Abstract The mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment.
The 2,660m-thick basalt pile described in this thesis represents one of the upper parts, which were extruded predominantly under subaerial conditions, of the Tertiary basalt sequence in East Greenland. The pile rests on Lower Cretaceous sediments and is overlain by the sediments of Kap Dalton Formation Chemically (XRF analysis), the basalts can be classified as ocean-island tholeiites and are broadly comparable to the Tertiary tholeiites from mid-west Iceland and the tholeiites of the Neovolcanic zone of Iceland. Although they have rather limited compositional ranges, they show evidence of two episodes of fractionation. The magma compositions were controlled largely by olivine fractionation in the initial stage and subsequently by plagioclase fractionation. Comparisons between bivariate and multivariate analyses have been made and it is found that in the case of rather narrow compositional ranges, the multi variate analysis (R-mode factor analysis) is the best method for illustrating geochemical patterns. Petrographically, the basalt suite consists chiefly of plagioclase (average labradorite), augite and Fe-Ti oxides with small amounts of olivine, pigeonite and interstitial glass, except for pillow lavas where the groundmass is entirely glass. The textures are variable from glassy to coarse-grained types. Thus they can be divided, in terms of textures and field observations, into four main groups and then subdivided into a number of rock types. The majority appear to be aphyric types. The principal microphenocryst and/or phenocryst assemblages are plagioclase, plagioclase + olivine, plagioclase + olivine + augite, Fe-Ti oxides, or Fe-Ti oxides + plagioclase. The plagioclase compositions (electron probe analysis) vary from An(_83.4)Ab(_16.3)Or(_0.3) to An(_27.3)Ab(_68.7)Or(_4.0). The great majority of analysed clinopyroxenes are augite and they follow the equilibrium fractionation trend typified in the Skaergaard tholeiitic intrusion. Olivine microphenocrysts and groundmass olivines were analysed and have chrysolitic and hyalosideritic compositions, respectively. Temperatures and oxygen fugacities, which have been estimated from the coexisting phases of titaniferous magnetite and ilmenite, are comparable to those of the Icelandic Thingmuli tholeiitic suite. The origin of these East Greenland tholeiites is attributed to low-degree partial melting of undepleted mantle beneath the region on the present seaward side of the Blosseville coast and, subsequently, low-pressure crystal fractionation.
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.
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