Assimilation And Fractional Crystallization Research Articles

Multistage Evolution of Dolerites in the Karoo Large Igneous Province, Central South Africa

EarlyJurassic sheet-like intrusions (sills and dykes) are abundant in the Karoo Basin in South Africa, and were emplaced as a part of the Karoo Large Igneous Province. Here we discuss the evolutionary history of dolerite sills and dykes in different parts of the basin on the basis of new major and trace element analyses of dolerite samples collected from drill-cores (five sites spanning 1700 m of basin stratigraphy) and previously published data on sills and dykes in the Golden Valley Sill Complex (GVSC). In addition, we present Sr^ Nd isotope data for selected samples.The dolerites are subalkaline tholeiitic basalts and basaltic andesites characterized by enriched trace element patterns, variable degrees of depletion in Nb^Ta relative to light rare earth elements, negative to positive Pb anomalies, and mild to moderate enrichment in initial Sr^Nd isotopic ratios. The aim of this study is to unravel the evolutionary history of the melts that gave rise to the dolerites. We propose that the primary melts were derived from sub-lithospheric mid-ocean ridge basalt (or ocean island basalt) source mantle and had acquired a weak subduction signature (relative depletion in Nb^Ta, mildly enriched Sr^Nd isotopic ratios) through interaction with metasomatized lithospheric mantle. In the deep crust the magmas underwent assimilation and fractional crystallization (AFC) processes involving up to 10% assimilation of granulites with strong arc-type geochemical signatures. The AFC processes may alternatively have taken place in the uppermost mantle. Distinct geochemical characteristics among the GVSC and drill-core units reflect different amounts of AFC. During and/or after intrusion into the sedimentary rocks in the Karoo Basin the magmas underwent a second stage of fractional crystallization (50^60%) and local contamination by their sedimentary wall-rocks. High U concentrations and U/Th ratios in some dolerites in the southwestern part of the Karoo Basin were probably caused by fluids released from shales rich in organic material (e.g. Ecca Group shales) during devolatilization and contact metamorphism. Contamination in a GVSC unit may reflect interaction withTa^Th^U-rich minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or fluids that have interacted with such rocks. Considering that continental flood basalts are emplaced through continental crust and sedimentary basins, it is likely that other LIPs have similar evolutionary histories to that proposed for the Karoo Basin.

The Chogat-Chamardi subvolcanic complex, Saurashtra, northwestern Deccan Traps: Geology, petrochemistry, and petrogenetic evolution

The Saurashtra region in the northwestern part of the Deccan continental flood basalt province (India) is notable for its many volcano-plutonic complexes, compositional diversity, and the abundance of rhyolite and granophyre. In this first detailed study of the Chogat-Chamardi subvolcanic complex in eastern Saurashtra, we report microgranite and granophyre plutons, gabbro plutons cut by basaltic andesite dykes, as well as rhyolite plugs. Bulk-rock geochemical (including Nd–Sr isotopic) data suggest a strong contribution from ancient, Rb-rich basement crust to the silicic magmas. The mafic rocks are also crustally contaminated but less than the silicic rocks, in conformity with observations elsewhere (e.g., the Skye and Skaergaard granophyres). The Chogat-Chamardi silicic rocks have initial 87Sr/ 86Sr (for t = 65 Ma) as high as 0.72914, and initial ε Nd values as low as −13.9. Trends defined by the Chogat-Chamardi and other Deccan rhyolitic suites on plots of Sr concentration vs. Sr isotopic ratio are modeled with concurrent assimilation and fractional crystallization (AFC) processes involving a basaltic magma and granitic basement rocks. Considering both Nd and Sr isotopic variations, the Chogat-Chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of Deccan rocks.

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

Measurements of 238U– 230Th– 226Ra disequilibria, Sr–Nd–Pb–Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr–Nd–Pb–Hf isotopes ( 87Sr/ 86Sr = 0.70319–0.70322; 143Nd/ 144Nd = 0.51302–0.51305; 206Pb/ 204Pb = 19.04–19.06; 207Pb/ 204Pb = 15.53–15.54; 208Pb/ 204Pb = 38.61–38.65; 176Hf/ 177Hf = 0.28311–0.28312). All the samples possess ( 230Th/ 238U) disequilibrium with 230Th excesses, and they show systematic variations in ( 230Th/ 232Th) and ( 238U/ 232Th) ratios. The highest 226Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have ( 226Ra/ 230Th) ratio that are close to equilibrium. The 238U– 230Th– 226Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of 230Th and 226Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent–daughter nuclides in 230Th– 238U and 226Ra– 230Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.

