concentric zoning that is typical of igneous growth [Han- char and Miller, 1993]. .... Kyaukse [Maung Thein and Soe Win, 1970] indicate that. Carboniferous to ...
TECTONICS, VOL. 22, NO. 3, 1019, doi:10.1029/2002TC001398, 2003
Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India-Eurasia collision in Myanmar M. E. Barley and A. L. Pickard Centre for Global Metallogeny, The University of Western Australia, Crawley, Western Australia, Australia
Khin Zaw Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania, Australia
P. Rak and M. G. Doyle Centre for Global Metallogeny, The University of Western Australia, Crawley, Western Australia, Australia
Received 12 April 2002; revised 25 November 2002; accepted 5 March 2003; published 22 May 2003.
[1] Situated south of the eastern Himalayan syntaxis at
the western margin of the Shan-Thai terrane the highgrade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia. The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (170 Ma) zircons that have partly recrystallized during 43 Ma high-grade metamorphism. A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 30.9 ± 0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. MidCretaceous to earliest Eocene (120 to 50 Ma) I-type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra. Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma. Similar syntectonic syenites and leucogranites intruded the Ailao Shan-Red River shear belt in southern China and Vietnam during the Eocene-Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time. The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it Copyright 2003 by the American Geophysical Union. 0278-7407/03/2002TC001398
may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting INDEX TERMS: extrusion or rotation of Indochina. 9320 Information Related to Geographic Region: Asia; 9699 Information Related to Geologic Time: General or miscellaneous; KEYWORDS: SE Asia, Myanmar, granitoids, metamorphism, zircon dating. Citation: Barley, M. E., A. L. Pickard, Khin Zaw, P. Rak, and M. G. Doyle, Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the IndiaEurasia collision in Myanmar, Tectonics, 22(3), 1019, doi:10.1029/ 2002TC001398, 2003.
1. Significance and Setting of the Mogok Metamorphic Belt [2] The Mesozoic to Tertiary tectonic evolution of Southeast Asia is the result of the convergence and collision of fragments of Gondwanaland with Eurasia culminating in the collision of India. There is general agreement that the Indian collision indented the margin of Eurasia and resulted in substantial crustal thickening and the uplift of Tibet. However, there is considerable controversy about the amount and distribution of strain and its effect on Southeast Asian tectonics [e.g., Tapponnier et al., 1982; Peltzer and Tapponnier, 1988; Dewey et al., 1989; England and Molnar, 1990, 1997; Huchon et al., 1994; Lee and Lawver, 1995]. A rapidly growing geochronological database is placing tight constraints on the timing and duration of magmatic episodes and tectonic events in the Himalayas, Tibet and eastern Indochina [e.g., Xu et al., 1985; Scharer et al., 1994; Searle et al., 1999; Zhang and Scharer, 1999; Nagy et al., 2000]. However, there is very little comparable modern highprecision geochronology for the Myanmar region. [3] The sigmoidal Mogok metamorphic belt (MMB) is exposed at the margin of the Shan-Thai (Sibumasu) terrane (Figures 1 and 2), along the northwestern margin of the Shan Plateau and southwards between the north-south trending Sagaing fault and Shan Scarp [Searle and Haq, 1964; Mitchell, 1993]. It is approximately 50 km wide and consists of marbles, schists and gneisses of upper amphib-
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BARLEY ET AL.: MAGMATISM AND METAMORPHISM IN THE MOGOK BELT, MYANMAR
competing models for the tectonic evolution of Southeast Asia. In this paper, sensitive high-resolution ion microprobe (SHRIMP) U-Pb in zircon dating of selected granitoids and orthogneisses in the MMB provides the initial step in defining the geochronology of its tectonic evolution as a key part of the southern margin of Eurasia prior to, during, and since the collision with India.
2. Analytical Methods
Figure 1. Schematic tectonic map of eastern Asia showing major Cenozoic fault zones and tectonostratigraphic terranes. olite and locally granulite facies that are intruded by variably deformed granitoids and pegmatites. Pyroxeneand silimanite-bearing gneisses are interlayered with marbles near Mogok, which is famous for its ruby workings. North of Mogok, the MMB can be traced into the eastern Himalayan syntaxis. In the south it is either truncated by, or links with, belts of high-grade metamorphic rocks adjacent to the Wang Chao and Three Pagodas faults of northern Thailand [Lacassin et al., 1997]. [4] Although originally considered to be Precambrian, K-Ar and Ar-Ar age determinations from the MMB provide evidence of Mesozoic and younger metamorphism and magmatism [Mitchell, 1993; Bertrand et al., 1999]. The MMB potentially links metamorphic and magmatic belts at the southern margins of the Lhasa and Karakoram terranes in Tibet and Pakistan (Figures 1 and 3), that directly faced the Indian collision, with major strike-slip shear zones are thought to have accommodated the extrusion or rotation of Indochina away from the collision. These shear zones include the Ailao Shan-Red River shear belt in southern China, and the Wang Chao and Three Pagodas faults in Thailand [Scharer et al., 1994; Lacassin et al., 1997]. Consequently, the magmatic, metamorphic and structural evolution of the MMB may place important constraints on
[5] With the exception of sample MH1, samples analyzed, in this study were collected by R. D. Beckinsale as part of the British Geological Survey’s study of granites of the Southeast Asian tin belt. Sample locations, petrography and geochemistry are reported by Cobbing et al. [1992]. KZ and MEB undertook follow up fieldwork in the Mandalay area in 1998 and 2000 to obtain supplementary samples and confirm field relationships. The samples were crushed, milled, washed and separated into mineral concentrates using conventional magnetic and gravity procedures. Zircon grains were then hand-picked from the heavy-mineral concentrate and mounted on a 25 mm diameter epoxy disc. The mounts were polished to expose a cross section of the crystals, cleaned, photographed and gold coated. Backscattered electron (BSE) and cathodoluminescence (CL) images of all zircon crystals analyzed were acquired with a scanning electron microscope (SEM) to provide information about zircon internal morphologies (Figure 4). This assists the understanding of zircon growth history and thus data interpretation and subsequently governed the choice of sites for SHRIMP analysis. [6] The data were collected during several 24-hour sessions, using six scans of eight isotopic species and one background position with a counter dead time of 32 nanoseconds. Apart from sample MH1, a 100 mm aperture was used to analyze all samples. Whereas a 70-micron Kohler aperture was used to achieve a beam size between 15 to 20 microns in order to analyze the thin rims on MH1 zircons. The CZ3 zircon standard (564 Ma; 206Pb*/238U = 0.0914) was used as a reference, and Pb*/U precision (calibration) errors (1s), which are the dominant source of uncertainty for Phanerozoic dates, were
