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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

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Panjasawatwong, Yuenyong and Panjasawatwong, Yuenyong.Empirical study of the effects of Ca/Na, Al/Si and H2O (1991) 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. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Petrology, Geochemistry, Plagioclase
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1993. Includes bibliographical references (leaves 206-224)

Date Deposited: 03 Feb 2015 03:09
Last Modified: 19 Feb 2017 20:57
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