Tectonic implications of Early Miocene OIB magmatism in a near-trench setting: The Outer Zone of SW Japan and the northernmost Ryukyu Islands
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Diorites, granites, and associated magmatic enclaves and dykes constitute the bulk of the Ladakh Batholith, which is an integral part of the Trans-Himalayan magmatic arc system. In this paper, geometry of microgranular enclaves hosted in the granites has been examined from the Leh-Sabu-Chang La and surrounding regions of the eastern Ladakh Batholith to infer the mechanism and schedule of mafic to hybrid magma injections into evolving felsic magma chambers and the resultant enclave geometry. Mafic and/or hybrid magmas injected into felsic magma at low volume fraction (<0.35) of crystals and form the rounded to elongated microgranular enclaves in the Ladakh Batholith. Angular to subangular (brecciated) rounded to elongated pillow-like microgranular enclave swarms can also be documented as disrupted synplutonic mafic to hybrid dykes and sheets, when intruding the felsic magma with high volume fraction (>0.65) of crystals. A large rheological difference between coeval felsic and mafic magmas inhibits much interaction. Mafic magma progressively crystallizes and evolves while minimizing thermal and rheological differences. Consequently, the felsic-mafic magma interaction process gradually becomes more efficient causing dispersion of enclave magma globules and undercooling into the partly crystalline felsic host magma. Thus, the evolution of the Ladakh Batholith should be viewed as multistage interactions of mafic to hybrid magmas coeval with felsic magma pulses in plutonic conditions from its initial to waning stages of evolution.
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Abstract: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO 2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO 2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H 2 O.
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ABSTRACT The mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber. Tilting has exposed a 5-km section through the GZ and adjacent portions of the HZ. A porphyry is interpreted as a remnant of a chilled roof zone that marks the first stage of felsic GZ intrusion. Subsequent recharging by felsic and mafic magma, reflected by repeated cycles of crystal accumulation and melt segregation in the GZ and emplacement of mafic flows in the HZ, rejuvenated and maintained the chamber. Kilometre-scale lobes of mafic HZ material were deposited as prograding tongues into the GZ during periods of increased mafic input. Thus, they are lateral equivalents of the cumulate GZ granites with which they interfinger. Conglomerate-like units comprising rounded, matrix-supported intermediate clasts in cumulate granite are located immediately above the lobes. These ‘conglomerates’ appear to represent debris flows shed from sloping upper surfaces of the lobes. Thus, the GZ can be viewed as comprising distal facies, remote from the site of mafic recharging in the HZ, and the HZ as comprising proximal facies. Elemental chemistry suggests that the GZ cumulate granites represent a second-stage accumulation from an already evolved melt, and that coarse, more mafic, feldspar+biotite+accessory mineral ± hornblende rocks trapped between mafic sheets in the HZ are the initial cumulates. Fractionated melt accumulated roofward and laterally, and was the direct parent of the ‘evolved’ GZ cumulates. The most highly fractionated, fluid-rich melts accumulated at the roof.
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ABSTRACT Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe 3+ /Fe 2+ , and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks. S-type granites have higher δ 18 O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks. Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.
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In monomagmatic chambers density differences between overlying country rock and magma should, following development of ring fractures, result in mafic ring dikes above mafic magma, and in foundering of the roof into felsic magma with the development of felsic stocks. The observed prevalence of felsic ring dikes may be due to shallow floors in felsic magma chambers, or to shallow constructed floors at the top of the mafic fraction in polymagmatic chambers.
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Petrographic studies on the mafic and felsic dykes from the Piratini region, RS, reveal a porphyritic texture with an aphanitic to fine-grained matriz, with glomeroporphyritic and spherulitic textures in the case of felsic, and ophitic to sub-ophitic with myrmekitic intergrowths in the case of mafic dykes. These dykes are intrusive in granitic rocks of the Pelotas Batholith along NW-SE and N-S trends associated with high angle shear zones. They are strongly metaluminous (mafic) and slighty metaluminous to peraluminous (felsic), with an alumina saturation index between 0.60 and 0,65 and 0,75 and 1,2, respectively. The Si02 content in the mafic dykes varies from 44 to 48 wt % and in the felsic varies from 67 to 75 wt %. The mafic dykes present higher Ti, Mg, Ca, Fe, Mn and P contents in comparison with the felsic dykes. The behavior of several major and trace elements (Fe, Mn, Mg, Ti, P, Ca and Sr, and, in minor degree, Ca and Mg show the importance of crystallization of iron-magnesium and feldspars in both dykes. The Nb/Ta e U/Th ratios of trace elements of these rocks show evidence of crustal contamination. The HREE display sub horizontallized patterns, showing Tb/Lu ratios from 1,5 to 2,7 in the samples of mafic dykes and from 1,6 to 2,5 in the felsic dykes. The samples have shown Lu concentration from 0,3 ppm to 0,6 ppm in mafic, and from 18 ppm to 41 ppm in the felsic dykes, respectively. The felsic dykes are more enriched in LREE, with La values from 73 ppm to 155 ppm, whereas mafic dykes present 18 ppm to 41 ppm of La and La/Sm ratios in the range of 3,0 to 5,0.
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