Formation and evolution of silicic magma plumbing system: Petrology of the volcanic rocks of Usu volcano, Hokkaido, Japan

The whole-rock composition of the historical rocks of Usu volcano, which has erupted nine times since AD 1663, has changed from rhyolite (SiO 2 = 75.8 in AD 1663) to dacite (SiO 2 = 69.1 in AD 2000). Temporal changes in the composition of the eruptive magma have been explained as due to repeated withdrawals from a single, zoned magma chamber. Nevertheless, new petrographical and geochemical data suggest that the juvenile materials are composed of three groups: Group-1 is nearly aphyric hornblende–augite-bearing hypersthene rhyolite (SiO 2 = 74.3–75.8) from the 1663 eruption; Group-2 is porphyritic augite–hornblende–quartz-bearing hypersthene rhyodacite (SiO 2 = 70.5–72.7) from four eruptions during the 18–19th centuries; and Group-3 is slightly porphyritic, augite-bearing hypersthene dacite (SiO 2 = 69.1–70.5) from three eruptions in the 20th century. All the rocks show wide compositional distributions of phenocrystic minerals and coexistence of normally and reversely zoned phenocrysts, which suggest that the rocks originated from magma mixing. Many variation diagrams of the whole-rock chemistry show distinctive mixing trends with possible compositional gaps. The groups differ—especially Group-3 from Group-1 and Group-2—not merely in terms of several oxide–oxide diagrams of matrix glass chemistry, but also in terms of SiO 2 vs. Sr isotopic ratios of the whole-rock chemistry. These geochemical features suggest that the end-member magmas of each group are different from those of the other groups. Felsic end-member magmas, which have been voluminous, could not have been formed from a single magma by fractional crystallization or assimilation and fractional crystallization (AFC) processes: the felsic end-member magma in the magma plumbing system must have been replaced twice, and the three resulting magmas could have been active ever since. Compositional variations of these felsic magmas could have been caused by partial melting of the same source by different degrees. During the 350 years of volcanic activity, the temperature and degree of melting of the magma progressively increased from Group-1 to Group-3, as reflected by successive melt segregations from partially melted crustal materials by a single heat source.

Origin of the Gangdise (Transhimalaya) Permian arc in southern Tibet: Stratigraphic and volcanic geochemical constraints

AbstractThe well‐studied Mesozoic and Cenozoic volcanic rocks of the Gangdise Terrane, southern Tibet, are widely interpreted to have resulted from subduction of the Neotethys; however, Late Paleozoic volcanic rocks and their tectonic setting remain poorly studied. Based on new geological data, we carried out stratigraphical and geochemical analyses of Permian volcano‐stratigraphic sequences within an east–west‐trending, fault‐bounded zone of uplift in the central Gangdise Terrane. Sedimentary rocks in this area consist of platform carbonates and terrigenous clastic rocks that represent widespread shallow‐marine sedimentary basins developed around northern Gondwana. A regression or tectonic uplift event is recorded in Permian sedimentary rocks that show the local development of fluvial environments. The sedimentary succession contains evidence of two volcanic stages: a period of basaltic extrusions and younger explosive felsic magmatism. The first volcanic stage is Early and Middle Permian in age. Tholeiitic basaltic lavas are exposed around Maizhokunggar (Tangjia) and Lhunzhub in central Gangdise. The Lower Permian basalts are relatively enriched in MgO (4.58–12.19%), whereas the Middle Permian basalts are characterized by high Al2O3 contents (11.75–21.22%). Rocks of both ages are enriched in large‐ion lithophile elements (LILE) and light rare earth elements (LREE), and show pronounced negative Nb and Ta anomalies. Total REE contents and light (LREE)/heavy (HREE) ratios increased from the Early to Middle Permian. Observed variations in initial Sr, Nd, and Pb isotopes (87Sr/86Sri = 0.7013–0.7066, 207Pb/204Pbi = 15.53–15.63, and 208Pb/204Pbi = 38.04–38.64 for a given 206Pb/204Pbi; εNd = +0.69 to −11.55) can be explained by crustal interaction with mantle sources, as is characteristic of metasomatism by slab‐derived fluids or assimilation and fractional crystallization (AFC) processes during magmatic evolution. The observed geochemical signatures, coupled with stratigraphic constraints, support the hypothesis that an initial arc formed during the Permian due to southward subduction of the Paleotethys, predating the well‐known Mesozoic arc preserved in the Gangdise Terrane.

