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Preview: Journal of Petrology - current issue

Journal of Petrology Current Issue





Published: Tue, 10 Oct 2017 00:00:00 GMT

Last Build Date: Fri, 10 Nov 2017 08:45:45 GMT

 



Enhancing Maficity of Granitic Magma during Anatexis: Entrainment of Infertile Mafic Lithologies

2017-10-10

ABSTRACT
Most studies of migmatites examine how anatexis occurred in the most fertile units and what happened to that melt, whereas the associated minor lithologies are typically ignored. The Kinawa migmatite in the southern São Francisco Craton of Brazil is the product of water-fluxed melting of a leucogranodiorite that contained dykes of amphibolite. The migmatite consists mostly of pink diatexites, metatexites and leucosomes, but it also contains schollen of amphibolite. This study examines the behaviour of these minor mafic rocks during anatexis to determine what role they play in the formation of migmatites and development of granitic magmas in their source region.The amphibolites are massive or banded Hbl + Pl, and rarely Hbl + Pl + Cpx, schollen in the diatexite migmatite. The amphibolite schollen melted very little, and show a complex morphology suggesting mechanical and chemical interaction with the enclosing leucocratic pink diatexite migmatite. Diatexites and leucosomes immediately adjacent to the schollen have a considerably higher proportion of amphibole (up to 12%) and/or biotite (up to 10%) compared with the diatexite a few tens of centimeters farther away. Six stages of disaggregation and interaction of mafic schollen with the enclosing diatexite magma are recognized: (1) amphibolite layers break up to form schollen, but are mineralogically and texturally unchanged; (2) melt infiltrates into fractures and foliation in the schollen; (3) schollen disaggregate into swarms of single amphibole crystals within the diatexites; (4) amphibole is partially replaced by biotite; (5) flow of the enclosing diatexite magma arranges the detached amphibole crystals into schlieren and aggregates of biotite; (6) detached crystals are completely replaced by biotite and dispersed by magmatic flow to produce a mesocratic to melanocratic homogeneous diatexite. Geochemical modelling indicates that the composition of the diatexites and leucosomes is changed by the wholesale entrainment of the disaggregated mafic schollen or in some cases by the preferential entrainment of detached hornblende or plagioclase crystals. This contamination increases the maficity of initially felsic, leucodiatexite magma, by the addition of FeO + MgO, CaO and TiO2 (which results in a concomitant decrease in SiO2), to become a mesocratic to melanocratic diatexite magma that is comparable with typical I-type granites found around the world. Entrainment of mafic material and hornblende in particular strongly influences the behaviour of the rare earth elements, lowering LaN/YbN ratios. Thus, non-protolith mafic lithologies within migmatites represent a source of contamination for anatectic melts that results in a significant increase in maficity.






The Provenance of the Lithospheric Mantle in Continental Collision Zones: Petrology and Geochemistry of Peridotites in the Ulten–Nonsberg Zone (Eastern Alps)

2017-10-05

ABSTRACT
We report petrographic descriptions, modal mineralogy estimates and major and trace element analyses of whole-rocks and minerals for 36 spinel and garnet–spinel peridotites from the Ulten–Nonsberg Zone (UNZ) in the Eastern Alps in Italy. We seek to constrain the origin and evolution of their source region in the mantle using a comprehensive geochemical dataset on representative, large, modally homogeneous samples from six UNZ sites. This complements earlier work on metamorphism, exhumation history and trace element residence. The samples range from coarse- to dominant fine-grained peridotites affected by syn-tectonic recrystallization and formation of amphibole ± chlorite as well as late-stage alteration (loss on ignition from 0·3 to 8·6 wt %). The UNZ rocks show a rather limited major oxide range (e.g. 1·8–2·8 wt % Al2O3 in ∼80% of the samples) and include neither very fertile nor highly refractory peridotites. Their range and average composition are distinct from those in several massifs from the western Alps, further indicating that the mantle beneath the Alps is heterogeneous, consistent with the tectonically active, plate boundary setting in which mantle domains of different origins may be juxtaposed. Comparison of the Al–Fe–Mg relationships in the UNZ peridotites with experimental data on melting of fertile mantle, together with modelling of REE contents in bulk-rocks, indicates that their mantle protoliths were formed by ∼10–20% polybaric melting in upwelling mantle that began at 2–4 GPa and ended close to the surface, possibly in an oceanic setting. The melting may have started in the presence of garnet, but mainly proceeded in the spinel stability field. Many UNZ peridotites are enriched in silica relative to continental off-craton xenoliths and experimental dry melting trends at similar Al2O3 and MgO. These enrichments are similar to those observed in suprasubduction-zone peridotites, suggesting their potential origin and/or evolution in a subduction-related setting. Modal and cryptic metasomatism is widespread in the UNZ suite, with a broad range of enrichments in incompatible trace elements. It took place mainly in the mantle wedge above a subduction zone, consistent with low high field strength elements and high light rare earth elements, Th, U, Ba and Pb, and probably incorporated slab components. Amphibole is the major host of highly incompatible trace elements whereas garnet, texturally equilibrated with the amphibole, hosts much of heavy rare earth elements and Zr and shows broad grain-to-grain variations of these elements consistent with its growth during tectonic recrystallization and hydrous modal metasomatism.



