Subduction Component Research Articles

A Late Oligocene Suprasubduction Setting in the Westernmost Mediterranean Revealed by Intrusive Pyroxenite Dikes in the Ronda Peridotite (Southern Spain)

AbstractContrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margin...

The Late Triassic Kataev volcanoplutonic association in western Transbaikalia, a fragment of the active continental margin of the Mongol-Okhotsk Ocean

The Kataev volcanoplutonic association has been recognized in western Transbaikalia. It unites the volcanosedimentary rocks of the Kataev Formation and associated granites localized within the lower plates of the Buteel-Nuur and Zagan metamorphic-core complexes. The rocks of the Kataev association are dynamometamorphosed to different degrees, which is due to the tectonic exposure of metamorphic-core complexes in the Early Cretaceous. The U-Pb zircon dating of the Kataev Formation rhyolites yielded an age of 226 ± 3 Ma. The U-Pb zircon age of the granites intruding the Kataev Formation rocks is 223.4 ± 5.0 Ma. The volcanics of the Kataev Formation belong to the subalkalic basalt-andesite-dacite-rhyolite series. The trachybasalts and trachyandesite-basalts of the Kataev Formation have geochemical characteristics of igneous rocks formed as a result of subduction, e.g., they show distinct negative Nb and Ti and positive Ba and Sr anomalies on multielemental patterns. The specific composition of mafic volcanics points to their formation through the melting of a mantle source resulted from the mixing of depleted mantle and subduction components. Trachyandesites have higher Th and U contents than basaltoids. They can result from the contamination of a mantle source, similar in composition to the Kataev Formation basaltoids, with crustal material. The felsic volcanics of the Kataev Formation and granites intruding them show nearly identical geochemical characteristics corresponding to both A-and I-type granites. These rocks might have formed through the melting of a moderately water-saturated magmatic source of diorite-tonalite composition at 742–833°C. We have established that the rocks of the Kataev volcanoplutonic association in western Transbaikalia and Northern Mongolia formed in the Late Triassic synchronously with the calc-alkaline granitoids of the Henteyn–Daurian batholith and the alkali granites and bimodal volcanic associations of the Kharitonovo and Tsagaan-Hurtey volcanoplutonic associations. The synchronous formation of volcanoplutonic associations of normal and high alkalinity agrees with the geodynamic setting of the Andean-type active continental margin existing in the area of present-day western Transbaikalia and Northern Mongolia in the Early Mesozoic. This setting was the result of the subduction of the Mongol-Okhotsk oceanic plate beneath the Siberian continent.

Tectonomagmatic Constraints on the Sources of Eastern Mediterranean K-rich Lavas

Potassium-enriched mafic lavas, and in particular basalts of the high-K series, are important end-members of subduction zone volcanism. Two petrogenetic models can explain the generation of mafic K-rich lavas, involving either melting of ancient, enriched lithospheric mantle sources (single-stage model) or melting triggered by recent refertilization by subduction-related components derived from subducted sediments or oceanic crust (multi-stage model). These two models are tested for post-collisional high-K rocks of Eocene-Oligocene age from the Rhodopes, SE Bulgaria, based on new major element, trace element and Sr-Nd-Hf-Pb isotope data. The single-stage model is evaluated by Sr-Nd isotope modelling assuming the presence of ancient enriched lithospheric domains whereas the multi-stage model is assessed by comparing the compositions of the Bulgarian lavas with those of lavas from Santorini. Santorini lavas are considered to sample the current trace element and isotope inventory of the long-lived Aegean subduction zone system. This northward facing system has been active most probably since late Jurassic or early Cretaceous times and was potentially involved in refertilizing the mantle sources of the Bulgarian lavas. In addition to the data for Bulgarian lavas, we present new major element, trace element and Sr-Nd-Hf-Pb isotope data for Santorini. These are evaluated together with previously published data to infer the mode of source enrichment in the Aegean realm. The Bulgarian lavas exhibit a broad range of compositions from medium-K to high-K and shoshonitic with radiogenic Sr-87/Sr-86 (0.706-0.709), and unradiogenic epsilon Nd (-5.7 to -1.9) and epsilon Hf (-3 to +3) isotope signatures. The trace element budget was apparently well buffered against shallow-level crustal assimilation as documented by the trace element and Sr-Nd isotope systematics. Modelling of the Sr-Nd isotope compositions of the Bulgarian lavas argues for recent (Mesozoic to Cenozoic) source enrichment. Therefore single-stage models involving melting of ancient (>1 Ga) lithospheric mantle can be confidently ruled out, in agreement with tectonic models for the region. The enriched isotope signatures, together with a pronounced enrichment of incompatible elements, rather indicate mantle refertilization by a subduction component similar to continent-derived sediments subducted at the Hellenic Trench at present. The Bulgarian lavas record a predominant influx of subduction fluid components in comparison with the Santorini lavas. Collectively, the data presented for the Bulgarian lavas are clearly in favour of a multi-stage model.

