Slab partial melts from the metasomatizing agent to adakite, Tafresh Eocene volcanic rocks, Iran

Island Arc ◽  
2011 ◽  
Vol 20 (2) ◽  
pp. 188-202 ◽  
Author(s):  
MOHAMMAD R. GHORBANI ◽  
RASOUL N. BEZENJANI
Keyword(s):  
Volcanic Rocks ◽  
Partial Melts

Tectonic evolution of southwestern Newfoundland as indicated by granitoid petrogenesis

1985 ◽  
Vol 22 (7) ◽  
pp. 1080-1092 ◽  
Author(s):  
Derek H. C. Wilton
Keyword(s):  
Fault Zone ◽  
Point Group ◽  
Volcanic Rocks ◽  
Early Carboniferous ◽  
Late Devonian ◽  
Partial Melt ◽  
Show Evidence ◽  
Partial Melts ◽  
Felsic Volcanic ◽  
Host Rocks

Four granitoid suites are recognized in the region of the Cape Ray Fault Zone of southwestern Newfoundland. The two oldest (Ordovician–Silurian (?)) suites represent partial melts of their enclosing host rocks. The Port aux Basques granite is modelled as a partial melt of the gneissic component of its host, Port aux Basques Complex. The Cape Ray granite forms a dominantly tonalitic terrane derived by partial melting of ophiolitic material. The Red Rocks granite and a megacrystic phase of the Cape Ray granite form coherent lines of geochemical descent from the parental tonalite but show evidence of some continental crust contamination.The Late Devonian Windowglass Hill granite is a subvolcanic equivalent of felsic volcanic rocks in the Windsor Point Group. Both units were derived as partial melts of continental crust.The post-tectonic, Late Devonian to Early Carboniferous Strawberry and Isle aux Morts Brook granites constitute the youngest granitoid suite in the region. These A-type granitoids were derived as partial melts of an underlying depleted granulitic (felsic) crust. The depleted nature of the source may have resulted from previous generation of the Windowglass Hill granite and Windsor Point Group. The only possible protolith for the granulitic source is Precambrian Grenvillian gneiss. The presence of this gneiss beneath the Cape Ray Fault Zone of southwestern Newfoundland implies that the complete series of lithologies is allochthonous.

scholarly journals A short, sharp pulse of potassium-rich volcanism during continental collision and subduction

Geology ◽  
2019 ◽  
Vol 47 (11) ◽  
pp. 1079-1082 ◽  
Author(s):  
M.R. Palmer ◽  
E.Y. Ersoy ◽  
C. Akal ◽  
İ. Uysal ◽  
Ş.C. Genç ◽  
...  
Keyword(s):  
High Pressure ◽  
Isotope Composition ◽  
Volcanic Rocks ◽  
Continental Collision ◽  
Western Anatolia ◽  
Slab Rollback ◽  
Tectonic Zones ◽  
Geochemical Indices ◽  
Partial Melts ◽  
Potassic Volcanism

Abstract Potassic volcanic rocks are characteristic of collisional tectonic zones, with recycling of continental crust playing an important role in their generation. Potassium-rich partial melts and/or fluids derived from subducted continental material initiate and/or mix with mantle-derived melts and then erupt at the surface with varying degrees of interaction with the overlying lithosphere. The details of how continental material incorporates into mantle melts are, however, uncertain. In particular, the depths from which the potassium-rich fluids and/or melts are released from the continental material and then react with the mantle-derived melts remain a subject of debate. We have measured the boron isotope composition of volcanic rocks from Western Anatolia (Turkey) that erupted between 52 and 0.1 Ma, and span the lifetime of collisional events from initial arc-type eruptions to post-collisional volcanism. These data and other geochemical indices show that ultrapotassic volcanism was mainly confined to a narrow window between ca. 20 and 15 Ma, consistent with recycling of high-pressure phengite, with the timing of the potassic volcanism coincident with slab rollback and breakoff.

The geochemistry and petrogenesis of ophiolitic volcanic rocks from Lac de l'Est, Thetford Mines Complex, Quebec, Canada

1986 ◽  
Vol 23 (2) ◽  
pp. 202-213 ◽  
Author(s):  
I. O. Oshin ◽  
J. H. Crocket
Keyword(s):  
Fractional Crystallization ◽  
Spatial Association ◽  
Volcanic Rocks ◽  
Minor Role ◽  
Lower Unit ◽  
Complete Section ◽  
Depleted Mantle ◽  
Partial Melts ◽  
A Minor ◽  
Back Arc

The Cambro-Ordovician age Thetford Mines Complex from the Quebec Appalachians, Canada, preserves a remarkably complete section of ophiolites at Lac de l'Est, where mafic volcanics overlie a plutonic mafic–ultramafic plate. The basaltic volcanics consist of a lower unit, representing the extrusive component of the ophiolite assemblage, and an upper unit, whose petrogenetic and tectonic relationships with the ophiolitic volcanics are problematic.The lower unit ophiolitic volcanics include high- and low-TiO2 basalts. The upper unit volcanics, of which the basal 80 m was sampled, are low-TiO2 basalts. Fractional crystallization was important in the evolution of high-TiO2 lower unit magmas but played only a minor role in the formation of other magmas. Partial melting processes were dominant, or much more important than fractional crystallization, in controlling the composition of other magmas. The parental magmas of the high-TiO2 lower unit basalts were partial melts of undepleted mantle, whereas the low-TiO2 volcanics were partial melts of residual, depleted mantle. Despite different mantle sources, the high- and low-TiO2 basalts of the lower unit are interbedded in the field.The close spatial association of chemically diverse magma types is best accounted for by generation in a back-arc or marginal basin environment. This interpretation is supported by the geochemistry of argillaceous sediments in the Lac de l'Est pile and the absence of a sheeted dike facies in the Thetford Mines ophiolites.

