Source Of Parental Magmas Research Articles

Thermodynamic constraints on the petrogenesis of massif-type anorthosites and their parental magmas

Development of computational modeling tools has revolutionized studies of magmatic processes over the last four decades. Their refinement from binary mixing equations to thermodynamically controlled geochemical assimilation models has provided more comprehensive and detailed modeling constraints of an array of magmatic systems. One of the questions that has not yet been vigorously studied using thermodynamic constraints is the origin of massif-type anorthosites. The parental melts to these intrusions are hypothesized to be either mantle-derived high-Al basaltic melts that undergo crustal contamination or monzodioritic melts derived directly from lower crust. On the other hand, many studies suggest that the monzodioritic rocks do not represent parental melts but instead represent crystal remnants of residual liquids left after crystal fractionation of parental melts. Regardless of the source or composition, magmas that produce massif-type anorthosites have been suggested to have undergone polybaric (~1000–100 MPa) fractional crystallization while ascending through the lithosphere. We conducted lower crustal melting, assimilation-fractional crystallization, and isobaric and polybaric fractional crystallization major element modeling using two thermodynamically constrained modeling tools, the Magma Chamber Simulator (MCS) and rhyolite-MELTS, to test the suitability of these tools and to study the petrogenesis of massif-type anorthosites. Comparison of our models with a large suite of whole-rock data suggests that the massif-type anorthosite parental melts were high-Al basalts that were produced when hot mantle-derived partial melts assimilated lower crustal material at Moho levels. These contaminated basaltic parental magmas then experienced polybaric fractional crystallization at different crustal levels (~40 to 5 km) producing residual melts that crystallized as monzodioritic rocks. Model outcomes also support the suggestion that the cumulates produced during polybaric fractional crystallization likely underwent density separation, thus producing the plagioclase-rich anorthositic rocks. The modeled processes are linked to a four-stage model that describes the key petrogenetic processes that generate massif-type anorthosites. The presented framework enables further detailed thermodynamic and geochemical modeling of individual anorthosite intrusions using MCS and involving trace element and isotope constrains. • Thermodynamic constraints on the origin of massif-type anorthosites using MCS. • Models provide indications for the composition and source of parental magmas. • The parental magmas were generated by AFC processes of mantle-derived melts. • The models are linked to general magmatic processes during anorthosite formation. • The general models set the foundation for detailed modeling of individual intrusions.

Nd-Sr-Hf isotopes and U-Pb ages of mesoproterozoic Três Estradas Alkaline-Carbonatite Complex, Brazil: Implications for Sul-Riograndense Shield evolution and rodinia break-up

The first occurrence of Mesoproterozoic carbonatite in the Rio de la Plata Craton – RDPC indicates extensional event prior to the opening of the Charrua Ocean. Nd-Sr-Hf Isotopic and U-Pb on zircon geochronological data are presented for the Três Estradas Alkaline-Carbonatite Complex – TEC, located in southernmost Brazil. The complex consists predominantly of ultramafic rocks, carbonatites and, subordinate, deformed fenitized syenites metamorphosed under greenschist to amphibolite facies conditions. U-Pb ages between 1110 ± 4.8 Ma and 1123 ± 15 Ma, respectively derived from a dolomite metacarbonatite and an associated metasyenite, constrain the TEC emplacement timing. The Nd-Sr-Hf isotopic signatures indicate that the TEC rocks were derived from a heterogeneous mantle, isotopically depleted in Sm-Nd and slightly depleted to enriched in Rb-Sr, with little influence of ancient crust and that these rocks are derived from the same source of parental magma probably submitted to intense metasomatism. The Nd-Sr isotopic signatures and crystallization ages of the TEC allow correlating it to the carbonatitic intrusions of the Alkaline provinces of Ontario and Grenville, Canada, in Laurentia, associated with the Keweenawan magmatic event (1.11–1.09 Ga), to which extension efforts would be related causing ruptures in RDPC edge and Laurentia, probably juxtaposed in that time interval. According to paleogeographic reconstructions of Rodinia, the Kalahari Craton would only have joined Laurentia and the RDPC at 1050 Ma. Around 920 Ma there is the generation of an oceanic crust in the Rio de la Plata Craton with the installation of the Charrua Ocean and the beginning of the formation of the Dom Feliciano Belt, concluded at 540 Ma, with peak deformation and metamorphism at 650–630 Ma.