Submarine Basaltic Glasses from the Galapagos Archipelago: Determining the Volatile Budget of the Mantle Plume

2017-09-22

ABSTRACTWe report new major, trace and volatile element contents (H2O, CO2, F, S, Cl), and new Sr, Nd, Pb and He isotopes on submarine glasses from the Galapagos Archipelago from several dredging expeditions. Four groups are distinguishable on the basis of composition and geographical distribution: the Fernandina group (3He/4He > 22 RA), which is similar to the less degassed primitive mantle; the Sierra Negra group (enriched Pb and Sr isotopes, 3He/4He = 8–20 RA), produced by mixing the Floreana (HIMU-type) and Fernandina end-members; the Pinta group (high Δ7/4, Δ8/4 and Th/La ratios, 3He/4He = 6–9 RA), an enriched mantle (EM)-type mantle indicative of recycled material in the source; and the depleted mantle (DM) group, characterized by an isotopic composition similar to mid-ocean ridge basalts (MORB). Only a single submarine glass with the isotopic composition of the Floreana end-member has been identified in the sample suite. Degassing has significantly lowered the glass CO2 content with little effect on the H2O concentration. Volatile data for oceanic basalts reveal that CO2–H2O gas–melt equilibration at eruption depth is common in ocean island basalts (OIB) and rare in MORB, suggesting different ratios of melt transport to bubble formation and gas–melt equilibration. The Galapagos glasses range from sulfide saturated to undersaturated, and a subset of samples indicate that S degasses at pressures ≤ 400 bars. Assimilation of hydrothermally altered material affected the volatile contents of a number of samples in the groups. Once shallow-level processes have been accounted for, we evaluate the volatile contents in the different Galapagos mantle sources. Ratios between volatile and refractory elements with similar incompatibilities are used to estimate the volatile budget of the Galapagos mantle plume. Most of the glasses from the Fernandina, Sierra Negra and Pinta groups have high volatile/refractory element ratios, whereas a few pristine DM group lavas have ratios similar to those measured in MORB. The volatile/refractory element ratios are consistent with previous reports for the high 3He/4He, HIMU and MORB components. The values measured for the Pinta group, however, are higher than those found in other OIB associated with the presence of recycled material (EM-type). Our data suggest that mixing between the different mantle components is pervasive throughout the archipelago, which acts to normalize the volatile data between the groups. The Fernandina component can be modeled by a 6–20% mixture of the high 3He/4He primitive mantle component with the MORB source, assuming a two-layered mantle and using existing estimates of helium concentrations. The resulting estimated volatile content and H/C mass ratio for the high 3He/4He primitive mantle are consistent with previous estimates, but calculated C/3He ratios are lower than the canonical ratio. This indicates the following: (1) the estimates require ∼20–50 times higher C or lower 3He contents, which is difficult to reconcile with the measured volatile/refractory ratios in oceanic basalts; (2) the C/3He ratio is not constant throughout the mantle; (3) an impact erosion model, rather than a two-layered mantle model, is more consistent with the relatively constant C/3He ratios observed in oceanic basalts, although it is unclear how representative oceanic basalts are of the lower mantle. The high volatile content of the high 3He/4He component will affect mantle dynamics and melt migration during plume–ridge interaction as this component would be predicted to be less viscous than the ambient mantle. The lower viscosity material would have an enhanced vertical upwelling, which could explain the buoyancy flux of the Galapagos plume without the need for a temperature anomaly. A lower viscosity, high 3He/4He component could also provide an explanation for the lack of high 3He/4He in Galapagos Spreading Center lavas erupting in the vicinity of the Galapagos[...]



