Origin of Tholeitic Intermediate Rocks in the Early Cretaceous Comei LIP

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27 ноября - 30 ноября 2018 года

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Томск 2018 ORIGIN OF THOLEIITIC INTERMEDIATE ROCKS IN THE EARLY CRETACEOUS COMEI LIP, SE TIBET

Ying Xia1’ 3, Di-Cheng Zhu1, R. E. Ernst2- 3

1 State Key Laboratory o f Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China 2 Department o f Earth Sciences, Carleton University, Ottawa K1S5B6, Canada 3 Department o f Geology and Geography, Tomsk State University, Tomsk, Russia

A newly found tholeiitic intrusive suite in Chigu Tso of the Comei LIP (large igneous province), S E Tibet, provides new insight into the petrogenesis of tholeiitic intermediate rocks in LIPs. Zircon U-Pb dating results suggest that Chigu Tso intrusive suite is coeval to the Comei LIP at c. 132 Ma. The compositions of Chigu Tso intrusive suite exhibit a compositional continuum from dolerite, diorite to tonalite (SiO2=51.2-62.6 wt %). M ass balance calculations suggest that high FeO^/MgO (> 3.4) tholeiitic intermediate rocks (diorites and tonalites) in Chigu Tso could be generated by large degrees (~60-85 wt %) of fractional crystallization from low FeO^/MgO (~0.7) primary magmas. The occurrence of significant amounts of halogen-rich amphibole in tholeiitic intermediate rocks indicates volatile-rich conditions in the late differentiation stage and requires a prolonged shallow crustal magma chamber provides room for cooling and differentiation (up to 75 wt % fractional crystallization). Extensional stress field in rift zones are not favorable to generate shallow crustal magma chamber which results in limited site for producing highly evolved tholeiitic intermediate rock. The paucity of intermediate rocks in LIPs suggest that most silicic rocks should derived from partial melting of rather than fractional crystallization of basaltic materials.

Key words; tholeiitic intermediate rocks; fractional crystallization; amphibole; Comei LIP; S E Tibet

A large igneous province (LIP) is the manifestation of exten remains enigmatic. LIPs are mainly tholeiitic basalts (Bryan, Er sive mafic magmatism (> O.lMkm2 and > 0.1 Mkm3) in a short nst, 2008). If fractional crystallization of tholeiitic basalts could duration (< 5 Ma) pulse (or in some cases multiple pulses), and produce tholeiitic intermediate rocks, then the following question with intraplate characteristics (Coffin, Eldhoim, 1994; Bryan, is why tholeiitic intermediate rocks are so rare in LIPs? Ernst’ 2008; Ernst, 2014). LIPs are dominantly mafic composi Previous equilibrium crystallization experiments of anhy tions, but also show significant compositional variations form ul drous tholeiitic basalts provide little constraint on the generation tramafic to silicic. Both globally and regionally, the distribution of tholeiitic intermediate rocks (e.g. Grove, Bryan, 1983; Yang of SiO2 content in LIPs is bimodal with chemical groupings at et. al., 1996). Recently, fractional crystallization experiments of 45-56 wt % and 65-75 wt % (Bryan, Ernst, 2008; Bryan, Ferrari, low FeO'/MgO (~0.5) anhydrous tholeiitic basalts suggest that 2013). The paucity of intermediate compositions in LIPs, gener tholeiitic intermediate rocks are generated at very late fraction ally recognized as “Bunsen-Daly gap”, is frequently manifested ation stage (> 80 wt %) by crystallization of spinel and plagi- as bimodal suites in the field (e.g. Kirstein et. al., 2000; Ewart et. oclase at 1.0 GPa and ilmenite at 0.7 GPa, resulting in FeO'/ al., 2004; Pinto et. al., 2011). Generation of such a silica gap is a MgO >2.6 and >22, respectively (Villiger et. al., 2004, 2007). subject of much debate, with focus on the origin of silicic rocks However, fractional crystallization of primary magmas during derived from partial melting of basaltic precursors or lower crust magma ascent through the crust or storage in staging chambers is (e.g. Suneson, Lucchitta, 1983; Mahoney et. al., 2008; Colon et. a polybaric process (Putirka, Condit, 2003; Putirka et. al., 2009; al., 2018), fractional crystallization from basaltic melts (e.g. Ay- Putirka, 2017; Hole, 2018) which is difficult to simulate accu alew et. al., 2002; Shellnutt, Jahn, 2010; Hutchison et. al., 2016) rately by laboratory experiments. More investigations, in particu or silicate-liquid immiscibility (e.g. McBirney, Nakamura, 1974; lar integrated natural examples of tholeiitic intermediate rocks, Jakobsen et. al., 2005). However, the occurrence and petrogene are needed for better understanding the generation of tholeiitic sis of intermediate rocks has been ignored. intermediate rocks in LIPs. Here, we offer a case study from the Although intermediate rocks are volumetrically minor in Comei LIP. LIPs (Bryan, Ernst, 2008), previous studies provide important This study focus on a compositional diverse tholeiitic intru constraints on the petrogenesis of these rocks (e.g. Lightfoot et. sive suite including dolerite, diorite and tonalite dykes and sills in al., 1987; Turner et. al., 1999; Ewart et. al., 2004; Natali et. al., Chigu Tso, central Comei LIP, SE Tibet. Zircon U-Pb age dating 2013). Intermediate rocks in LIPs are mainly alkaline rocks (e.g. results suggest Chigu Tso intrusive suite is coeval to the Comei syenite, trachyte and trachyandesite) with subordinate tholeiitic LIP at c. 132 Ma. Mass balance calculations suggest that the high and calc-alkaline andesites and trachyandesites (Fig. 1a). Based FeO‘/MgO tholeiitic intermediate rocks (diorites and tonalites) upon field and geochemical relations to the associated basalts, al in Chigu Tso are highly evolved magmas which could be gen kaline and tholeiitic intermediate rocks are commonly interpret erated at the late differentiation stage from low FeO‘/MgO pri ed as a consequence of fractional crystallization from basaltic mary magma by amphibole-dominated fractional crystallization magmas, possibly with the involvement of crustal contamination (Fig. 2). Large amounts of halogen-rich amphiboles in tholeiitic (Holland, Brown, 1972; Turner et. al., 1999; Natali et. al., 2013; intermediate rocks indicate a volatile-rich condition in the late Owen-Smith et. al., 2013). Calc-alkaline intermediate rocks are differentiation stage which may result from progressing frac characterized by low FeO'/MgO ratios (Fig. 1b) which are likely tional crystallization (Fig. 3). Such large degrees of fractional to derive from metasomatized sub-continental lithosphere man crystallization of the tholeiitic intermediate rocks require magma tle (SCLM) (Cai et. al., 2010; Zhang et. al., 2013). Tholeiitic chamber in shallow crust (Putirka, 2009). intermediate rocks are characterized by higher FeO'/MgO ratios However, LIPs are generally associated with continental breakup than calc-alkaline intermediate rocks in continental arcs (Fig. and formation of new ocean basins, proving the critical role of rift lb). However, where and how tholeiitic intermediate rocks form ing (White, Mckenzie, 1989; CourtiUot et. al., 1999; Ernst, 2014).

