Petrology, geochemistry and tectonic implications of igneous rocks in the Nan Suture, Thailand and An empirical study of the effects of Ca/Na, Al/Si and H2O on plagioclase - melt equilibria at 5-10 kb pressure

Part I: Igneous Rocks in the Nan Suture The Nan ma:fic-ultrarnafic belt is widely believed to represent a continental suture between the Shan-Thai (to the west) and Indochina (east) cratons. Geological mapping of selected areas has shown for the first time that this belt is an extensive melange, made up of variably sized blocks (up to a few km across) of igneous, metamorphic and sedimentary rocks in a serpentinite matrix. Igneous rocks as blocks within the Nan Suture melange consist of lavas, dolerites, microgabbros and cumulate gabbroic and ultrarnafic rocks. Lavas, microgabbros and dolerites may be chemically separated into three main compositional groups that reflect their tectonic settings of eruption, namely intraplate ocean-island basalts (Group A), backarc basin basalts and andesites (Group B) and island-arc basalts and andesites (Group C). Cumulates include gabbros/amphibolites and associated ultrarnafics and chromitites. A brief assessment of mineral equilibria in blueschists blocks is also presented. Group A lavas are intraplate basalts that include tholeiites (Subgroup A-1), and transitional tholeiites and alkalic basalts (Subgroup A-2). Subgroup A-1 tholeiites are chemically analogous to tholeiites from the Hawaiian and Tasmantid intraplate seamount chains. Subgroup A-2 basalts show markedly greater LREE enrichment relative to HREE and are comparable with transitional tholeiitic and alkalic basalts such as those erupted in the postshield stages of Haleakala and Mauna Kea in the Hawaiian chain. Group B includes basalts and andesites that have been chemically subdivided into four compositionally distinct subgroups that all show geochemical features transitional from arc basalts to backarc basin basalts. They are thus assigned to an eruption setting associated with the incipient rifting of an oceanic arc and development of an immature backarc basin. Group C basalts and andesites are subdivided into 2 subgroups. Subgroup Cl includes basalts and andesites with compositional characteristics most similar to many oceanic island-arc low-K to medium-K basalts and andesites. Subgroup C-2 samples are compositionally transitional between island arc tholeiitic and calc-alkalic basalts and andesites, and are typical of calc-alkalic lavas formed in an island arc or active continental margin. Cumulate gabbros are compositionally most similar to the Subgroup C-1 arc suite, but field relationships rule against direct affinities with the Group C arc suite. The gabbros probably represent basement rocks of an earlier arc sequence subsequently invaded by Subgroup C-1 arc magmas following reorganisation of plate boundaries related to arc-continent collision (see later). Ultramafic rocks associated with the gabbros show the crystallisation sequence olivine+chromite+orthopyroxene - clinopyroxene - plagioclase. This, together with spinel compositions, supports affinities with the low-Ti ophiolitic association, and implies generation in a suprasubduction zone setting. Chromitites contain highly refractory chromites comparable to those in high Ca-boninites, and were also probably generated in a supra-subduction environment. Occasional blocks of blueschist are probably derived from tuffaceous sediments associated with the intraplate Group A magma suite. They probably formed at about 7 kb in temperatures range 390 - 450°C, i.e. geothermal gradients 15- 180C/km. Based on these new data regarding the tectonic settings of eruption of igneous rocks in the Nan suture melange, and other available geological and age constraints, a plate-tectonic scenario for the Shan Thai - Indochina continental collision is proposed. In the Late Carboniferous, the Shan-Thai and Indochina cratons were separated by a major ocean basin, and an island arc probably formed above an east-dipping subduction zone at the leading edge of the Indochina craton. Ocean-island basalts (Group A samples) in seamount volcanoes on the subducting ocean crust were transported into the subduction zone and partly scraped off on to the forearc slope of this arc. Arc rifting possibly occurred in Early Permian time, giving rise to a backarc basin floored in its earliest phase of opening by Group B tholeiites. The thinned leading margin of the Shan-Thai craton eventually arrived at the subduction zone fronting this arc - backarc basin system that had developed at the leading edge of the Indochina craton, resulting in arc-continent collision, occurring probably initially in the Middle Permian. As the result of collision, backarc spreading terminated and a reversal of subduction polarity may have taken place. Arc magma Subgroup C-1 was produced above this new subduction zone and intruded the older arc represented by the foliated mafic-ultramafic plutonic rocks. This subduction episode eventually dragged the Shan-Thai craton into collision with the Middle to Late Permian foldbelt formed during arc-continent collision involving the Indochina craton. This continentcontinent collision, culminating in the Late Triassic, produced the Nan Suture. The original geometry of this suture has been largely modified by movements of transcurrent faults that formed widespread pull-apart basins in the Late Triassic, and the Cainozoic. The Nan Suture itself has probably acted as a locus of transcurrent fault motion since the final collision. Part II: Plagioclase-Melt Equilibria xix The presence of highly calcic plagioclase (>An80) in arc lavas and cumulates and some mid-ocean ridge basalts is well known, but there is presently no adequate model explaining such occurrences. An experimental study has been carried to test the effects of bulk composition, pressure, temperature and water pressure on the composition of liquidus or near-liquidus plagioclase formed in a variety of starting compositions. Experiments were carried out on synthetic starting mixes covering the calcic basalt to andesite range. Probably the strongest single control of the composition of crystallising plagioclase is the Ca# (molecular Ca/(Ca+Na)) of the bulk composition. The KnCa# values for plagioclase crystallising from any bulk composition from anhydrous experiments at 5 kb are always significantly higher than those from anhydrous experiments at 10 kb, but lower than those for hydrous experiments at 5 or 10 kb, or anhydrous experiments at 1 atmosphere. The effect of water on the composition of plagioclase is most marked for bulk compositions with Ca# ranging from 60 to 80, in which plagioclase is up to 10 mol% An more calcic than plagioclase crystallising from the same bulk composition under anhydrous conditions at similar pressure. These results form the basis of a discussion of the various models for the existence of highly calcic plagioclase in many arc lavas and cumulates, and provide a more thorough assessment of the petrogenetic significance of highly calcic plagioclases in mid-ocean ridge basalts.