The Origin of Rapakivi Feldspar by a Fluid-induced Coupled Dissolution–Reprecipitation Process

2017-09-04

ABSTRACT
The rapakivi texture found in many granitoids comprises alkali feldspar megacrysts mantled by plagioclase, usually of oligoclase composition. The existing models for their genesis involve magmatic or dry subsolidus processes. Here, we describe the occurrence of rapakivi feldspars in A-type granites from the Malani Igneous Suite in western India and use microtextural and geochemical evidence to argue that rapakivi textures can form by subsolidus deuteric alteration of feldspar megacrysts through a coupled dissolution–reprecipitation replacement process. The feldspars in granites from the Malani Igneous Suite crystallized at temperatures >720°C and subsequently underwent coherent exsolution, producing strain-controlled braid microperthite/antiperthite. At temperatures of 465–490°C, the feldspar megacrysts reacted with deuteric fluids, which led to the dissolution of the braid perthite/antiperthite along an inward-moving reaction interface and coupled precipitation of an oligoclase/albite mantle. As the rapakivi replacement front progressed inward, the fluids infiltrated into the interiors of the relict megacrysts along fractures and braid boundaries and reacted with the braid perthite via a dissolution–reprecipitation replacement process. This resulted in the formation of patch perthite/antiperthite. The replacement reactions were incomplete, preserving patches of the unreacted braid perthite. At temperatures of 253–283°C, the feldspars were partially albitized, whereby the oligoclase patches and the plagioclase mantle were partially pseudomorphically replaced by albite. Mass-balance constraints indicate that the replacement processes leading to the formation of the plagioclase mantle and the patch perthite/antiperthite were not isochemical. The fluid composition was externally buffered for many of the elements, but internally controlled by feldspar dissolution–reprecipitation reactions for those elements that are normally incorporated in the feldspar structure. These results conclusively demonstrate for the first time that in addition to magmatic processes, rapakivi feldspars can form by subsolidus, fluid-induced, dissolution–reprecipitation replacement reactions.



Clockwise and Anticlockwise P–T Paths of High-pressure Rocks from the ‘La Pioza’ Eclogite Body of the Malpica–Tuy Complex, NW Spain

2017-08-30

ABSTRACT
The Malpica–Tuy complex (MTC) in northwestern Spain is a key area for the understanding of geodynamic processes related to the early collision of Gondwana and Laurussia. To better understand this collisional situation in Variscan times, we deciphered mass-flow paths in terms of pressure (P) and temperature (T) for five samples (three eclogites, a glaucophanite and a tonalitic gneiss) occurring in a large eclogite body at the ‘La Pioza’ site in the central MTC. The following three contrasting P–T paths resulted from the application of P–T pseudosections, calculated with PERPLE_X, and Zr-in-rutile geothermometry. (1) Two eclogites and the gneiss yielded a similar anticlockwise P–T path. In particular, one of these eclogites recorded an extended prograde path from about 8·5 kbar at 575°C to peak pressures of ∼24·5 kbar at 630°C. The retrograde path passed through P–T conditions of ∼17·5 kbar and 650°C. (2) A clockwise P–T loop was derived for an eclogite starting at ∼18 kbar at 580°C to peak pressures of ∼23 kbar at 620°C. (3) The clockwise P–T path of the glaucophanite is characterized by a temperature increase from 610 to 680°C at nearly constant pressure around 19 kbar. The peak P of rocks in the eclogite body is much higher than that of the surrounding gneisses (≤13 kbar). Major and trace element geochemical features demonstrate that the protoliths of one eclogite and the glaucophanite were calc-alkaline igneous rocks. The other two eclogites have a tholeiitic affinity. The tonalitic gneiss is characterized by a subalkaline affinity. All the mafic rocks are characterized by a Nb anomaly. The protoliths of the eclogite with calc-alkaline affinity and the gneiss can be assigned to a continental magmatic arc formed in Late Cambrian times according to previous age dating results. The protoliths of the eclogites with tholeiitic affinity were related to basalt/gabbro of thickened oceanic crust (island arc in the Rheic Ocean). The contrasting P–T paths and different nature of the protoliths are explained by different upwards-directed mass flows and, thus, mixing of various types of rocks in a subduction channel. Our findings also support the hypothesis that the MTC represents subduction-related rocks embedded in high-pressure orthogneisses from the downgoing tip of a continental plate during initial continent–continent collision.



