The 132 Ma Comei-Bunbury Large Igneous Province: Remnants Identifi Ed in Present-Day Southeastern Tibet and Southwestern Australia
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The 132 Ma Comei-Bunbury large igneous province: Remnants identifi ed in present-day southeastern Tibet and southwestern Australia Di-Cheng Zhu1*, Sun-Lin Chung2, Xuan-Xue Mo1*, Zhi-Dan Zhao1, Yaoling Niu3, Biao Song4, and Yue-Heng Yang5 1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China 2Department of Geosciences, National Taiwan University, Taipei 106, Taiwan 3Department of Earth Sciences, Durham University, Durham DH1 3LE, UK 4Beijing SHRIMP II Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 5Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China ABSTRACT and Cona areas; Fig. 1B) of southern Tibet. The We report 11 new U-Pb zircon ages obtained by sensitive high-resolution ion microprobe new data obtained in this study together with (SHRIMP) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP–MS) fi eld observations and consideration in a global for a large province of Early Cretaceous Comei igneous rocks consisting of basaltic lavas, context further suggest that the rocks in south- mafi c sills and dikes, and gabbroic intrusions together with subordinate layered ultramafi c eastern Tibet and the Bunbury Basalt in south- intrusions and silicic volcanic rocks exposed in the Tethyan Himalaya, southeastern Tibet. western Australia are temporally related and Available zircon U-Pb ages obtained from various rocks in this province, which has an areal constitute the erosional and/or deformational extent of ~40,000 km2 (~270 km × 150 km), indicate that the magmatism occurred ca. 132 Ma remnants of a large igneous province, which ago, coeval with the Bunbury Basalt in southwestern Australia. Such a striking similarity we call the Comei-Bunbury LIP. Like other pre- in emplacement age, in combination with the tectonic reconstruction of eastern Gondwana Cenozoic LIPs that have lost most of their vol- ca. 132 Ma ago, allows us to propose that the extensive Comei igneous rocks in southeastern canic components owing to tectonic deforma- Tibet and the Bunbury Basalts in southwestern Australia may represent the erosional and/or tion or erosion (Ernst, 2007; Bryan and Ernst, deformational remnants of a large igneous province, which we call the Comei-Bunbury LIP. 2008), the remnant Comei LIP in southeastern We argue that this newly identifi ed LIP was likely caused by the Kerguelen mantle plume, Tibet is dominated by dismembered mafi c lava which started in the Early Cretaceous and may have played a role in the breakup of eastern fl ows, sills, and dikes, with subordinate ultra- Gondwana and the development of the 132 Ma old Weissert oceanic anoxic event. mafi c and silicic rocks of Early Cretaceous age belonging to Sangxiu and Lakang Formations INTRODUCTION obtained by sensitive high-resolution ion micro- (Wan et al., 2005; Zhu et al., 2008a) (Fig. 1B). The term large igneous province (LIP) is used probe (SHRIMP) and laser ablation–inductively In this study, we collected igneous rock sam- to represent a variety of mafi c igneous prov- coupled plasma–mass spectrometry (LA-ICP– ples along three north-south transects from 28°N inces with areal extents >100,000 km2 (Coffi n MS) for the extensive Early Cretaceous igneous to 29°N and 90°30′E to 92°E in the Comei LIP and Eldholm, 1994; Bryan and Ernst, 2008) rocks around the Comei area in southeastern (Fig. 1B). Of all the 70 samples with geochemi- or >50,000 km2 (Sheth, 2007) as the result of Tibet. We argue that the contemporaneous igne- cal data, 11 samples that include 5 diabasic extensive decompressive melting in response ous rocks dispersed in southeastern Tibet and dikes, 4 gabbros, a pyroxenite, and a dacite were to ascending mantle plumes and lithospheric southwest Australia are genetically related and successfully extracted for zircons for SHRIMP extension (e.g., Richards et al., 1989; White constitute the erosional and/or deformational and LA-ICP–MS U-Pb dating. Sample details and McKenzie, 1989). The generation of many remnants of a large igneous province, which we and zircon U-Pb age data are summarized in plume-related LIPs has been linked to continen- call the Comei-Bunbury LIP, emplaced above a GSA Data Repository Table DR1.1 Actual analy- tal breakup, as exemplifi ed by the Kerguelen mantle plume in eastern Gondwana ca. 132 Ma ses and data reduction procedures are given in mantle plume initiation that played a role in ago. Its possible link with the