Petrology and geochemistry of Mesozoic granitic rocks from the Nansha micro-block, the South China Sea: Constraints on the basement nature

There are several micro-blocks dispersed in the South China Sea (SCS), e.g., Xisha-Zhongsha block, Nansha block and Reed-Northeastern Palawan block, etc., but detailed petrological constraints on their basement nature were previously lacking. The magmatic ages for granitic rock samples from two dredge stations in the Nansha micro-block vary from 159 to 127 Ma, which are comparable to magmatic activities occurred in the northern margin (Pearl river mouth), HongKong and East China. Petrographic characteristics, major-, trace element and Sr–Nd isotopic data of nine samples from two dredged station performed in the Nansha micro-block, the SCS, are reported. Petrographically, these granitic rocks can be divided into two groups which underwent a complex history of magmatic process, i.e., tonalitic rock (Group I) and monzogranitic rock (Group II). The Rittmann index ( σ) for these rocks (1.9–3.1) suggest that they belong to calc-alkaline rocks. Group I rocks which is of typical I-type, have higher contents of TiO 2, Al 2O 3, FeO, MgO, CaO, Na 2O and P 2O 5, but lower values of SiO 2 and K 2O, when compared with those of Group II with I-type characteristics. Group I rocks are produced by partial melting of older Precambrian basement with the variable influence of mantle-derived magma which results from the interaction of released fluids from the subducted slab and the overlying mantle wedge in a general convergent margin setting, and Group II rocks result from partial melting of lower crustal basic rocks (amphibolite) and/or further partial melting of the Group I rocks associated with the variable influence from the underplating mantle-derived magma resulting from lithospheric extensional regime. Both Groups I and II have undergone assimilation and fractional crystallization (AFC) processes during its petrogenesis. This study therefore demonstrates that there exists a continental basement within micro-blocks in the South China Sea, and further supports the idea that a Middle Jurassic to Mid-Cretaceous subduction zone existed across the temporary Taiwan, Palawan to Southern Vietnam, which was associated with westward to northestward convergence of the Pacific Plate during Late Mesozoic. We suggest that this subduction zone may have been connected with the paleo-Pacific plate subduction zone offshore eastern China during Mesozoic era. This study provides petrologic data for the pre-Cenozoic tectonic evolution of the South China Sea.

Geochemistry and petrogenesis of basaltic rocks from the Develidağ volcanic complex, Central Anatolia, Turkey

The Develidağ volcanic zone (DVZ) is a part of the Middle Miocene-Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia. The DVZ is located at the eastern side of the Plio-Quaternary Sultansazlığı pull-apart basin, that opened along the Ecemiş left-lateral strike-slip fault. Towards the south, the volcanic rocks of the DVZ overlie the Paleozoic metamorphic rocks of the Taurus range. Regional stratigraphic studies indicate that volcanic rocks of the DVZ crop out in a ∼N–S trending rectangular-shaped exposure in the middle of which Middle Miocene andesitic rocks are found surrounded by Upper Miocene basaltic rocks. The purpose of this paper is to present basic geochemical data for the DVZ rocks and discuss possible processes of magma generation. The Develidağ basalts are characterized by low LILE (Rb, K, Ba, Th) and high HFSE (Nb,Zr,Hf,Y) contents, whereas the andesites generally have high LILE and HFSE values except for Nb and Zr. Variable abundances of Pb (3.11–12.09 ppm) and U (0.36–2.64 ppm) are associated with high Ba content within the rock suites. Although low Nb/La (0.6–0.7) and relatively high Ba/Nb ratios indicate crustal involvement for the basalts, high Zr/Ba (0.5), Zr/Hf (42–47) and Th/U (3.13–4.69) values imply contributions from an asthenospheric source component. Furthermore, the high Zr/Hf values (>36) are the diagnostic feature of metasomatized mantle (Dupuy et al., 1992; Rudnick et al., 1993). Moreover, multi-element patterns show that Develidağ basalts have similar trace element signatures to those of the US Cascades tholeiites. The ratios of Zr/Hf, Zr/Ba, Nb/Th and Sr/Ce indicate that basaltic rocks are derived from a MORB-like mantle, and calculated melting model reflects generation from a spinel peridotite source (3–4% melting), but the combined effects of melting and assimilation and fractional crystallization (AFC) processes seem to be partially responsible for the relatively evolved rocks. Typical tholeiitic-calk-alkaline associations are also observed among the volcanic rocks as a consequence of melting processes and crustal involvement, or due to the changes in the melting rate and degree. The lack of alkaline rocks that is conventionally observed during the final stage of many central Anatolian volcanic centers may be attributed either to the lack of extensional development or to a deficient amount of extension before the late Miocene.

