Magma hybridization in the middle crust: Possible consequences for deep-crustal magma mixing (original) (raw)
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Proc. Estonian Acad. Sci. Geol, 1995
Iceland, with the plate boundary exposed on land, represents a natural laboratory for observing the petrology and tectonics of the mid-ocean ridge environment interacting with a deep mantle plume. The specific chemical characteristics ol Icelandic igneous rocks, such as high abundance of incompatible trace elements, high 8iSr/865r, 1o* us567ru5d and radiogenic Pb contents result lrom plume influences on the magma generation and its subsequent evolution in crustal magma chambers inside the anomalously thick Icelandic crust (8-20 km) and interaction with the altered old crust.
Journal of Petrology, 2001
of the Pannonian Basin and the tensional stress field may have Almandine garnet-bearing andesites and dacites occur frequently in enhanced their fast ascent from lower-crustal depths, allowing the Neogene calc-alkaline volcanic series of the northern Pannonian preservation of early-formed almandine phenocrysts. Basin (Hungary and Slovakia). They were erupted during the early stage of volcanism and occur along major tectonic lineaments. On the basis of petrographic and geochemical characteristics, garnets from these rock types are classified into (1) primary phases, KEY WORDS: almandine; garnet; calc-alkaline volcanism; geochemistry; (2) composite minerals containing xenocrystic cores and magmatic Pannonian Basin overgrowths and (3) garnets derived from metamorphic crustal xenoliths. Coexisting phenocrysts of primary garnets include Carich plagioclase, hornblende (magnesiohastingsite to tschermakite) and/or biotite. The primary garnets have high CaO (>4 wt %) and low MnO contents (<3 wt %). They have strongly light rare INTRODUCTION earth element depleted patterns and are enriched in heavy rare earth Almandine garnet is a relatively rare mineral phase in elements. Negative Eu anomalies occur only in garnets in the more calc-alkaline volcanic rocks worldwide (Table 1). Its rarity silicic host rocks. 18 O values for primary garnets are 6•1-7•3‰, may be due to the restricted conditions under which whereas composite garnets have elevated 18 O values (>8‰). such garnets can form (hydrous mantle source, high-Chemical compositions of the primary garnets and coexisting minerals pressure crystallization from hydrous Al-rich magma) suggest that they crystallized at high pressures (7-12 kbar) and and to the particular geodynamic setting (tensional stress temperatures (800-940°C) from mantle-derived magmas. Sr-Nd field), which enhances the rapid ascent of garnet-bearing isotopic compositions of their host rocks and O isotopic values of melts. Green & Ringwood (1968) demonstrated that the garnets are consistent with two-component mixing between almandine-rich garnet could be a liquidus or nearmantle-derived magma and lower-crustal metasedimentary material. liquidus mineral in silicic magmas at high pressure (9-18 kbar). Further experimental studies (e.g. Hensen & The garnet-bearing silicic magmas were erupted during extension
Journal of Geology, 2008
We report new laser fluorination oxygen isotope analyses of selected samples throughout the Skaergaard intrusion in East Greenland, particularly relying on ∼1-mg separates of the refractory, alteration-resistant minerals zircon, sphene, olivine, and ferroamphibole. We also reexamine published oxygen isotope data on bulk mineral separates of plagioclase and clinopyroxene. Our results show that the latest-stage, strongly differentiated magmas represented by ∼3 to 6 km 3 of ferrodiorites around the Sandwich Horizon (SH), where the upper and lower solidification fronts met, became depleted in 18 O by about 1.5‰-2‰ relative to the original Skaergaard magma and the normal mantle-derived mid-ocean ridge basalt. Earlier studies did not recognize these low-δ 18 O ferrodiorite magmas (δ 18 O ¼ ∼3‰-4‰) because after the intrusion solidified, much of the intrusion and its overlying roof rocks were heavily overprinted by low-δ 18 O meteorichydrothermal fluids. We consider three possible ways of producing these low-δ 18 O ferrodiorite magmas. (1) At isotopic equilibrium, liquid immiscibility may cause separation of a higher-δ 18 O, higher-SiO 2 granophyric melt, thereby depleting the residual Fe-rich ferrodiorite magma in 18 O. However, such a model would require removal of many cubic kilometers of coeval granophyre, a greater proportion than is observed anywhere in the intrusion; there is no evidence that any such magmas erupted to the surface and were eroded. (2) While direct migration of low-δ 18 O water seems implausible, we consider a model of "self-fertilization," whereby oxygen from meteoric waters entered the SH magma by devolatilization and exchange with hydrated, low-δ 18 O stoped blocks of the upper border series. Such reactive exchange between residual melt and adjacent hydrothermally altered, water-saturated rocks contributed low-δ 18 O crystalline components and low-δ 18 O pore water to the residual melt. The low-δ 18 O zircon and sphene may have crystallized directly from this contaminated low-δ 18 O melt, even though the entire mineral assemblage did not, simplifying the mass balance problem. (3) Finally, after SH crystallization, fracturing, and subsequent subsolidus meteoric-hydrothermal alteration depleted these rocks in 18 O, intrusion of the 660-m-thick Basistoppen sill, emplaced 150-200 m above the still hot SH, may have reheated and partially melted these late-stage differentiates. In this scenario, zircon and sphene could have crystallized from a low-δ 18 O partial melt, while other minerals may have simply reequilibrated. We favor models 2 and 3 and discuss their strengths and weaknesses.