Petrogenesis of scapolite-rich gabbro from the alkaline Cho Don complex in north-eastern Vietnam - mineralogical and geochemical implications

A unique scapolite-rich gabbro has been found in the Cho Don complex (north-eastern Vietnam) and investigated using cathodoluminescence microscopy and spectroscopy, electron-microscopic observations, and Raman micro-spectroscopy. The scapolite, occupying up to ca. 30 vol% of rock matrix, displays distinctive brownish to greenish (S2_activated) luminescence, while its chemical composition is marked by the enrichment in Ca (~Me73–80) and CO2 (~4.60 wt%). Meionitic scapolite is accompanied by such magmatic phases as: clinopyroxenes (partially altered to uralite), oligoclase-andesine, K-feldspars, biotite, as well as fluorapatite and titanite. On the basis of micro-textural observations and phase-relations including interstitial form of scapolite, the high Ca content relative to adjacent Na-rich plagioclase and high-angle contacts between pyroxenes and scapolites, it may be inferred that the investigated Me-scapolite represents a high-temperature igneous phase. The presence of wollastonite, vesuvianite and calcite within the rock matrix gives a clue to the interaction between parental melt and country rocks that could act as a source for the formation of a specific CO2-enriched magma. Furthermore, scapolite breaks down into: (1) intergrowths of Ca-rich plagioclase with worm-like calcite that may reflect isobaric cooling following the high-temperature stage of rock evolution, and (2) prehnite-vuagnatite clusters, which in turn correspond to the late-stage low-temperature hydrothermal alteration. The formation of the latter could be induced by alteration of primary clinopyroxenes due to the release of Ca2+ and Ti4+. According to major and trace element data of whole-rock samples, the scapolite-rich gabbro originated from slightly evolved alkaline melt, but also shows enrichment in LREE relative to HREE ([La/Yb]CN ~ 10.24), as well as strong depletion in Nb-Ta-Ti relative to U and Th content. These geochemical signatures point to the enriched lithospheric mantle source of parental magma, which is likely to have been imprinted by subduction components (possibly sediment-derived melts) and/or crustal contamination.

Evolution of Late Cretaceous to Palaeogene basalt–andesite–dacite–rhyolite volcanic suites along the northern margin of the Ladakh magmatic arc, NW Himalaya, India

This paper describes a comprehensive geochemical study of basalt–andesite–dacite–rhyolite volcanic association in the Khardung volcanic suite along the northern margin of the Ladakh magmatic arc. This volcanic association is outcropped mainly in the segment of the further north of the Khardung village to Khalsar delineating from the Ladakh magmatic arc by ~2 km thick porphyritic sill. The closed association of basalt–andesite–dacite–rhyolite volcanic within a volcanic suite suggests that these rocks may be genetically inter-related and might have derived from the same parental magma source. Felsic lavas (dacite–rhyolite) show SiO2 range from 64.75 to 79.11 wt.%, while intermediate lavas (basaltic andesite–andesite) ranges from 50.80 to 51.81 wt.% with mafic lavas (basalt) span from 53.39 to 62.05 wt.%. These volcanic suites show enrichment in LIL elements (Rb, Ba, Th, U, and Pb) and depletion in Nb, P, and Ti, which can be evident in spider diagrams with pronounced to mild Eu negative anomalies in REE patterns. Previous reports on zircon U–Pb ages of the Khardung volcanics range between 60 and 69.7 Ma confirm an upper bound eruption age of this volcanic suite as pre-collision continental arc magmas. Hence, the results of geochemical modelling suggest that the Khardung mafic–intermediate-felsic lavas were generated from the melting of 1–4% spinel and garnet-bearing lherzolite sources. The generated parental magmas were modified by crustal materials during the magma ascent along with fractional crystallization and were metasomatized by slab-derived fluids released from the subducting Neo-Tethyan oceanic crust during the Late Cretaceous to Palaeogene in the northern margin of the Ladakh magmatic arc.

