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Lithosphere Erosion and Crustal Growth in Subduction Zones: Insights from Initiation of the Nascent East Philippine Arc
Colin G. Macpherson
- Vol. 2008
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TLDR
In this paper, the East Philippine Arc was studied and local variations in lithosphere thickness suggest that thinning is rapid and may be piecemeal, and the evolution of the arc lithosphere has been observed over time.Abstract:
The Philippine Trench marks a nascent plate margin where subduction initiation is propagating from north to south. Magma compositions in the East Philippine Arc record thinning of arc lithosphere as it is eroded from below. Lithosphere is thicker beneath the younger, southern part of the arc, causing basaltic magma to stall and fractionate garnet at high pressure. In the mature, northern section, basaltic magma differentiates at shallower levels, at pressures where garnet is not stable. Local variations in lithosphere thickness suggest that thinning is rapid and may be piecemeal. Fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust. Varying fractionation depths of hydrous basalt may help to explain the andesitic composition of bulk continental crust.read more
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HIGH Sr/Y ARC MAGMAS AND PORPHYRY Cu ± Mo ± Au DEPOSITS: JUST ADD WATER
Abstract: Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H 2 O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content.
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Geochemical, Sr-Nd-Pb, and zircon Hf-O isotopic compositions of Eocene-Oligocene shoshonitic and potassic adakite-like felsic intrusions in Western Yunnan, SW China: Petrogenesis and tectonic implications
Yongjun Lu,Robert Kerrich,Campbell Mccuaig,Zheng-Xiang Li,Craig J.R. Hart,Peter A. Cawood,Peter A. Cawood,Zengqian Hou,Leon Bagas,John B. Cliff,Elena Belousova,Suo Han Tang +11 more
TL;DR: In this article, a coeval potassic adakite-like and shoshonitic felsic intrusions in the western Yunnan province of SW China are spatially and temporally associated with Eocene-Oligocene shoshoniitic mafic volcanic rocks.
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Enriched and depleted arc basalts, with Mg-andesites and adakites: A potential paired arc–back-arc of the 2.6 Ga Hutti greenstone terrane, India
TL;DR: In this article, two tholeiitic basalt populations have been identified within the Hutti greenstone belt: (1) those with enriched LREE at 20−50 times chondrite, and (2) an depleted LREE population at 12−20 times Chondrite.
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A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)
TL;DR: In this article, it was shown that peraluminous tonalite can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process.
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Adakites in the Truong Son and Loei fold belts, Thailand and Laos: Genesis and implications for geodynamics and metallogeny
TL;DR: In this paper, the authors showed that the Phu Kham and Puthep 1 adakites are characterized by low HREE and Y contents, but elevated LREE, Sr, Sr/Y, and La/Yb values.
References
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Derivation of some modern arc magmas by melting of young subducted lithosphere
Marc J. Defant,Mark S. Drummond +1 more
TL;DR: In this paper, the trace-element geochemical properties of the adakites (termed "adakites") of modern island and continental arcs are shown to be consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.
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Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa
TL;DR: In this paper, a set of experiments on natural, hydrous basalts at 1-4 GPa constrain the composition of "unadulterated" partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones, where these same adakite melts are allowed to infiltrate and react with an overlying layer of peridotite.
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Adakites without slab melting: High pressure differentiation of island arc magma, Mindanao, the Philippines
TL;DR: In this article, geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr/Y and depletion of the heavy rare earth elements.
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Amphibole “sponge” in arc crust?
TL;DR: In this article, amphibole is used as a filter for water dissolved in mantle-derived arc magmas, and amphibole cumulates may act as a fertile source of intracrustal melts and fluids.
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The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study
TL;DR: In this article, the effects of variable H2O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks were examined.