The formation of mantle phlogopite in subduction zone hybridization
Contributions to Mineralogy and Petrology, 1982
Extrapolation and extension of phase equilibria in the model system KA1SiO4-Mg2SiO4-SiOz-H20 suggests that at depths greater than 100 km (deeper than amphibole stability), hybridism between cool hydrous siliceous magma, rising from subducted oceanic crust, and the hotter overlying mantle peridotite produces a series of discrete masses composed largely of phlogopite, orthopyroxerie, and clinopyroxene (enriched in jadeite). Quartz (or coesite) may occur with phlogopite in the lowest part of the masses. The heterogeneous layer thus produced above the subducted oceanic crust provides: (1)aqueous fluids expelled during hybridization and solidification, which rise to generate in overlying mantle (given suitable thermal structure) HzO-undersaturated basic magma, which is the parent of the calc-alkalic rock series erupted at the volcanic front; (2) masses of phlogopite-pyroxenites which melt when they cross a deeper, high-temperature solidus, yielding the parents of alkalic magmas erupted behind the volcanic front; and (3) blocks of phlogopite-pyroxenites which may rise diapirically for long-term residence in continental lithosphere, and later contribute to the potassium (and geochemically-related elements) involved in some of the continental magmatism with geochemistry ascribed to mantle metasomatism.
European Journal of Mineralogy, 2011
We present evidence that the 1674 Ma Borås Mafic Intrusion of the Swedish Eastern Segment experienced high-pressure metamorphism related to a Sveconorwegian subduction-exhumation cycle. Mg-rich staurolite is found as inclusions in garnet in metaluminous amphibolites. The inclusion assemblages include staurolite (X Mg 0.34-0.40), kyanite, euhedral anorthite, clinozoisite and quartz. The thermodynamic packages winTWQ and Theriak-Domino were used to investigate the P-T conditions of the matrix and inclusion mineral parageneses. The bulk composition of the rock does not have a stability field for staurolite-bearing parageneses. In our samples minerals of an eclogite-facies paragenesis became isolated from the whole rock in the first stage as inclusions in garnet. High Zn levels in the staurolite (0.6-1 wt% ZnO) show that it must have formed as either chloritoid or staurolite, both of which concentrate Zn. Euhedral anorthite inclusions have trace-element compositions including high Sr and insignificant Eu anomalies, which support their interpretation as pseudomorphs after lawsonite in plagioclase-out conditions. Rutile lamellae in the garnet are also indicative of a high-pressure origin. Calculated phase diagrams show that the most likely original paragenesis was garnet þ clinopyroxene þ Mg-rich chloritoid þ lawsonite þ kyanite þ quartz, which has a stability field for the whole-rock composition at 600 C and 2.23-2.45 GPa. These conditions correspond to depths greater than 75 km, thus the Borås Mafic Intrusion was situated in the mantle at that time, implying subduction of the crustal block in which it was situated. The minerals now observed in the inclusions and in the rock matrix formed under amphibolite-facies conditions at lower pressures of 0.6-0.9 GPa and slightly increased temperatures around 650 C, reflecting rapid exhumation from the mantle. Sm-Nd dating of garnet gives 957.1 AE 9.4 Ma, consistent with less precise Lu-Hf data, and represents either garnet growth during subduction or resetting during exhumation. Our investigations of staurolite in amphibolites documented in the literature show that staurolite cannot form in equilibrium with amphibolitefacies parageneses in normal metabasic rocks, which always have metaluminous compositions. A two-stage process is required in which a peraluminous assemblage with kyanite and possibly chloritoid first forms, due to plagioclase-out reactions in eclogite-facies conditions. Staurolite forms in the second stage during exhumation as pressure decreases, in domains which are not in contact with the common amphibolite-facies assemblage, for example by hydration reactions involving kyanite and garnet or by breakdown of chloritoid at higher temperatures. The pressures estimated for garnet growth and the development of inclusions correspond to minimum depths of 75-83 km (for basaltic or granitic overburden) and the Borås Mafic Intrusion is an integral part of the Eastern Segment in which retrograde eclogite metabasic bodies occur within orthogneisses in at least four other localities. This implies that a major part of the Eastern Segment experienced a high-pressure metamorphic event and the entire block of continental crust was involved in a subduction-exhumation cycle during the Sveconorwegian orogeny.
Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of [11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4-6 km. The crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where H 2 O-rich (*6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the H 2 O-saturated liquidus at *4 km. H 2 O contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently 'hot' and 'cold' granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons.