Geochemistry of Pliocene/Pleistocene basalts along the Central Anatolian Fault Zone (CAFZ), Turkey

Pliocene–Pleistocene volcanism accompanied strike-slip-related transtensional deformation along the Kızılırmak fault segment of the Central Anatolian fault zone (CAFZ) in the west of Şarkışla (Sivas–central Turkey). These volcanic rocks are represented by alkali olivine basalts. They can be divided into four different sub-groups on the basis of their Zr, Nb, TiO₂ contents. A primitive mantle-normalized incompatible trace element diagram for four subgroups shows close similarity to typical OIB pattern. Some of the incompatible trace element ratios (Ce/Y, Zr/Nb, La/Ba, La/Nb) are also akin to OIB values. Highly fractionated REE patterns (La/Yb_N=24.7–9.2) with no Eu anomaly are the main features of the alkali basalts and are comparable to alkaline volcanism in continental rift zones. On the basis of Al₂O₃/TiO₂, Nb/Y, Zr/Y Zr/Nb ratios, the geochemical differences among four sub-groups can be explained by variable degrees of partial melting of compositionally similar mantle source. Th/Nb, Th/Y, Nb/Y ratios and the primitive mantle-normalized trace element diagram suggests significant amount of crustal involvement for most of the alkali olivine basalts erupted along the CAFZ. Rupture of the continental lithosphere by strike-slip-related transtensional deformation might have caused decompressional partial melting of the asthenospheric mantle and generating alkali olivine basalts in this region.     

Geochemistry of Pliocene/Pleistocene basalts along the Central Anatolian Fault Zone (CAFZ), Turkey

Abstract

Pliocene-Pleistocene volcanism accompanied strike-slip-related transtensional deformation along the K?z?l?rmak fault segment of the Central Anatolian fault zone (CAFZ) in the west of ?ark??la (Sivas-central Turkey). These volcanic rocks are represented by alkali olivine basalts. They can be divided into four different sub-groups on the basis of their Zr, Nb, TiO₂ contents. A primitive mantle-normalized incompatible trace element diagram for four subgroups shows close similarity to typical OIB pattern. Some of the incompatible trace element ratios (Ce/Y, Zr/Nb, La/Ba, La/Nb) are also akin to OIB values. Highly fractionated REE patterns (La/Yb_N=24.7–9.2) with no Eu anomaly are the main features of the alkali basalts and are comparable to alkaline volcanism in continental rift zones. On the basis of Al₂O₃/TiO₂, Nb/Y, Zr/Y Zr/Nb ratios, the geochemical differences among four sub-groups can be explained by variable degrees of partial melting of compositionally similar mantle source. Th/Nb, Th/Y, Nb/Y ratios and the primitive mantle-normalized trace element diagram suggests significant amount of crustal involvement for most of the alkali olivine basalts erupted along the CAFZ. Rupture of the continental lithosphere by strike-slip-related transtensional deformation might have caused decompressional partial melting of the asthenospheric mantle and generating alkali olivine basalts in this region. © 2001 Éditions scientifiques et médicales Elsevier SAS.     

Petrogenesis of the Late Cenozoic collision volcanism in the central part of the Lesser Caucasus (Azerbaijan)

The Late Cenozoic volcanics of the Lesser Caucasus have similar trace-element and REE patterns with negative anomalies of Nb, Ta, Hf, and Zr. They are highly enriched in Rb, Ba, Th, and La and depleted in Ti, Yb, and Y with respect to N-MORB, which indicates their formation from the subduction-metasomatized lithospheric mantle. Partial melting of the subcontinental mantle lithosphere and crustal assimilation and fractional crystallization controlled the magma evolution in the collisional magmatic belts.     