New Insights into the Evolution and Age of the Neoproterozoic Jebel Ohier Porphyry Copper Deposit, Red Sea Hills, Northeastern Sudan

AbstractNew SHRIMP U-Pb data from dioritic to granodioritic synmineral intrusions associated with the Jebel Ohier porphyry copper deposit (mineral inventory, including NI43-101-compliant total inferred and indicated resources, of 593 million tonnes [Mt] at 0.33% Cu and 0.05 ppm Au, for 1.953 Mt of contained Cu and 933,600 oz of Au at 0.15% Cu cutoff) in the Red Sea Hills of northeastern Sudan have bracketed the age of mineralization to ca. 816 to 812 Ma. This age range, as well as constraints from new and existing lithogeochemical data, is consistent with the deposit’s formation from a productive parental magma source during the early stages in the evolution of an intra-Mozambique Ocean island arc. The Jebel Ohier porphyry copper deposit bears many similarities to well-documented Phanerozoic analogues elsewhere in terms of (1) the mapped style and zonation of hydrothermal alteration (i.e., proximal K-silicate–dominated, to sericitic, to distal propylitic alteration), (2) the occurrence of intense Cu-bearing A- and B-type vein stockwork, as well as sulfide-only C-type veins, anhydrite veins, and younger, peripheral D-type veins, and (3) the geochemical fingerprint of the associated porphyry, which is akin to those of ore-related Tertiary porphyries in the Escondida area in northern Chile. The multiphase intrusion hosting the Jebel Ohier porphyry copper deposit has been intruded by several generations of mafic to felsic postmineralization dikes and voluminous plutons, with a peak in magmatic activity coinciding with the suturing of the Gebeit terrane at ca. 724 Ma. In spite of, or perhaps because of, the occurrence of extensive postmineralization magmatism, and regardless of subsequent deformation, regional metamorphism, uplift, and erosion, the deposit has remained remarkably intact. The discovery of a relatively ancient, yet well-preserved porphyry copper deposit in the Neoproterozoic Arabian-Nubian Shield has major implications for the exploration potential of this resource-rich geologic terrain.

Contemporaneous Paleogene arc-magmatism within continental and accreted oceanic arc complexes in the northwestern Andes and Panama

The basement rocks of the northwestern Andes are composed of accreted, fault bound, slivers of the Cretaceous Caribbean Large Igneous Province (CLIP). Previous geologic mapping, geochronological and geochemical data show that Paleogene arc-like units make up part of the accreted CLIP in SW Colombia (Timbiquí Complex), NW Colombia (Santa Cecilila La Equis Complex), and at the boundary between Colombia and Panama (Acandí Region). Similar Paleogene igneous units have been reported in Ecuador (Silante and Macuchí arc), and Panama (Chagres-Bayano Arc). These arc-like rocks are commonly interpreted to have formed as part of an intra-oceanic arc before accretion to the continental margin. Alternative explanations suggest that the Paleogene igneous rocks in Ecuador were formed within a continental magmatic arc. We compare Paleogene igneous rocks in the San Blas Complex (Chagres-Bayano Arc and Acandí Region), Santa Cecilia la Equis Complex, and the SW Complex (Timbiquí Complex, Silante and Macuchí arcs) using geochemical (major and trace elements), geochronological (zircon U-Pb) and isotopic (zircon Hf, and whole rock Pb, Sr, Nd isotopes) data. Zircon U-Pb analyses yield ages of 46–42 Ma for the Acandí Region, 59–51 Ma for the Santa Cecilia La Equis Complex, and 44–42 Ma for the Timbiquí Complex. Major and trace element data show low-K to calc-alkaline compositions with negative Nb anomaly and LREE/HREE enrichment typically attributed to arc-like signatures, for the three complexes. Pb, Sr, and Nd whole rock and zircon Lu-Hf isotopic data for the Santa Cecilia La Equis and the San Blas complexes, are consistent with a depleted mantle source mixed with Pacific sediments. However, the presence of inherited Proterozoic zircons, elevated 207Pb/204Pb ratios in some evolved igneous rocks, and crystallization under low pressure conditions, suggest assimilation of continental-derived sediments. These data indicate that the Santa Cecilia La Equis and the San Blas complexes likely originated in an intra-oceanic island arc setting, in a close proximity to the South American margin before accretion to the Andean margin. In the case of the SW Complex, the absence of Eu anomaly, slightly depleted HREE, as well as increased Sr/Y values with fractionation, indicate that these magmas formed in a moderately mature arc within a mid-shallow crust level. Moreover, radiogenic Pb and Sr isotopic values for the less evolved igneous rocks of the SW Complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source, and assimilation of the mafic basement crust. This indicates that the Paleogene magmatic rocks of the SW Complex originated in a continental arc-like setting within previously accreted slivers of the CLIP.

Zircon U–Pb geochronology, mineral and whole‐rock geochemistry of the Khardung volcanics, Ladakh Himalaya, India: Implications for Late Cretaceous to Palaeogene continental arc magmatism

In this study, we present new mineral and whole‐rock geochemical data with zircon U–Pb ages of the Khardung volcanics (KV) from the western Himalaya and discuss their tectono‐magmatic evolution. These volcanics are sandwiched between the Ladakh batholith and Karakoram batholith and classified as intermediate volcanics (basaltic andesite‐andesite) and felsic volcanics (dacite‐rhyolite). The intermediate volcanics are marked by low SiO2 (52.80–61.31 wt.%), enriched LREEs, and depleted HFSEs (Nb, Ti, Zr), whereas more evolved felsic volcanics exhibit quartz, K‐feldspar, and plagioclase as dominant mineral phases and felsic compositions are characterized by high SiO2 (64.52–79.19 wt.%) content with pronounced negative Eu anomalies, enriched LREEs, and depleted HREEs and HFSEs (Nb, Ti). New zircon U–Pb ages of intermediate volcanics (andesite) yield 69.71 Ma, whereas felsic volcanics (rhyolites) range between 62.49 and 66.55 Ma, indicating that the Khardung magmatism overlaps with the last phase of the Ladakh batholith magmatism. Geochemical characteristics indicate that the KV were generated from a same parental magma source through fractional crystallization along with crustal assimilation from an older crust, and they show genetic affinity with the adjacent Ladakh batholith. Therefore, the KV and Ladakh batholith could be considered as a product of the mature stage arc magmatism generated during subduction of the Neo‐Tethyan oceanic crust prior to the main collision between the Indian and Eurasian continents.

Geochemistry and zircon U-Pb geochronology of Miocene plutons in the Urumieh-Dokhtar magmatic arc, east Tafresh, Central Iran

ABSTRACT The Tafresh plutons that include Ahmadabab diorite, Vasfonjerd monzonite, Mehrezamin diorite and Chahak diorite, located to the east of Tafresh city, north-central Iran, are part of Urumieh-Dokhtar magmatic arc. U-Pb dating of zircon grains provides emplacement ages of 22.3 ± 1 Ma for the Ahmadabad diorite, and tightly clustered ages of 22.2 ± 0.2 Ma, 21.3 ± 0.2 Ma, and 21.7 ± 0.4 Ma for Vasfonjerd monzodiorite, Mehrezamin diorite-monzonite, and Chahak diorite-monzonite plutons, respectively. These rocks are metaluminous to weakly peraluminous, calc-alkaline, and characterized by enrichment in light rare earth elements, Nb-Ta negative anomalies, and high LILE/HFSE ratios. Tafresh plutonic rocks originated from a parental magma source and experienced different degrees of partial melting. Geochemical signatures of Tafresh plutonic rocks, such as a wide range of Y/Nb (2.7–8.4) and low Zr/Nb (19.5–35.) ratios, Nb/Ta (11.46–18.15), argue for mantle–crust interaction during generation of Tafresh magmas. Relatively low Nb/La ratios further indicate that the lithospheric mantle played a significant role in melt generation. HREE signatures (i.e. decrease Dy/Yb with increasing SiO2) preclude substantial involvement of garnet either in the residue, both during partial melting and fractionation of the magma. The plutons are a product of final stages of subduction-related magmatism prior to the collision between the Arabian and Eurasian tectonic plates.

The Neoproterozoic volcanic complex of the Boumalne inlier (Saghro massif, Eastern Anti-Atlas; Morocco): petrological and geochemical characteristics

Neoproterozoic volcanic series are exposed in the northern edge of the Saghro massif (Eastern Anti-Atlas, Morocco). Four volcanic rock types (basalt, andesite, dacite, and rhyolite) were distinguished in the Boumalne inlier within the so-called Saghro volcanic sequences based on petrographic and geochemical observations. Boumalne volcanic rocks contain high Al2O3, Fe2O3, Ba, Sr, Zr, Rb, and Nb contents, including calc-alkaline affinity in composition. Boumalne volcanic rocks are similar to other lower-Neoproterozoic volcanic rocks such Agouiniy formation in Sirwa inlier and in other parts of Bou-Azzer inlier. Indeed, they indicate an active subduction signature. The geochemical data show a LREE enrichment compared to HREE. The fractional crystallization has played a major role during the evolution of the magma. The less-siliceous dacitic rocks could have been formed after a low degree of partial melting of mafic parental magma source, whereas the rich-siliceous rhyolite may have been derived from dacitic magma source by a higher degree of fractional crystallization.

Garnet-Pyroxenite-Derived End-Member Magma Type in Kamchatka: Evidence from Composition of Olivine and Olivine-Hosted Melt Inclusions in Holocene Rocks of Kekuknaisky Volcano

Late Quaternary volcanoes of Sredinny Range (Kamchatka) attract geoscientists’ attention by their unusual geochemical features and geodynamic setting. They produced volcanic rocks that are enriched relative to N-MORB in most of incompatible trace elements (except HREE), including strong enrichment in large-ion lithophile elements, and show a negative Nb–Ta anomaly, which is typical for rocks formed in supra-subduction settings. However, modern subduction of the Pacific Plate does not reach the most part of Sredinny Range, as inferred by mapping of Wadati–Benioff zone or seismic tomography. We constrain the source of parental magmas for Sredinny Range volcanic rocks by combining major and trace element geochemical data for olivine and naturally quenched olivine-hosed melt inclusions for Holocene tephra layers of the Kekuknaisky field. Composition of the most magnesian olivine (Ni > 2000 ppm, Fe/Mn ≈ 75 at Mg# ~ 84–85 mol %) and geochemical characteristics of the most primitive melts (FC3MS = 0.61 ± 0.04 (2s)) are consistent with their derivation from a pyroxenite source, while elevated LREE/HREE ratios in lavas indicate that it contained garnet. This garnet-bearing pyroxenite likely originated from the lower crust or lithospheric mantle. Its melting could have occurred due to delamination and sinking into the hotter mantle.

Formation of the Permian Taipinggou igneous rocks, north of Luobei (Northeast China): implications for the subduction of the Mudanjiang Ocean beneath the Bureya–Jiamusi Massif

ABSTRACTControversy has long surrounded the tectonic framework and evolution of the Mudanjiang Ocean between the Bureya–Jiamusi–Khanka Massif and Songnen–Zhangguangcai Range Massif, which are located in the easternmost segment of the Central Asian Orogenic Belt. To address these issues, we present zircon U-Pb ages, geochemical data, and zircon Hf isotopic compositions of the Taipinggou amphibolite and metagabbro exposed along the boundary area of Bureya–Jiamusi Massif and Songnen–Zhangguangcai Range Massif. Magmatic zircons from the amphibolite and metagabbro yield 206Pb/238U ages of 267 ± 2 Ma and 264 ± 2 Ma, respectively, which are interpreted as protolith ages. The geochemical data of the amphibolite samples show transitional characteristics of calcalkaline to tholeiitic series, with high MgO concentrations (9.44–10.48 wt.%) and Mg-numbers (73–75). These samples are enriched in large ion lithophile elements (e.g. Rb, Ba, and K) and light rare earth elements and are depleted in high-field-strength elements (e.g. Nb, Ta, and Ti) and heavy rare earth elements, with εHf(t) values of −6.63 to −3.26. It is inferred that the parental magma originated from an enriched lithospheric mantle that had been metasomatized by fluids derived from subducted oceanic slab. During magma evolution, the magma that formed the amphibolite mainly experienced accumulation with a shallow-level evolutionary process involving fractional crystallization. The Taipinggou metagabbro samples are subalkaline series and also characterized by enrichment in large ion lithophile elements (e.g. Rb, Ba, and K) and light rare earth elements and by depletion in Nb–Ta–P–Ti, with εHf(t) values of −3.09 to +1.16. The Taipinggou metagabbro and amphibolite have similar geochemical and Hf isotopic compositions, indicating a common parental magma source but with different degrees of magmatic differentiation. Based on the new geochronological and geochemical data presented in this study, we propose that both the Taipinggou metagabbro and amphibolite formed in a Middle Permian continental arc setting, closely related to eastward subduction beneath the Bureya–Jiamusi Massif. Combined with previous studies and regional geological observations, we suggest that a double-side subduction model is favoured for the Late Palaeozoic–Early Mesozoic geodynamic processes along the boundary area of Bureya–Jiamusi–Khanka Massif and Songnen–Zhangguangcai Range Massif.

The Sr, Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in eNd(T) = 8.6–9.7 and eHf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: eNd(T) = 3.7–6.0, eHf(T) = 11.1–12.7, and T DM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary 87Sr/86Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.

On the role of contamination and hybridism in formation of the composite Raumid pluton granites (Pamir Mnts.): Results of Sm–Nd Isotope Study

This work presents isotope Sm-Nd data obtained for bulk samples of granites of all 8 emplacement phases of the Raumid granite massif, which occurred 35 Ma ago at a hypabyssal depth during the orogenic stage of development of Southern Pamir fold system. The 147Sm/144Nd ratio in studied collection of granite samples ranges between 0.091 and 0.323; the eNd(T) value is–4.0. The Sm-Nd isotope study results suggest that all granite varieties distinguished in the Raumid massif are comagmatic formations and contamination and hybridization processes did not play any role in REE distribution in granites. At this, the source of parental magma did not change during granite generation. We assume that the only process, resulted in the trace element evolution in granites, was differentiation of three batches of magma sequentially uplifted from the source.

Geochemistry of the volcanic rocks from Bioko Island (“Cameroon Hot Line”): Evidence for plume-lithosphere interaction

Bioko Island (3008 m a.s.l) is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine (83% < Fo < 87%) and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase (An62-67Ab35-32Or3-1). Major element variation diagrams show an increase in SiO2, Al2O3, Na2O and K2O with increasing MgO for the studied rock groups. The rocks are characterized by low (86Sr/87Sr)i ratios (0.70320–0.70406), high ɛNd(t) values (2.56–4.33) and high (206Pb/204Pb)i ratios (20.032–20.035) values. Basalts are enriched in LILE and LREE, and have (Hf/Sm)N = 0.57–1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.