Geochemistry and 40Ar/ 39Ar geochronology of Miocene volcanic rocks from the Karaburun Peninsula: Implications for amphibole-bearing lithospheric mantle source, Western Anatolia

New geochemical and 40Ar/ 39Ar age data are presented from the Neogene volcanic units of the Karaburun Peninsula, the westernmost part of Western Anatolia. The volcanic rocks in the region are associated with Neogene lacustrine deposition and are characterized by (1) olivine-bearing basaltic-andesites to shoshonites (Karaburun volcanics), high-K calc-alkaline andesites, dacites and latites (Yaylaköy, Armağandağ and Kocadağ volcanics) of ~ 16–18 Ma, and (2) mildly-alkaline basalts (Ovacık basalt) and rhyolites (Urla volcanics) of ~ 11–12 Ma. The first group of rocks is enriched in LILE and LREE with respect to the HREE and HFSE on N-MORB-normalised REE and multi-element spider diagrams. They are comparable geochemically with volcanic rocks in the surrounding regions such as Chios Island and other localities in Western Anatolia. The Ovacık basalt is geochemically similar to the first stage early–middle Miocene volcanic rocks but differs from NW Anatolian late Miocene alkali basalts. Trace element models indicate that the Kocadağ and Armağandağ volcanics were produced from the Yaylaköy volcanics by assimilation and fractional crystallization (AFC). Melting models of several trace elements of the most primitive mafic volcanics indicate that their source was metasomatized by subduction-related fluids and was amphibole-bearing garnet–lherzolitic.

Trace element modelling of the origin and evolution of the Oyaca‐Boyalik volcanic rocks (NE of Haymana, Ankara, Turkey)

Lower Miocene Boyalik volcanic rocks, situated approximately 80 km south of Ankara, exhibit both alkaline and calc‐alkaline characteristics. Alkaline products are trachybasaltic and trachyandesitic, whereas calc‐alkaline products are dacitic. The phenocrysts in the dacites consist primarily of plagioclase and hornblende, with lesser amounts of biotite. The groundmass contains plagioclase and quartz microcrysts. Trachyandesites are mainly composed of plagioclase and biotite phenocrysts with a groundmass of alkali feldspar microlites and minor clinopyroxene microcrysts. Trachybasalts are mainly composed of olivine and plagioclase phenocrysts, with minor clinopyroxene phenocrysts associated with alkali feldspar, plagioclase and clinopyroxene microlites and microcrysts in the groundmass. Oxides are common accessory phases in all products. Boyalik volcanic rocks have essentially homogeneous incompatible trace element patterns with variable Nb and Th anomalies, enrichment in Rb, Ba, K, La, Ce and Nd, and positive Sr anomalies. Some trace element ratios (e.g. Ba/Ta, Ba/Nb, Th/U and Ce/Pb) are variable among the series. For instance, dacites and trachyandesites have higher Ba/Ta (724–2509), Ba/Nb (45–173) and Th/U (3.5–8.7) and lower Ce/Pb (7.1–3.9) values than the trachybasalts. Trace element data indicate that the series are chemically distinct but probably were derived from a common lithospheric mantle source via variable degrees of partial melting. The magmas then underwent a process of evolution involving assimilation and fractional crystallization (AFC) during ascent to the surface. Although trachyandesites and dacites were generated from a lithospheric mantle source via ∼1% and ∼1.5% to ∼5% degrees of partial melting, respectively, trachybasalts were derived from the same source via higher degrees of partial melting (∼20%) with neglegible crustal contamination. Boyalik volcanism is linked to an intracontinental transpressional setting. However, the overall geochemical features are consistent with derivation from a mantle source that records earlier Eocene subduction between the Sakarya continental fragment and the Kırşehir block during time.

Zuni–Bandera volcanism, Rio Grande, USA — Melt formation in garnet- and spinel-facies mantle straddling the asthenosphere–lithosphere boundary

The Zuni–Bandera Volcanic Field (ZBVF) is a late-Neogene volcanic field on the boundary of the stable Colorado Plateau and the active Rio Grande Rift. Alkalic and tholeiitic magmas have erupted through Proterozoic continental crust with the tholeiitic magmas having undergone shallow-level fractional crystallization of olivine ± clinopyroxene ± spinel. The alkaline–tholeiitic lava flows lack elemental and isotopic correlations usually indicative of concomitant crust assimilation and fractional crystallization (AFC) and appear to have inherited their geochemistry from sub-Moho depths. Consideration of isotopic data, and modelling of REE and U–Th data constrains the mantle-melting history. The tholeiite basalts are primarily spinel-facies mantle melts (95–100% for spinel-facies), whereas the alkali basalts have a much higher proportion of garnet-facies mantle melts (15–25% for all samples except QBO 607 assumed to be 100% garnet-facies). The increased contribution from garnet-facies mantle in the alkali basalts is supported by U–Th isotopic data, where a shift towards 230Th excess is observed. Since the lithospheric thickness increases from 45–55 km beneath the Rio Grande Rift, to 120–150 km beneath the Colorado Plateau, the ZBVF volcanic rocks are most likely a mixture of asthenosphere-derived (garnet-facies) alkali basalts and lithosphere-derived (spinel-facies) tholeiitic basalts. This is further supported by Sr–Nd isotopic data with alkali basalts having isotopic compositions similar to depleted-mantle values ( 87Sr/ 86Sr = 0.702986 to 0.70378, 143Nd/ 144Nd = 0.512712 to 0.512978), while the tholeiite basalts have higher 87Sr/ 86Sr (0.704725 to 0.706003) and lower 143Nd/ 144Nd (0.512379 to 0.512913) typical of basaltic magmas derived from ancient, LREE-enriched lithospheric mantle. Overall, melting is observed to be polybaric in nature, with mixing of melts over an extended depth range with the development of melting columns that span (a) the garnet-to spinel-facies phase boundaries in the mantle, and, (b) the asthenosphere–lithosphere boundary.

Petrochemistry of the south Marmara granitoids, northwest Anatolia, Turkey

Post-collision magmatic rocks are common in the southern portion of the Marmara region (Kapidag, Karabiga, Gonen, Yenice, Can areas) and also on the small islands (Marmara, Avsa, Pasalimani) in the Sea of Marmara. They are represented mainly by granitic plutons, stocks and sills within Triassic basement rocks. The granitoids have ages between Late Cretaceous and Miocene, but mainly belong to two groups: Eocene in the north and Miocene in the south. The Miocene granitoids have associated volcanic rocks; the Eocene granitoids do not display such associations. They are both granodioritic and granitic in composition, and are metaluminous, calc-alkaline, medium to high-K rocks. Their trace elements patterns are similar to both volcanic-arc and calc-alkaline post-collision intrusions, and the granitoids plot into the volcanic arc granite (VAG) and collision related granite areas (COLG) of discrimination diagrams. The have high 87Sr/86Sr (0.704–0.707) and low 143Nd/144Nd (0.5124–0.5128). During their evolution, the magma was affected by crustal assimilation and fractional crystallization (AFC). Nd and Sr isotopic compositions support an origin of derivation by combined continental crustal AFC from a basaltic parent magma. A slab breakoff model is consistent with the evolution of South Marmara Sea granitoids.

On the onset and evolution of the Ross-orogeny magmatism in North Victoria Land — Antarctica

An extensive geochemical (major-, trace-element and oxygen isotope) and geochronological (U/Pb geochronology on zircon) characterisation of Ol-bearing ultramafic–mafic cumulates and gabbro-diorites recently discovered in the southwestern region of the Wilson Terrane (North Victoria Land — Antarctica) was undertaken in order to constrain source characteristics, magma evolution and emplacement history in this sector of the Cambro–Ordovician Ross Orogeny. Textural and chemical data indicate disequilibria among mineral phases of these Ol-bearing cumulate rocks. Corroded Cr-rich clinopyroxene with high Mg# (0.82) and Ca-rich plagioclase likely represent xenocrysts from an old magmatic system, and are not in equilibrium with the host cumulate assemblage (Ol + Opx + Amp + Bt + Pl). The calculated liquid in equilibrium with the xenocrysts (“melt 1”) exhibits exceptionally high La/Yb ratios and Th–U concentrations, suggesting a strong sediment influx in the mantle source, possibly from the subducted slab. Because of the extensive fractional crystallisation (Fo 70; Mg# = 76) and crustal contamination (e.g., δ 18O Opx = 7.13–7.47‰) the trace element composition of the liquid in equilibrium with the cumulate assemblage (“melt 2”) does not reveal the nature of the mantle source not the differentiation processes. The gabbroic parental liquid for the main cumulate assemblage was not produced by assimilation and fractional crystallisation (AFC) of the equilibrium “melt 1” calculated from the xenocrystic paragenesis. Rather, it likely represents a new pulse of magma originated from a different mantle source, which then evolved through AFC into gabbro-diorites. Zircon U–Pb dating of one gabbro-diorite yields a crystallisation age of 489 ± 4 Ma. Zircon in the Ol-bearing cumulates yielded two ages populations at 521 ± 2 Ma and 502 ± 3 Ma, which we interpret to represent the actual age difference between the two magmatic systems. This age difference suggests that poorly differentiated melts with adakite-type signature intruded the crust prior to the generation of the large volumes of gabbroic and dioritic magmas. These early pulses represent up to now the oldest proof of subduction related mantle melts in North Victoria Land and predate the diffuse igneous activity dominated by intermediate- to felsic products.

40Ar– 39Ar age, geochemistry and Sr–Nd–Pb isotopes of the Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic complex, SW China

The Lengshuiqing Cu–Ni sulfide deposit of the Yanbian area in Sichuan Province, SW China, is hosted by relatively small mafic–ultramafic bodies that intruded Mesoproterozoic metavolcanic rocks and schists. The mafic–ultramafic complex is mainly composed of the olivine-hornblende pyroxenite and hornblende gabbro units. 40Ar/ 39Ar age, elemental and Sr–Nd–Pb isotopic results of the complex are reported in this paper. 40Ar/ 39Ar dating of hornblende yielded an age of 821 ± 1 Ma for the mafic–ultramafic complex. This age is similar to the SHRIMP U–Pb zircon age of 825 ± 12 Ma for the adjacent Gaojiacun mafic–ultramafic complex. This suggests that the Lengshuiqing mafic–ultramafic complex originated from Neoproterozoic mafic magma. The olivine-hornblende pyroxenite unit is characterized by high MgO contents (24.25–28.08%), Mg # values (74.5–78.0) and compatible elements (Cr = 1179–2336 ppm, Ni = 732–1303 ppm), low ΣREE (19.5–44.6 ppm) and other incompatible element contents. The hornblende gabbro unit has MgO = 7.32–12.51%, Mg # = 55.0–69.3, Cr = 37.7–1131 ppm, Ni = 50.0–243 ppm and ΣREE = 37.2–75.4 ppm. Most of the rocks are moderately enriched in LREE ((La/Yb) N = 2.28–7.67), and significantly depleted in Nb–Ta ((Nb/La) PM = 0.17–0.47; (Ta/La) PM = 0.19–0.58), with slightly moderately positive Eu anomalies ( δ Eu = 0.95–1.65). The trace element feature of the ore-bearing samples is similar to that of the rocks in the complex. The variations in major and trace elements can be well explained by magma crystal fractionation. Isotopically, the rocks in Lengshuiqing mafic–ultramafic complex are characterized by low 87Sr/ 86Sr(i) (0.7039–0.7044), positive ɛ Nd(i) (+2.2 − +4.3) and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.382–17.913, 207Pb/ 204Pb(i) = 15.518–15.578, 208Pb/ 204Pb(i) = 37.227–38.019). The ores also have positive ɛ Nd(i) (+2.4 − +5.4), and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.256–17.733, 207Pb/ 204Pb(i) = 15.484–15.555, 208Pb/ 204Pb(i) = 36.924–37.665). All the samples exhibit positive ɛ Nd(i) values, suggesting that their parental magmas were derived from a long-term depleted asthenospheric mantle. The relatively large variations in 87Sr/ 86Sr(i) of the ores (0.7012–0.7046) could be ascribed to the interaction between magma and country rocks. A good positive correlation between Nb/La and Nb/Th reveals that crustal assimilation was involved in their genesis. This contamination resulted in the observed depletion of Nb and Ta in rocks and ores. The crustal contamination and fractional crystallization, similar to the process of magmatic assimilation and fractional crystallization (AFC), are thus critical for the formation of the Lengshuiqing Cu–Ni ore deposit. The Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic intrusions formed in an intraplate continental rift, which was most likely related to a mantle plume underneath the supercontinent Rodinia.

The petrogenesis of the Apollo 14 high-Al mare basalts

In this paper, we report analysis of various basaltic lunar samples including 14053 and 14072, KREEP basalt 15386, thirty basalt clasts from Apollo 14 breccia 14321, as well as impact-generated samples (matrix from breccia 14168, olivine vitrophyres 14321,1180 and 14321,1539, and impact melt 14310) using a combination of solution and laser ablation inductively coupled plasma mass spectrometry (ICP-MS). The basalt clast samples were previously analyzed by instrumental neutron activation. On plots of incompatible trace elements (ITEs) vs. compatible trace elements, the Apollo 14 high-Al basalts form three approximately subparallel trends that, on the basis of current data, are also separated by age. Plots of ITE ratios (i.e., Nb/Ce vs. Zr/Y) can be used to indicate source composition, and also divide the basalts into three groups: Group A (~4.3 Ga); Group B (~4.1 Ga); and Group C (~3.9 Ga). New data for 14072 suggest the sample does not fit with any of the three groups defined here, and may indicate the presence of a fourth group of high-Al basalts in the proximity of the Apollo 14 site. The Apollo 14 high-Al basalts are compositionally distinct from known Apollo 14 impact melts and impact-generated lithologies. The three groups cannot be related by varying degrees of partial melting of a single, KREEP-contaminated source and, therefore, require three separate source regions. The new data indicate that Group A basalts evolved through closed-system crystal fractionation. However, the new data from basalts forming Groups B and C require open-system evolution that involves combined assimilation and fractional crystallization (AFC). Unlike previous AFC modeling of the Apollo 14 high-Al basalts, an assimilant composed of KREEP is not sufficient to generate the compositional ranges of each basalt group. The modeling of both groups requires a mixture of KREEP and granite as the assimilant, which supports the notion of a genetic relationship between these two lunar components.

Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from 238U– 230Th– 226Ra systematics

In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469–2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show 238U– 230Th– 226Ra disequilibria with excess 238U and 226Ra, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest ( 230Th/ 232Th) ratio compared to older Miyakejima eruptives, yielding the youngest 238U– 230Th model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a ( 230Th/ 232Th)–( 238U/ 232Th) diagram and (2) a positive linear correlation in a ( 226Ra/ 230Th) 0 − 1/ 230Th diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of 230Th and 226Ra, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in ( 230Th/ 232Th) and ( 226Ra/ 230Th) 0 ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr–Nd–Pb isotopic systems. In such cases, Th isotopes and ( 226Ra/ 230Th) ratio are excellent geochemical tracers of magmatic evolution.

Post-collisional, Potassic and Ultrapotassic Magmatism of the Northern Tibetan Plateau: Constraints on Characteristics of the Mantle Source, Geodynamic Setting and Uplift Mechanisms

Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east–west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-related magmas, and range in SiO2 from 44 to 66 wt %. There is a relative enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) in the most primitive magmatic rocks (MgO >6 wt %) in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in the North Qiangtang terrane than in the Songpan–Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India–Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5–10%) compared with those in the Songpan–Ganzi and North Kunlun terranes (<2%). The degrees of partial melting required to generate the primitive potassic and ultrapotassic magmas from the enriched mantle sources range from <0·1% to ∼15% in the three major basement terranes. Energy-constrained AFC model calculations show that the more evolved magmatic rocks (MgO <6 wt %) are the results of AFC processes in the middle crust in the North Qiangtang terrane and the upper crust in the Songpan–Ganzi and North Kunlun terranes. We propose that the ultimate driving force for the generation of the post-collisional potassium-rich magmatism in north Tibet is the continuous northward underthrusting of the Indian continental lithosphere following India–Asia collision. This underthrusting resulted in upwelling of hot asthenosphere beneath north Tibet, squeezed up between the advancing Indian lithosphere and the backstop of the rigid Asian continental lithosphere. Asthenospheric upwelling may have also contributed to uplift of the northern Tibetan Plateau.