Trace and rare earth elements as indicators of provenance and depositional environments of Lias cherts in Gumushane,NE Turkey

Trace elements and rare earth elements (REEs) of Lias-aged cherts in the Gumushane area were studied in order to understand their origin and depositional environment. Twenty three chert samples from five stratigraphic sections were analysed by inductively coupled plasma-mass spectrometry, X-ray diffraction, and mineralogical investigation. Lias cherts in the study area are microcrystalline, cryptocrystalline quartz, and megaquartz depending on mineralogical content. Trace elements of the cherts were compared with PAAS, Co, Y, and Th had stronger depletions in the five sections, whereas V, Ni, Zr, Nb, and Hf had smaller depletions. The distribution of Zr, Hf, and Ta yields Zr/Hf, Zr/Ta and Hf/Ta ratios (25/645, 37/665, and 0.18/3, respectively) that differ from those of chondrites and average upper continental crust, suggesting that these elements are likely non-detrital but are sourced from seawater. Th/U ratios range from 0.04 to 0.45 and are lower than those of the upper continental crust (average: 3.9). Lias-aged cherts have low total REE abundances and stronger depletions in five sections of the PAAS and chondrite-normalised plots. The cherts are characterised by a positive Eu anomaly (average: 4.9) and LREE-enrichment (La_N/Yb_N = average: 3.5). In addition, about one-half of the cherts exhibit positive Ce anomaly (range: 0.25–2.58), chondritic Y/Ho values (range: 3.3–60), and low (La/Ce)_N values (average: 1.8). REE and trace element abundance in Lias cherts indicate that these elements were likely derived from hydrothermal solutions, terrigenous sources, and seawater. The REE patterns of the cherts show that they were probably deposited close to a continental margin.     

Geochemical Characteristics and LA-ICP-MS Zircon U-Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

Geochemical studies on the arnphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The arnphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N=0.33-0.55, (La/Sm)N= 0.45-0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N-MORB. The second group of rocks have (La/Yb)N=0.63-0.95, (La/ Sm)N = 0.69--0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82, 0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N-MORB and E-MORB but closer to the former. The two groups of rocks both exhibit flat patterns from Th to Yb in th     

Metasomatism of the Pan-African lithospheric mantle beneath the Damara Belt,Namibia, by the Tristan mantle plume: geochemical evidence from mantle xenoliths

In situ trace element analyses of constituent minerals in mantle xenoliths occurring in an alnöite diatreme and in nephelinite plugs emplaced within the central zone of the Damara Belt have been determined by laser ablation ICP-MS. Primitive mantle-normalized trace element patterns of clinopyroxene and amphibole indicate the presence of both depleted MORB-like mantle and variably enriched mantle beneath this region. Clinopyroxenes showing geochemical depletion have low La/Sm_n ratios (0.02–0.2), whereas those showing variable enrichment have La/Sm_n ranging up to 3.8 and La/Yb_n to 9.1. The most enriched clinopyroxenes coexist with amphibole showing similar REE patterns (La/Sm_n = 1.3–4.1; La/Yb_n = 4.5–9). Primitive mantle-normalized trace element patterns allow further groups to be distinguished amongst the variably enriched clinopyroxenes: one having strong relative depletion in Rb–Ba, Ta–Nb and relative enrichment in Th–U; another with similar characteristics but with additional strong relative depletion in Zr–Hf; and one showing no significant anomalies. Amphiboles show similar normalized trace element patterns to co-existing clinopyroxene. Clinopyroxene and amphiboles showing LREE_N enrichment have high Sr and low Nd isotope ratios compared to clinopyroxene with LREE-depleted patterns. Numerical simulation of melt percolation through the mantle via reactive porous flow is used to show that the chromatographic affect associated with such a melt migration process is able to account for the fractionation seen in La–Ce–Nd in cryptically metasomatized clinopyroxenes in Type 1 xenoliths, where melt–matrix interactions occur near the percolation front, whereas REE patterns in clinopyroxenes proximal to the source of metasomatic melt/fluid match those found in modally metasomatized Type 2 xenoliths. The strong fractionation between Rb–Ba, Th–U and Ta–Nb shown by some cryptically metasomatized xenoliths can be also accounted for by reactive porous flow, provided amphibole crystallizes from the percolating melt/fluid close to its source. The presence of amphibole in vein-like structures in some xenoliths is consistent with this interpretation. The strong depletion in Zr–Hf in clinopyroxene and amphibole in some xenoliths cannot be accounted for by melt migration processes and requires metasomatism by a separate carbonate-rich melt/fluid. When taken together with published isotope data on these same xenoliths, the source of metasomatic enrichment of the previously depleted (MORB-like) sub-Damaran lithospheric mantle is attributed to the upwelling Tristan plume head at the time of continental breakup.     

Neoarchean crust-mantle interactions in the Yishui Terrane,south-eastern margin of the North China Craton: Constraints from geochemistry and zircon U-Pb-Hf isotopes of metavolcanic rocks and high-K granitoids

The Neoarchean Yishui Terrane (YST) is situated in the east of Western Shandong Province (WSP), south-eastern margin of the North China Craton (NCC). The metavolcanic rocks of the YST are fine-grained hornblende plagioclase gneisses (Group #1) and fine-grained amphibolites (Group #2) in the Yangzhuangzhen area and fine- to medium-grained amphibolites (Group #3) in the Leigushan area. The high-K granitoids associated with Groups #1 and 2 are dominated by fine- to medium-grained monzogranitic gneisses. Zircon LA-ICP-MS U-Pb dating reveals that the magmatic precursors of Groups #1 and #2 were formed at 2641 Ma and the magmatic precursors of concomitant monzogranitic gneisses were emplaced from 2615 to 2575 Ma, whereas Group #3 represents a later 2500 Ma volcanic eruption, and all these metamorphic volcanic rocks and monzogranitic gneisses were subjected to subsequent 2470–2460 Ma metamorphism.The metamorphic volcanic rock samples in Group #1 exhibit the chemical compositions of calc-alkaline andesites, showing fractionated chondrite-normalized REE patterns ((La/Yb)_N = 10.48–19.30) and negative Nb, Ta and Ti anomalies ((Nb/La)_PM = 0.13–0.22), which are akin to those of typical high-Mg andesites (HMAs) in the subduction-related settings. The magmatic precursors of the Group #1 samples were derived from partial melting of a fluid- or melt-metasomatized depleted mantle wedge at deep levels in the upper mantle. Samples in Group #2 show calc-alkaline chemical compositions with less fractionated chondrite-normalized REE patterns ((La/Yb)_N = 2.24–3.34) and negative Nb, Ta and Ti anomalies ((Nb/La)_PM = 0.47–0.76), which are consistent with those of the volcanic rocks in the Aleutian island arc. The magmatic precursors of Group #2 were generated by partial melting of a fluid-metasomatized depleted mantle wedge at shallow levels in the upper mantle. The monzogranitic gneisses exhibit high SiO₂ and K₂O contents with high-K calc-alkaline affinities and peraluminous characteristics. Based on their distinct HREE contents and chondrite-normalized REE patterns, these granitoid samples are subdivided into low-Yb monzogranitic gneisses (LYMGs) and high-Yb monzogranitic gneisses (HYMGs). The LYMG magma was derived from partial melting of a mixed source of juvenile two-mica pelites and minor basic-intermediate igneous rocks at lower crustal levels with pyroxene + amphibole + garnet as the main residual phases, and the HYMG magma was derived from partial melting of multi-sourced juvenile two-mica pelites at middle to lower crustal levels with pyroxene + amphibole and subordinate plagioclase and garnet as the main residual phases. In addition, Group #3 resembles tholeiitic back-arc basalts in the Okinawa Trough and displays flat chondrite-normalized REE patterns ((La/Yb)_N = 1.22–2.08) and slightly negative Nb and Ta anomalies ((Nb/La)_PM = 0.35–0.59). This group was most likely derived from partial melting of a depleted mantle source that had been modified by the addition of subducted slab-derived fluids at shallow levels in the upper mantle. These metavolcanic rocks and concomitant high-K granitoids record important Neoarchean crust-mantle interactions involving the first modification and partial melting of the lithospheric mantle induced by oceanic crust subduction; then, upwelling and underplating of mantle-derived magmas triggered partial melting of the middle to lower crust and mixing between crust- and mantle-derived magmas. These processes imply that Neoarchean crust-mantle interaction played a crucial role in the evolution of the southeastern margin of the NCC.Available whole-rock Sm-Nd and zircon Lu-Hf isotopic data from metamorphic volcanic rocks and plutonic granitoids from this study and previous studies reveal that YST experienced three crucial juvenile crustal growth events from ~2.78–2.69 Ga, ~2.64–2.56 Ga and ~2.54–2.50 Ga.     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Peridotite xenoliths in alkali basalts from the Sikhote-Alin, southeastern Siberia, Russia: trace-element signatures of mantle beneath a convergent continental margin

Abundant spinel peridotite xenoliths occur in late Cenozoic alkali basaltic rocks in the Sikhote-Alin region at the Pacific margin of the Asian continent. Major- and trace-element compositions of representative peridotite xenolith are documented for four occurrences located in different structural units of the continental margin. In each locality, the majority of xenoliths have distinctive microstructures, modal and chemical compositions that are typical for a given xenolith suite. Significant textural and compositional differences between the four xenolith suites suggest that the upper mantle beneath the Sikhote-Alin consists of distinct domains with contrasting composition. The inferred large-scale mantle heterogeneities may be due to juxtaposition of lithospheric blocks of different provenance during accretion of the Sikhote-Alin to the Asian continent.

Trace-element patterns of the xenoliths and their minerals obtained ICP-MS technique provide evidence of depletion and enrichment events and indicate contrasting behaviour of REE, HFSE and other incompatible trace elements. The HFSE behave non-concordantly, in particular, some xenoliths have highly fractionated Zr/Hf, Ti/Zr, Nb/Ta, La/Nb and U/Th ratios relative to their values in the primitive mantle. The fractionated compositions may be related to the interaction of evolved subduction-related fluids and melts with lithospheric mantle at the Mesozoic-early Cenozoic active continental margin or to metasomatism during later continental rifting.     

Sr–Nd isotope geochemistry and tectonomagmatic setting of the Dehsalm Cu–Mo porphyry mineralizing intrusives from Lut Block,eastern Iran

The Dehsalm Cu–Mo-bearing porphyritic granitoids belong to the Lut Block volcanic–plutonic belt (central eastern Iran). These rocks range in composition from gabbro-diorite to granite, with dominance of monzonites and quartz monzonites, and have geochemical features of high-K calc-alkaline to shoshonitic volcanic arc suites. Primitive mantle-normalized trace element spider diagrams display strong enrichment in large-ion lithophile elements such as Rb, Ba and Cs and depletions in some high-field strength elements, e.g., Nb, Ti, Y and HREE. Chondrite-normalized plots display significant LREE enrichments, high La_N/Yb_N and a lack of Eu anomaly. High Sr/Y and La/Yb ratios of Dehsalm intrusives reveal that, despite their K-rich composition, these granitoids show some resemblances with adakitic rocks. A Rb–Sr whole rock–feldspar–biotite age of 33 ± 1 Ma was obtained in a quartz monzonite sample and coincides, within error, with a previous geochronological result in Chah-Shaljami granitoids, further northwest within the Lut Block. (⁸⁷Sr/⁸⁶Sr)_i and εNd_i isotopic ratios range from 0.70481 to 0.70508 and from +1.5 to +2.5, respectively, which fits into a supra-subduction mantle wedge source for the parental melts and indicates that crustal contribution for magma diversification was of limited importance. Sr and Nd isotopic compositions together with major and trace element geochemistry point to an origin of the parental magmas by melting of a metasomatized mantle source, with phlogopite breakdown playing a significant role in the geochemical fingerprints of the parental magmas; small amounts of residual garnet in the mantle source also help to explain some trace element patterns. Geochemical features of Dehsalm porphyries and its association with Cu–Mo mineralization agree with a mature continental arc setting related to the convergence of Afghan and Lut plates during Oligocene.     

Isotopic and trace-element indications of lithospheric and asthenospheric components in Tertiary alkalic basalts, northeastern Brazil

Plate reconstructions at 30 Ma place northeastern Brazil over the Fernando de Noronha hotspot, presently 250 km offshore northeastern Brazil. Tertiary basaltic centers in northeastern Brazil may therefore be products of the Fernando de Noronha hotspot. We examined alkalic basalt from seven 30–13 Ma old centers in Rio Grande do Norte and Pernambuco states to assess this possible link. Compositions are primitive, where MgO concentrations range from 12.5 to 17 wt.%, and SiO₂ from 41 to 48 wt.%. Trace-element abundances and Sr, Nd, and Pb isotopic compositions compare well with those of ocean island basalt: =0.7038–0.7051, =0.51266–0.51280, and =18.52–19.35. There are correlations among SiO₂-undersaturation, incompatible-element abundances, relative percentages of partial melting (based on La/Yb and La/Y ratios), and the degree of isotopic ‘enrichment' inherited from mantle sources. There is also a negative correlation for La/Nb (0.6–0.86) vs. Ba/Nb (6.6–10.6), where lower La/Nb samples represent larger percentages of melting of a source comparatively enriched in radiogenic Sr. We attribute these compositional relationships to the lavas representing mixing of melts mainly from asthenosphere of largely HIMU plus DM characterization, probably the Fernando de Noronha plume, with melts from subcontinental lithosphere that was isotopically closer to EM1. Isotopic and trace-element compositions of the northeastern Brazil basalts are generally similar to those of Fernando de Noronha lavas (12–2 Ma), and some minor trace-element differences observed (e.g., more Zr, Nb, and less Th compared to northeastern Brazil basalts) are probably due to heterogeneity within the asthenospheric plume and to melt contributions from delaminated subcontinental lithosphere that may underlie Fernando de Noronha.     

Geochemistry of the Desur lavas,Deccan traps: Case study from the vicinity of Belgaum,Karnataka and their petrogenetic inferences

Geochemical characteristics of Desur-type basalt flows in the southern and southwestern part of Belgaum in Karnataka, India have been investigated to understand their petrogenesis. The basalts are compact, hard, massive, and show characteristic microporphyritic textures with abundant well-twinned and un-twinned plagioclase phenocrysts and minor clinopyroxene set in a fine-grained groundmass consisting of plagioclase, clinopyroxene, glass and Fe-Ti oxides. Thin sections show sub-ophitic, intergranular and intersertal textures. The basalts are Fe-rich tholeiites (13.4–13.8 wt %), characterized by high TiO₂ (3.64 to 3.94 wt %); moderate MgO contents (4.79 to 5.41 wt %), low K₂O contents (<0.58 wt %) and low Mg# (42.4–45.9). They are enriched in large ion lithophile elements, moderately enriched in the light rare earths (chondrite-normalized La_N/Yb_N 3.37–4.24), and exhibit nearly flat heavy rare-earth patterns that lack significant Eu anomalies (Eu/Eu* 0.86–1.10). Primitive-mantle-normalized element patterns for these rocks show characteristic troughs at K and Sr, absence of a Nb anomaly, and a low Zr/Nb ratio (<15), which suggest insignificant contamination by many types of continental crust, whereas, enrichments in the large ion lithophiles, La, P and Th could suggest enriched source characteristics. Based on the geochemical characteristics of the basalts, it is inferred that the Desur basalts representing the youngest flows of the Deccan Basalt Group are derived by partial melting of a peridotite source, and subsequent fractionation gave rise to the compositions of the basalts that are found in the Belgaum region.     

Geochemistry of mafic dikes in the Singhbhum Orissa craton: implications for subduction-related metasomatism of the mantle beneath the eastern Indian craton

Mafic dikes, which transect the Mesoarchaean Singhbhum Granitoid Complex, are the most abundant members of the Newer Dolerite dikes of the Singhbhum Orissa craton. These dikes are subalkaline and exhibit a tholeiitic differentiation trend. Studied dikes underwent fractional crystallization of clinopyroxene and plagioclase. They show enriched patterns for the light rare earth elements (LREE) and large ion lithophile elements (LILE). On primitive mantle-normalized multi-element patterns, they possess Ba, Nb, Sr, P, and Ti depletions similar to subduction-related basaltic rocks. The high (La/Yb) _n and (Gd/Yb) _n ratios suggest that the studied mafic dikes were derived by low degrees of partial melting of a garnet-bearing source. Judging by trace elemental ratios (e.g. Ba/Y, Nb/Y, Ba/Th and Th/Nb), the studied dikes were derived from a mantle source metasomatized by a subduction component (e.g. fluids derived by dehydration of the subducting slab). We conclude that interaction between these fluids and the overlying mantle was the main cause of (LREE and LILE) enrichment and Nb (high field strength elements) depletion in the mafic dikes.     

Early Miocene adakite-like volcanism in the Balkuyumcu region, central Anatolia, Turkey: Petrology and geochemistry

The Balkuyumcu region, located in the southwestern part of Ankara in the Izmir-Ankara suture zone (central Anatolia, Turkey), consists of basic andesitic, andesitic, dacitic and rhyolitic rocks extruded during the Early Miocene (20–22 Ma) as a result of post-collisional volcanism. Balkuyumcu volcanic rocks can be divided into two groups on the basis of their mineralogy and composition: The basic andesitic (BA) and andesitic, dacitic and rhyolitic (ADR) groups. The ADR and BA group of rocks have adakite-like and calc-alkaline characteristics, respectively. The ADR group has higher SiO₂ content, Sr/Y and La/Yb ratios and low MgO, Mg#, Y and Yb contents than the BA group. Both groups have nearly the same Sr, Nd isotopic compositions and display similar normalized multi-element patterns with enrichments in LILE and LREE, depletions in Nb, Ti, Zr, P and a lack of Eu anomalies. Major, trace element and Sr, Nd isotopic data indicate that both groups of rocks were derived from the same source but affected by different magmatic processes during ascent. The adakite-like rocks may have been produced by partial melting of thickened lower continental crust. Fractional crystallization also played a major role in their formation. However, the BA group rocks were derived from partial melting of lower continental crust that was probably delaminated. These rocks appear to have had limited interaction with mantle peridodite during ascent to the surface.     

Geochemical constraints on the tectonic setting of basaltic host rocks to the Windy Craggy Cu-Co-Au massive sulphide deposit,northwestern British Columbia