Late Devonian–Early Permian A-type granites in the southern Altay Range, Northwest China: Petrogenesis and implications for tectonic setting of “A 2-type” granites

The Altay Range is an important part of the Central Asian Orogenic Belt (CAOB) that contains A-type granites that have been attributed to a post-collisional setting during the Late Carboniferous–Permian. However, our new LA-ICP-MS zircon U–Pb age data demonstrates that there were two episodes of A-type granite magmatism in the southern Altay Range. A Late Carboniferous to Early Permian (308–291 Ma) suite does occur in the vicinity of Qiakuerte but a distinct Late Devonian (382–367 Ma) suite is present in the Kouan area. The early Kouan A-type granites are mainly composed of alkali-feldspar granites, which are approximately synchronous with adakites, boninites, high-Ti basalts, picrites, ophiolitic rocks, and high temperature-low pressure metamorphic rocks, whereas the late Qiakuerte A-type granites mainly consist of arfvedsonite and aegirine-bearing granites, which are approximately coeval with some mafic–ultramafic rocks in the southern Altay Range. A-type granites are commonly classified as A 1 and A 2 sub-types, which are considered to be generated in anorogenic (rift or plume-related) or post-collisional settings, respectively. Both suites of the southern Altay Range are geochemically similar to typical A 2-type granites, e.g., high K 2O + Na 2O, FeO/MgO, and Ga/Al, and Y/Nb values, and low CaO, Ba, Sr, and Eu contents. The Kouan A-type granites have relatively low (La/Yb) N (2.7–5.9), high ε Nd( t) (+6.7 to +7.7) and Nb/La (0.59–1.67), and variable ε Hf( t) (+5.0 to +14.1) values, whereas the Qiakuerte A-type granites have comparatively high (La/Yb) N (4.2–9.2), low ε Nd( t) (+5.3 to +6.0) and Nb/La (0.33–0.74), and variable ε Hf( t) (+10.0 to +18.7) values. The early Kouan A-type granites may have been generated by partial melting of a mafic source containing more depleted mantle-derived components than the late Qiakuerte A-type granites. We suggest that the Late Carboniferous to Early Permian Qiakuerte granites were formed in a post-collisional extensional setting, as is typical of A 2-type granites. However, the Late Devonian Kouan A-type granites were more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction. In this case, the upwelling of asthenosphere through the slab window provided the source of parental magmas, or the heat for the generation of Devonian magmas. Therefore, our results suggest that A 2-type granites can also form in a ridge subduction-related extensional setting.

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals

Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin. This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 33°S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U–Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 28·8–5·2 Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the Nazca–Farallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb)N between 3·6 and 7·0, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range of 18·358–19·023 for 206Pb/ 204Pb, 15·567–15·700 for 207Pb/ 204Pb and 38·249–39·084 for 208Pb/ 204Pb. Initial 87Sr/ 86Sr are mostly in the range of 0·70369–0·70505, with two more radiogenic values at 0·7066. Initial Hf isotopic compositions of zircons yield exclusively positive εHfi ranging between + 6·9 and + 9·6. The newly determined initial isotope characteristics of the Oligo-Miocene magmas suggest that the mantle source lithologies are different from both those of Pacific mid-ocean ridge basalt and ocean island basalt, plotting in the field of reference values for subcontinental lithospheric mantle, characterized by moderate large ion lithophile element–high field strengh element depletion and high 238U/ 204Pb. A Hf model age of 2 Ga is estimated for the formation of the subcontinental mantle–continental crust assemblage in the region, suggesting that the initial Sr and Pb isotope ratios inferred for the source of the Oligo-Miocene parental magmas are the result of later Rb and U enrichment caused by mantle metasomatism. A time-integrated model Rb/Sr of ≈0·039 and μ ≈ 16 are estimated for the source of the parental magmas, consistent with ratios measured in peridotite xenoliths from continental areas. Evolution from predominant (>90%) basaltic–gabbroic to andesitic–dioritic magmas seems to involve a combination of (1) original trace element differences in the metasomatized subcontinental mantle, (2) different degrees of partial melting and (3) fractional crystallization in the garnet- and spinel-peridotite stability fields. The genesis of more differentiated magmas reaching rhyolitic–granitic compositions most probably also includes additional crystal fractionation at both shallow mantle depths and within the crust, possibly leading to some very minor assimilation of crustal material.