Kerguelen man- Tables DR2 and DR3. Zircons from mafi c and rifting the Gondwana and opening the east- tle plume and implications for the breakup of ultramafi c samples show similar crystal forms ern Indian Ocean (Storey, 1995). Along with eastern Gondwana and the development of the with no resorption or inherited cores (Figs. DR1 the immense Kerguelen Plateau being formed Weissert oceanic anoxic event are also explored. and DR2) and exhibit low to very high uranium within the nascent Indian Ocean basin, this (16–11,853 ppm) and thorium (24–22,230 ppm) plume activity may have also resulted in the BACKGROUND AND SAMPLES contents, yielding Th/U ratios from 0.77 to 5.24 Rajmahal Traps in northeastern India (Kent et The Early Cretaceous igneous rocks exposed (Tables DR2 and DR3). Zircons from a dacite al., 2002) and the Bunbury Basalt in southwest- in southeastern Tibet have been documented sample exhibit different crystal forms, including ern Australia (Frey et al., 1996) during the rift- for years (Bureau of Geology and Mineral long columnar and inherited cores (Fig. DR2e). ing event. However, correlation of the Bunbury Resources of Xizang Autonomous Region, Basalt with the other two outputs has been often 1993) as an important component within the 1GSA Data Repository item 2009138, summary of questioned because the former began erupt- eastern Tethyan Himalayan sequence located zircon U-Pb isotopic ages (Table DR1), methods and ing ca. 132 Ma ago (Frey et al., 1996; Coffi n between the Yarlung Zangbo suture to the zircon SHRIMP U-Pb data (Table DR2), methods and zircon LA-ICP–MS U-Pb data (Table DR3), cathodo- et al., 2002), whereas no rocks from both the north and the Greater Himalayas to the south luminescence (CL) images of zircon SHRIMP dating Rajmahal Traps and Kerguelen Plateau are (Fig. 1A). However, no good quality data on (Fig. DR1), CL images of zircon LA-ICP–MS dating older than 120 Ma (cf. Ingle et al., 2004). In these rocks have been available until recently (Fig. DR2), concordia plot of zircon SHRIMP dating this paper we present 11 new zircon U-Pb ages (Zhu et al., 2005; Jiang et al., 2006); these data (Fig. DR3), and concordia plot of zircon LA-ICP–MS formed a basis for subsequent investigations dating (Fig. DR4), is available online at www.geoso- ciety.org/pubs/ft2009.htm, or on request from edit- *E-mail: [email protected]; moxx38@yahoo. (Zhu et al., 2007, 2008a) that identifi ed plume [email protected] or Documents Secretary, GSA, com. signatures in local areas (e.g., Rimowa village P.O. Box 9140, Boulder, CO 80301, USA. © 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2009; 2009 v. 37; no. 7; p. 583–586; doi: 10.1130/G30001A.1; 3 fi gures; Data Repository item 2009138. 583 88°E Volcanic remnants: Intrusions: U-Pb dating methods: A Lhasa 92°E Fig. B Middle Triassic strata give a SHRIMP U-Pb age Sangxiu Formation Diabasic sills/dikes 132 SHRIMP dating YZS Zedong 29°N of 129.5 ± 1.3 Ma old and a LA-ICP–MS U-Pb Lakang Formation TH Comei Gabbroic intrusions 133 LA-ICPMS dating STDS age of 133 ± 2 Ma old. In the central Comei Basaltic rocks Cona GH LIP north of Dongjia village (Fig. 1B), gab- Σ 27°N Silicic rocks Ultramafic rocks MBT bros emplaced within the Middle Jurassic strata Rajmahal 0 100 200 km 90°00′E 90°30′E 91°00′E 91°30′E Quxu Zedong Traps give a SHRIMP U-Pb age of 129.7 ± 1.4 Ma B Shilong Rinbung Gonggar 25°N old and a LA-ICP–MS U-Pb age of 132 ± 3 Ma Zhanang Sylhet 134 Traps old. To the south of Dongjia village, a diabasic Baidi dike gives a LA-ICP–MS U-Pb age of 132 ± 2 92°00′E 92°30′E 93°00′E 133 Qonggyai Ma old. To the north of Comei County in the 29°00′N 132 Gyangze Nagarze Yamzho Yum Tso central Comei LIP, the Early–Middle Jurassic 130 strata were intruded by gabbros (Fig. 1B), one Dalong 133 133 131 of which gives a LA-ICP–MS U-Pb age of 131 Chigu Tso 28°40′N Rimowa 130 ± 1 Ma old. Farther north, a diabase dike yields 132 Σ 130 28°40′N Dongjia 132 a SHRIMP U-Pb age of 130.2 ± 2.0 Ma old. To 132 Kangmar Puma Yun Tso 131 130 the south of Chigu Tso, a gabbro that intruded Lhunze the Early Jurassic strata gives a LA-ICP–MS Comei 28°20′N U-Pb age of 132 ± 1 Ma old farther northeast 28°20′N Lhozhag 90°00′E of Chigu Tso (Fig. 1B). At the Qonggyai res- Σ ervoir of the northern Comei LIP, a gabbroic N Kada Lakang W E Gujue dike emplaced within the strongly deformed 131 28°00′N S Late Triassic strata gives a SHRIMP U-Pb age Cona 0 10 20 30 km of 132.1 ± 1.0 Ma old. To the south of Kada vil- 90°30′E 91°00′E 91°30′E 92°00′E 92°30′E lage in the southeastern Comei LIP (Fig.