32 Fig. 1. (a) Total alkalis vs silica diagram (Le Bas et. al., 1986) indicating intermediate rocks (56-65 wt %), such as andesite, syenite, latite, trachyte and trachyandesite, are predominately alkaline in LIPs [e.g. Central Atlantic Magmatic Province (Eby et. al., 1992; Sundeen et. al., 1992; Kennedy, Stix, 2007), Deccan (Tiwari, 1971; Lightfoot et. al., 1987; Devey, Stephens, 1992; Owen-Smith et. al., 2013), Emeishan (Zhou et. al., 2008; Cai et. al., 2010), Ethiopian Plateau (Kieffer et. al., 2004; Natali et. al., 2013), North Atlantic (Deer 1976; Holland, Brown, 1972); Parana-Etendeka (Piccirillo et. al., 1989; Turner et. al., 1999; Kirstein et. al., 2000; Ewart et. al., 2004; Marsh, Milner, 2007; Hartmann et. al., 2012) and Tarim (Zhang et. al., 2013; Zou et. al., 2015)]. (b) FeOt/MgO vs SiO2 diagram (Miyashiro, 1974) indicates that tholeiitic intermediate rocks have higher FeOt/MgO ratios than calc-alkaline intermediate rocks in LIPs and Andean arc (Barragan et. al., 1998; Samaniego et. al., 2005). I, B = Irvine, Baragar (1971).

33 Fig. 2. Mass balance calculations showing fractionation o f different mineral assemblages controlled the variation o f major elments from primary magma to the most evolved Chigu Tso tonalite. The tholeiitic intermediate compositions could be generated by a three- step fractional crystallization from primary magma. Stage 1: fractional crystallization o f 34 wt % mineral assemblages o f 96% olivine and 4% Cr-spinel from primary magma result in a composition of FeOt/MgO~2.2, MgO~6.54 wt % and SiO2~51.4 wt %, which may be the parental magam o f Cona OIB-type mafic rocks, Sangxiu basalts and Chigu Tso intrusive sutie. Stage 2: fractional crystallization o f 35 wt % of mineral assemblages o f 59% clinopyroxene, 33% plagioclase and 8% ilmenite from the end-product in stage 1 produce a composition o f FeOt/MgO~3.4, MgO~5.1 wt % andSiO2~53.0 wt %) which is similar to evolved Chigu Tso diorites. Stage 3: fractional crystallization o f 60 wt % o f mineral assemblages of 65% amphibole, 27% plagioclase and 8% ilmenite from the end-product in stage 2 generate a compositon o f FeOt/MgO~6.7, MgO~2.4 wt % and SiO2~63.5 wt %, which is similar to the most evolved tonalite in Chigu Tso. Data o f primary magma and Cona picrites (Xia et. al., 2014), Cona OIB-type mafic rocks (Zhu et. al., 2008) and Sangxiu basalts (Zhu et. al., 2007). Results of fractional crystallization experiments of anhydrous primary basalts at conditions o f 1 GPa (Villiger et. al., 2004) and 0.7 GPa (Villiger et. al., 2007) and mass balance calculations suggest that tholeitiic intermediate rocks are generated in the late differentiation stage.

The extensional regional stress field in rift zones is favorable (e.g. the Bushveld complex, Twist, Harmer, 1987; the Palisades to form elongated dyke swarms, rather than shallow magma sill in CAMP, Block et. al., 2015; the Red Hill dike in Ferrar chamber in the crust (Gudmundsson, 2011). Large intrusions LIP, McDougall, 1962). The intermediate compositions in these generally contain a significant portion of granophyre which is granophyres show similar high FeO'/MgO ratios to the tholeiitic considered as late-stage differentiates (Carmichael, 1964; Wager, intermediate rocks in Chigu Tso and other LIPs. Therefore, it is Brown, 1967). The granophyre in large intrusions, such as the reasonable for us to speculate that highly evolved tholeiitic in Palisades sill in CAMP, commonly hosts amphibole and is vol termediate rocks in LIPs may only be generated at large-sill-like atile-rich (Shirley, 1987; Block et. al., 2015). The compositions magma chambers in shallow crust and there is no intermediate of granophyres exhibit a continuum from intermediate to silicic compositional gap during the fractional crystallization of tholei-

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