Melts and metasomatic fluids: evidence from U-series disequilibria and Th isotopes

1993 ◽  
Vol 342 (1663) ◽  
pp. 79-90 ◽  
Keyword(s):  
Partial Melting ◽  
Chemical Equilibrium ◽  
Volcanic Rocks ◽  
Vapour Phase ◽  
Small Degree ◽  
Silicate Melts ◽  
Isotopic Equilibrium ◽  
Magma Genesis ◽  
Partial Melts ◽  
Subcontinental Lithosphere

Radioactive disequilibria and Th isotopes in volcanic rocks change in response to the extent and rate of partial melting, the attainment of chemical and isotopic equilibrium, and the presence and composition of a vapour phase before and during melting. Enrichments of 230 Th with respect to 238 U by 10-60% in MORB and intraplate alkalic magmas are attributed to the greater incompatibility of Th in small degree silicate partial melts. Inverse correlations between ( 230 Th)/( 238 U) and ( 230 Th)/( 232 Th) ratios indicate either that partial melting lasts more than 10 5 years, or that smaller degree melts take scores of millenia longer to move from the site of last chemical equilibrium with the source to the eruption site. Disequilibrium enrichments of 226 Ra with respect to 230 Th and of 231 Pa with respect to 235 U by 150-300% occur in oceanic island arc basalts and MORB. The excess 226 Ra in arcs is related to magma genesis, but the other disequilibria may reflect alteration or assimilation within the crust. When these large disequilibria accompany magma genesis they indicate transfer times between source and surface of a few decades to a few millenia and suggest an open system or chemical disequilibrium during melting. Th and U isotope and concentration systematics for lamproites suggest that melting was fast enough to preclude chemical equilibrium between source and melt. Non- silicate fluids may fractionate U-series radionuclides differently and to a greater extent than do silicate melts. This property may explain the U-Th-Ra characteristics of carbonate- and water-rich melts, and distinctive Th isotopic compositions in magmas derived from metasomes in the subcontinental lithosphere.

Petrogenesis of felsic I-type granites: an example from northern Queensland

1992 ◽  
Vol 83 (1-2) ◽  
pp. 115-126 ◽  
Author(s):  
David C. Champion ◽  
Bruce W. Chappell
Keyword(s):  
Partial Melting ◽  
Volcanic Rocks ◽  
Two Stage ◽  
High K ◽  
Depleted Mantle ◽  
Stage Process ◽  
Partial Melts ◽  
High Level ◽  
End Members ◽  
Isotope Systematics

ABSTRACTFelsic I-type granites and associated volcanic rocks of Carboniferous age are extensively developed over an area of 15,000 km2 in northern Queensland. These granites have been subdivided into four supersuites: Almaden, Claret Creek, Ootann and O'Briens Creek.Granites of the Almaden Supersuite are intermediate to felsic (56-72% SiO2) and are characterised by high K2O, K/K(K + Na), Rb, Rb/Sr, Th, U and relatively low Ba and Sr. The Claret Creek Supersuite granites are a little more felsic (65-77% SiO2), and are chemically distinctive, having higher A12O3, CaO, Na2O and Sr, and lower K2O, Rb, Th and U than granites of the Almaden Supersuite.Granites of the Ootann and O'Briens Creek supersuites all contain more than 70% SiO2 and these comprise more than 90% of the total area of granites. These two supersuites are characterised by low Sr, Sr/Y and large negative Eu/Eu*, with the more evolved rocks becoming strongly depleted in TiO2, FeO* MgO, CaO, Ba, Sr, Sc, V, Cr, Ni, Eu, CeN/YN and K/Rb, and enriched in Rb, Pb, Th, U and Rb/Sr. Granites belonging to the O'Briens Creek Supersuite contain significantly higher abundances of HFSE, HREE and F (0·2-0·5 wt%) than those of the Ootann Supersuite, and as such have developed some characteristics of A-type granites.Geochemical and isotopic properties suggest that all granites are of crustal derivation. The granites of all supersuites have very similar initial 87Sr/86Sr and εNd of 0·710 and −7·0–−8·0, respectively, except where they outcrop within Proterozoic country rocks, when they have more evolved εNd (−8·0–−11·0). Depleted-mantle model ages cluster around 1·5 Ga. The isotope systematics and geochemistry indicate that these granites were not derived from the equivalents of any exposed country rocks.Models for the petrogenesis of these granites all appear to require the involvement of a long-lived and isotopically homogeneous crustal protolith, that most probably underplated the crust in the Proterozoic. Granites of the two more felsic supersuites were either derived by varying degrees of partial melting from this protolith of andesitic to dacitic composition, and/or were produced by a two-stage process by remelting of intermediate rocks similar in composition to the mafic end-members of the Almaden Supersuite. The resulting primary partial melts for the Ootann and O'Briens Creek supersuites underwent extensive, high-level, feldspar-dominated, crystal fractionation.

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

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