Structure of the Plumbing System at Tungurahua Volcano, Ecuador: Insights from Phase Equilibrium Experiments on July–August 2006 Eruption Products

2017-08-28

ABSTRACT
Understanding the plumbing system structure below volcanoes and the storage conditions (temperature, pressure, volatile content and oxygen fugacity) of erupted magmas is of paramount importance for eruption forecasting and understanding of the factors controlling eruptive dynamics. Phase equilibria experiments have been performed on a Tungurahua andesite (Ecuador) to shed light on the magmatic conditions that led to the July–August 2006 eruptions and the parameters that controlled the eruptive dynamics. Crystallization experiments were performed on a representative August 2006 mafic andesite product between 950 and 1025°C, at 100, 200 and 400 MPa and NNO + 1 and NNO + 2 (where NNO is nickel–nickel oxide buffer), and water mole fractions in the fluid (XH2O) from 0·3 to 1 (water-saturation). Comparison of the natural phenocryst assemblage, proportions and phenocryst compositions with our experimental data indicates that the natural andesite experienced two levels of ponding prior to the eruption. During the first step, the magma was stored at 400 MPa (15–16 km), 1000°C, and contained c. 6 wt % dissolved H2O. In the second step, the magma rose to a confining pressure of 200 MPa (8–10 km), where subsequent cooling (to 975°C) and water-degassing of the magma led to the crystallization of reversely zoned rims on pre-existing phenocrysts. The combination of these processes induced oxidation of the system and overpressure of the reservoir, triggering the July 2006 eruption. The injection of a new, hot, volatile-rich andesitic magma from 15–16 km into the 200 MPa reservoir shortly before the eruption was responsible for the August 2006 explosive event. Our results highlight the complexity of the Tungurahua plumbing system in which different magmatic reservoirs can coexist and interact in time and are the main controlling factors of the eruptive dynamics.



Thermal History of the Upper Zone of the Kiglapait Intrusion

2017-08-28

ABSTRACT
A primary goal of igneous petrology is to understand the temperature range of crystallization for large intrusive bodies. Well-preserved layered intrusions, with their more or less stratigraphic mineral and compositional evolution, offer both a challenge and an opportunity to understand their thermometry. The challenge comes in a petrographic understanding of the appropriate rocks to study; the opportunity arises in the evolution of the intrinsic stratigraphic diversity. Whereas the crystallization temperatures of the troctolitic Lower Zone of the Kiglapait intrusion were determined experimentally in our laboratory at the liquidus of successively more evolved compositions, the temperatures of the Upper Zone are now determined at the solidus of adcumulate rocks. Ideal adcumulates have no zoning in plagioclase and so must have crystallized isothermally by rejection of interstitial solute to the main magma. The solidus thermometry of rock powders at pressure avoids any prospect of metastability. Here the solidus temperature was found in a piston-cylinder apparatus at 5 kbar. Melting experiments were made on rock powders held for 2 h or more in graphite capsules. Melt volumes were estimated visually in reflected light and by SEM, and scaled against temperature to determine the solidus at zero melt. The previously determined Lower Zone temperature limit and the first of the new Upper Zone determinations are the same at 1200°C, 5 kbar. The new results for temperature, plotted against stratigraphic progress, form a concave-up curve to a flat end point at 1010°C, 5 kbar, and 1000°C referred to 3 kbar. The entire magmatic history of the intrusion spans 250°C over a pressure gradient of 5·2–2·8 kbar and a duration of near 1 Myr of crystallization.



Origin and Evolution of Silicic Magmas in Oceanic Arcs; an in situ Study from St Lucia, Lesser Antilles

2017-08-21

ABSTRACT
Processes linked with the genesis, evolution and emplacement of silicic complexes in arcs are still poorly constrained. Of particular interest are the depth of magma production, the relative contribution of crystal fractionation versus crustal partial melting and the timescales involved. The Soufrière Volcanic Complex (SVC) on St Lucia is one of the largest silicic centres in the Lesser Antilles arc. Here we present the results of a detailed mineralogical study, including in situ Sr isotopes in plagioclase and in situ δ18O in dated zircons, of both SVC and Pre-SVC volcanic rocks to place constraints on the processes intrinsic to the development and evolution of the silicic complex. These data suggest that the production of silicic magma in the SVC occurs in two stages. The first stage involves differentiation of mafic magma by crustal assimilation and mineral fractionation in the middle–lower crust of the arc to produce magmas with intermediate compositions. These intermediate magmas are water-rich (∼7 wt %) and have high 87Sr/86Sr, Ba, Sr and La/Sm (∼5) compared with Pre-SVC lavas. Near-constant trace element and isotopic compositions throughout the SVC lifespan indicate that the same process was persistent over the last 600 kyr. In the second stage, the intermediate magmas are transferred to a shallower and more differentiated chamber (∼6 km depth). During ascent, any crystals or xenocrysts residual from stage one in the deeper chamber become fully resorbed and the magma crystallizes calcic amphibole microphenocrysts, followed by anorthite-rich plagioclase close to or at the water saturation depth. During mixing upon recharge within the shallow chamber, anorthite-rich plagioclase from the recharging magma is partially resorbed; so are the crystals in equilibrium with the resident differentiated magma. The recharge event probably causes chamber-wide convection. Mixing is thought to trigger eruption of the silicic complex magmas.