain of ellipsoidal dioritic enclaves and solid-state deformation suggest that the history of magma emplacement involved originally diapiric process followed by a combination of diapirism and fault-generated space. The
Acknowledgments
This study was supported by the Natural Science Foundation of China (40921062, 40872140), National Basic Research Program of China (2009CB421001) and the 111 Project (B07011). We have benefited from helpful discussions with Profs. Zhaoren Fu and Changhou Zhang during the study. Thanks are due to Ms. Xiao Fu, Dr. Weihua Sun and Prof. Yu Wang for laboratory work. Enlightening discussions and assistance throughout this study by Prof. P.T. Robinson and reading an earlier draft of this paper by
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