Windy Craggy is an approximately 300 Mt Cu-Co-Au volcanogenic massive sulphide (VMS) deposit in northwestern British Columbia, Canada. The Windy Craggy deposit is hosted by the Middle Tats Volcanics (MTV), a Late Triassic volcano-sedimentary sequence of intercalated mafic pillowed to massive volcanic flows and sills and calcareous argillite that are part of the Alexander terrane. The host footwall and hangingwall flows and sills are predominantly alkalic basalts (Nb/Y > 0.70). MTV alkali basalts at Windy Craggy are enriched in light rare earth elements (LREEs) >100X chondrite compared to chondrite, have steep REE patterns [(La/Yb)_cn = 7.1–25.4], and generally lack the Ta and Nb depletions relative to primitive mantle (e.g. [Nb/Th]_pm = 0.68–1.94) characteristic of arc environments, although most have [Nb/La]_pm < 1. By contrast, volcanic rocks away from the deposit (and regionally; Lower Tats Volcanics, LTV) as well as late dikes that cross-cut all lithologies including metamorphic and deformational fabrics are sub-alkalic tholeiitic to calc-alkaline basalts and basaltic andesites that are less enriched in the LREEs (10–100X chondrite), have less steep REE patterns [(La/Yb)_cn = 0.41–10.6], and show well-developed Ta and Nb depletions (arc signatures; [Nb/Th]_pm = 0.20–0.79), consistent with formation in an oceanic arc environment. The co-occurrence of tholeiitic/calc-alkaline arc rocks with alkalic rocks indicates that the LTV (former) and MTV (latter) formed from melts that were influenced to varying degrees by subducted oceanic crust, and likely formed within a back-arc basin setting formed on a rifted oceanic arc. There is no geochemical or isotopic evidence for major involvement of continental crust. The LTV basalts likely were produced by progressive depletion in the source by partial melting of mantle overlying the subducting oceanic crust. The presence of the MTV alkalic Windy Craggy rocks overlying the LTV is consistent with the presence of a slab-window, perhaps related to subduction of a spreading centre, which allowed more enriched magmas to reach the surface with only minimal interaction with subduction-modified mantle. The presence of this slab-window might have provided the mechanism for the generation of anomalously high heat flow close to the seafloor, which initiated and sustained vigorous, long-lived hydrothermal activity necessary for the precipitation of large accumulations of massive sulphide. To our knowledge, this is the first example of a large VMS deposit associated with a slab-window.     

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H₂O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).     

Tectonic significance of Triassic mafic rocks in the June Complex,Sanandaj–Sirjan zone,Iran

This study concentrates on the petrological and geochemical investigation of mafic rocks embedded within the voluminous Triassic June Complex of the central Sanandaj–Sirjan zone (Iran), which are crucial to reconstruct the geodynamics of the Neotethyan passive margin. The Triassic mafic rocks are alkaline to sub-alkaline basalts, containing 43.36–49.09 wt% SiO₂, 5.19–20.61 wt% MgO and 0.66–4.59 wt% total alkalis. Based on MgO concentrations, the mafic rocks fall into two groups: cumulates (Mg# = 51.61–58.94) and isotropic basaltic liquids (Mg# = 24.54–42.66). In all samples, the chondrite-normalized REE patterns show enrichment of light REEs with variable (La/Yb)_N ratios ranging from 2.48 to 9.00, which confirm their amalgamated OIB-like and E-MORB-like signatures. Enrichment in large-ion lithophile elements and depletion in high field strength elements (HFSE) relative to the primitive mantle further support this interpretation. No samples point to crustal contamination, all having undergone fractionation of olivine + clinopyroxene + plagioclase. Nevertheless, elemental data suggest that the substantial variations in (La/Sm)_PM and Zr/Nb ratios can be explained by variable degrees of partial melting rather than fractional crystallization from a common parental magma. The high (Nb/Yb)_PM ratio in the alkaline mafic rocks points to the mixing of magmas from enriched and depleted mantle sources. Abundant OIB alkaline basalts and rare E-MORB appear to be linked to the drifting stage on the northern passive margin of the Neotethys Ocean.     

The primary geochemistry of the monaro alkaline volcanics,southeastern Australia — evidence for upper mantle heterogeneity