Crustal assimilation in basalt and jotunite: Constraints from layered intrusions

To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that F, the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying r , the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr 0 (initial 87Sr/ 86Sr) and decreases in whole-rock εNd 0 (initial εNd). The r -values of the layered sequences range from ∼0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the bulk crust/ magma ratio, reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the instantaneous crust/ magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions. Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions. In the Hasvik Intrusion ( r = 0.27), the initial temperature of the country rocks was ∼450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion ( r = 0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion ( r = 0.12), the country rocks were cooler (∼300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced r -values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions. Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high r -values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.

Continental back-arc basin origin of some ophiolites from the Eastern Desert of Egypt

Geochemical and petrographical data of three ophiolitic pillow metavolcanic occurrences from the central Eastern Desert of Egypt are presented. The investigated rocks show a subalkaline, tholeiitic affinity. Chemical data indicate that the metavolcanics have transitional within-plate basalt to island-arc basalt features, which are characteristics of basalts formed in ensialic back-arc basins. The association of the investigated ophiolites with volcanoclastic metasedimentary rocks of marine to continental facies is a further confirmation of their ensialic evolution. This suggestion, along with the geochronologic, isotopic and crustal growth rate evidences, revives interest in models that involve contribution from a pre-Pan-African continental crust at least in the southern part of the Egyptian Shield. Mixing between a depleted mantle-derived magma and an enriched crustal melt, a process similar to AFC (assimilation and fractional crystallization), is suggested for the evolution of the investigated rocks. This study provides evidence for formation of some ophiolites in the Eastern Desert of Egypt in continental (ensialic) back arc basins.

Geochemical Constraints on the Petrogenesis of Cenozoic I-Type Granitoids in Northwest Anatolia, Turkey: Evidence for Magma Generation by Lithospheric Delamination in a Post-Collisional Setting

Extensive magmatic activity followed major plate convergence and Eocene continent-arc collision in northwest Anatolia, Turkey. This produced a considerable volume of eruptive products, as well as a large proportion of plutonic bodies along the suture zone. The main plutonic bodies in northwest Anatolia are exposed in the Armutlu, Kapidag, and Lapseki peninsulas where they form an E-W trending magmatic belt that comprises three individual intrusive suites. The plutonic bodies intrude Paleozoic—Mesozoic metamorphic basement rocks overthrust by Upper Cretaceous—Tertiary ophiolite fragments. Petrographic and geochemical characteristics of these three plutons are remarkably similar, indicating that the magmas that formed each of them were generated from the same source and experienced similar petrogenetic processes. The plutons are generally calc-alkaline, metaluminus, and I-type, and range in composition from hornblende-monzogranite and granite to granodiorite. The rocks are characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). They follow assimilation and fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source that had been modified by earlier metasomatic events. The chemical characteristics of the plutonic rocks are remarkably similar to those of subduction-related or active continental margin granites. Subduction-related dehydration metasomatism may therefore be inferred as the likely process that modified the mantle wedge source region from which the initial melts were generated. Late orogenic delamination of either the subducting slab or the lowermost part of the mantle lithosphere (e.g., the thermal boundary layer) and concomitant rise of hot mantle asthenosphere appears to be a suitable explanation of the heat that triggered partial melting within the metasomatized part of the mantle lithosphere after subduction waned

Cenozoic Adakite‐type Volcanic Rocks in Qiangtang, Tibet and Its Significance

Abstract Volcanic rocks in the study area, including dache, trachyandesite and mugearite, belong to the intermediate‐acid, high‐K calc‐alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that the rocks are characterized by SiO2&gt;59%, enrichment in Al2O3 (15.09–15.64%) and Na2O (&gt;3.6%), high Sr (649–885 μg/g) and Sc, low Y contents (&lt;17 μg/g), depletion in HREE (Yb&lt;1.22 μg/g), (La/Yb)N&gt;25, Sr/Y&gt;40, MgO&lt;3% (Mg#&lt;0.35), weak Eu anomaly (Eu/Eu*=0.84‐0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd and Sr isotope data (87Sr/86Sr=0.7062‐0.7079, 143Nd/144Nd=0.51166‐0.51253, εNd = −18.61‐0.02), show that the magma resulted from partial melting (10%‐40%) of newly underplated basaltic lower crust under high pressure (1–4 GPa), and the pedogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will give an insight into the uplift mechanism of the Tibetan plateau.