The Mg numbers [100 Mg/(Mg+Fe) atomic ratios] of the Tertiary Monaro alkaline volcanics in southeastern Australia indicate that many of these alkali basalts, basanites and nephelinites have undergone only limited crystal fractionation, and that a few may represent unmodified, unfractionated primary magmas. Fractionation involves essentially olivine and clinopyroxene; fractionation trends are identified by plotting trace-element abundances against Mg number, and are then extrapolated linearly back into the primary magma field to yield estimates of the primary geochemistry of the three rock types.The nephelinites, basanites and alkali basalts are interpreted as a partial melting series derived from a peridotitic upper mantle. The estimated primary abundances of Pb,Th, Ga, V, Cr and Ni are essentially the same for the three rock types, but the estimated primary abundances of K₂O, Rb, Sr, Ba, TiO₂, Zr, Nb, P₂O₅, La Ce, Pr Nd, Y, Cu, and Zn-overlap considerably between the three rock types and the coherence of the incompatible elements as a group is not preserved in the overlaps. These patterns are best accounted for by postulating a patchy distribution of accessory phases such as amphibole, mica and apatite, in the source regions for the Monaro volcanics. Heterogeneities of this kind allow local variation in the volume of partial melt generated at the solidus, and offer a possible solution to magma segregation problems in the upper mantle.     

From back-arc to rifted margin: Geochemical and Nd isotopic records in Neoproterozoic?-Cambrian metabasites of the Bystrzyckie and Orlickie Mountains (Sudetes,SW Poland)

Metamorphosed during the Variscan orogeny, sediments of the ca. 560 Ma M?ynowiec Formation and ca. 530 Ma Stronie Formation in the Bystrzyckie and Orlickie Mountains (Central Sudetes, Poland) contain metabasites with a range of basaltic compositions. Immobile trace element and Nd isotope features allow distinction of dominant, either E-MORB-like (Group 1: Zr/Nb 9–20, εNd₅₃₀ +2.6 to +6.7) or mildly enriched N-MORB-like tholeiites (Group 2: Zr/Nb 21–27, εNd₅₃₀ +0.2 to +6.7), and scarce but genetically important OIB-like alkaline (Group 3: Zr/Nb 5, εNd₅₃₀ +2.2) or depleted tholeiitic rocks (Group 4: Zr/Nb 67, εNd₅₃₀ +7.9). Neither the radiogenic age nor age relationships between these four groups are known. However, field evidence suggests that the metabasites are younger than the M?ynowiec Formation and that their emplacement must have been coeval with the accumulation of the Stronie Formation sediments. The OIB affinity of Group 3 is interpreted to reflect an enriched mantle (EM)-type asthenopheric source whilst the groups of tholeiitic rocks indicate involvement of depleted (locally slightly residual) MORB-type mantle (DMM). Several geochemical signatures, the decoupling between Nd isotope and trace element characteristics, and melting models indicate variable enrichment of the DMM-like source, here ascribed to asthenosphere-derived OIB-like melts (Group 1 and 2) and a contribution from a supra-subduction zone (Group 2 and 4). Based on contrasting back-arc basin (BAB)- and within-plate-like affinities of the metabasites, and on petrogenetic constraints from the spatially related infill of the Stronie Formation rift basin, the studied magmatic episode is suggested be related to cessation of the supra-subduction zone activity, presumably induced by ridge-trench collision. This event might have led to slab break-off, the development of a transform plate boundary, opening of a slab window and upward migration of sub-slab enriched asthenosphere. Decompression melting of the upwelling asthenosphere could then have produced OIB-like melts which segregated and infiltrated into the mantle of the former subduction zone, with randomly distributed slab-derived components. In an extensional regime, magmas generated at shallow levels from heterogeneous mantle regions were emplaced within sedimentary rocks of the overlying rift basin. The vestiges of subduction-related processes and within-plate style of mantle enrichment suggest that the metabasites could be related to final stages of the Cadomian orogeny and incipient Early Palaeozoic rifting of Gondwana that heralded the opening of the Rheic Ocean.     

A geochemical study of magmatism associated with the initial stages of back-arc spreading

Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading.Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with Ce_N/Yb_N = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although Ce_N/Yb_N ratios and ⁸⁷Sr/⁸⁶Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher ⁸⁷Sr/⁸⁶Sr (0.7039) and Ce_N/ Yb_N (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present ⁸⁷Sr/⁸⁶Sr ratios.Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.