Geochemistry of mafic dykes from the Southeast Anatolian ophiolites, Turkey: Implications for an intra-oceanic arc–basin system

The Late Cretaceous–Tertiary accretionary prism in Eastern Turkey includes several ophiolitic megablocks and/or tectonic slivers (Mehmetalan, Mollatopuz and Alabayir) within a mélange complex, mainly comprising harzburgite, dunite and cumulate-textured gabbro. The diabases, which are the main focus of this study, cut across the ophiolites as parallel and variably thick dyke-swarms. Geochemistry of the diabases reveals three distinct groups, including a) supra-subduction zone (SSZ) type, which is characterized by marked Nb-anomaly and normal mid-ocean ridge basalt (N-MORB) like HFSE distribution, b) enriched MORB (E-MORB) type, showing some degree of enrichment relative to N-MORB, c) oceanic-island basalt (OIB) type with characteristic hump-backed trace element patterns, coupled with fractionated REE distribution. Among these groups, SSZ- and E-MORB-type signatures are acquired from the Mehmetalan and Mollatopuz suites, whereas OIB-type characteristics are found in the Alabayir suite. The melting models indicate involvement of both depleted and enriched sources for the genesis of the studied dykes. The close spatial relationship, similar ages (based on Ar–Ar dating) and the presence of variable subduction component displayed by Mehmetalan and Mollatopuz suites may indicate melt generation in an intra-oceanic SSZ within the southern branch of Neotethys. In spite of the solely OIB-like character of the Alabayir suite, the similar age obtained from these dykes may suggest their formation in a similar SSZ setting. Alternatively, the Alabayir suite may have represented an oceanic island or seamount formed in an intra-plate setting with or without plume influence. We suggest that decompression melting triggered by slab roll-back mechanism during the closure of the southern branch of the Neotethys during the Late Cretaceous may have been the main process that led to generation of magmas of both depleted and enriched characteristics.

Major and trace element and Sr-Nd isotope signatures of the northern Lau Basin lavas: Implications for the composition and dynamics of the back-arc basin mantle

[1] We present new major element, trace element and Sr-Nd isotope analyses of volcanic glasses from Mangatolu Triple Junction (MTJ), Peggy Ridge (PR), Rochambeau Bank (RB), and Niuafo'ou Island (NF) within the northern Lau Basin (NLB). Lavas from MTJ range from tholeiitic basalts to basaltic andesites and andesites: such a lava series can be ascribed to fractional crystallization. Lavas from NF, RB and PR are mainly tholeiitic basalts save for two transitional basalts from RB. The lavas came from a compositionally heterogeneous mantle that exhibits compositional features similar to those of the mantle source of Indian mid-ocean basalt and bears the influence of both subduction and ocean island basalt (OIB) components. The subduction components consist mainly of fluid dehydrated from subducted oceanic crust and a minor amount of sediment melt. The geochemically enriched signature of enriched RB, PR and NF lavas comes from two OIB end-member components, most likely derived from enriched Samoan mantle plume materials leaking into NLB. The Rochambeau Rifts (RR)-RB corridor receives the greatest, although still variable, influence from the mantle source of Samoan shield magmatism whereas the outlying PR and NF regions experience a Samoan plume post-erosional type of magmatism. The relatively recent mixing of Samoan plume materials with the subduction-metasomatized Indian-type mantle may be responsible for some of the observed complex relationships between noble gases and other geochemical tracers in some NLB lavas.

Subduction zone dynamics in the SW Pacific plate boundary region constrained from high-precision Pb isotope data

This study presents high-precision Pb isotope data obtained by MC-ICP-MS for lavas from the Solomon island arc that covers ca. 1000 km of the SW Pacific plate boundary. Following a reversal of subduction polarity, the presence of the old subducted Pacific oceanic crust, and the presently subducting Indian–Australian plate beneath the entire island arc is confirmed by geophysical and geochemical evidence. New high-precision Pb isotope data now permit to distinguish more clearly between Indian and Pacific-type subduction components in this complex plate tectonic constellation. The values range from 206Pb/ 204Pb = 18.345 to 18.845, 207Pb/ 204Pb = 15.498 to 15.545, and 208Pb/ 204Pb = 38.120 to 38.372, indicating the absence of pelagic sediments in the sub-arc mantle. These data reveal a predominance of subduction components derived from ancient subduction of the Pacific plate. In contrast, lavas with Indian-type Pb isotope signatures are confined to the central New Georgia Group of the Solomon Islands, where the young Woodlark Ridge spreading centre is currently subducting. All other Solomon arc segments with a lower geothermal gradient have apparently not yet been overprinted by younger subduction components over the past 6 Myrs. Moreover, trace element and Pb isotope data for Woodlark Ridge basalts show that subduction components from the fossil Pacific slab have locally modified the source of Woodlark Basin lavas. Altogether, our study highlights the potential of high-resolution Pb isotope measurements to distinguish between Indian and Pacific-type subduction components along the SW Pacific plate boundary.

Crustal homogenization revealed by U–Pb zircon ages and Hf isotope evidence from the Late Cretaceous granitoids of the Agaçören intrusive suite (Central Anatolia/Turkey)

Geochemical and isotopic evidence from the Agacoren Igneous Association in central Anatolia-Turkey indicates that this suite of calc-alkaline granitic rocks have undergone crustal homogenization during regional metamorphic and related magmatic events. Whole-rock chemical and Sr–Nd isotopic data of the granitoids reveal crustal affinity with an earlier subduction component. Zircons show inherited cores and subsequent magmatic overgrowths. The laser ablation ICP-MS 206Pb/238U zircon ages are determined as 84.1 ± 1.0 Ma for the biotite-muscovite granite, 82.3 + 0.8/−1.1 Ma for the hornblende-biotite granite, 79.1 + 2.1/−1.5 Ma for the granite porphyry dyke, 75.0 + 1.0/−1.0 Ma for the alkali feldspar dyke, and 73.6 ± 0.4 Ma for the monzonite. This is interpreted as continuous magma generation, possibly from heterogeneous sources, from ca. 84 to 74 Ma during the closure of the northern branch of the Neotethyan Ocean. The oldest granitoids (84–82 Ma) were probably formed due to crustal thickening after obduction of the MORB-type oceanic crust onto the Tauride-Anatolide microplate. The younger granitoids are interpreted to be related to the subsequent post-collisional extension after lithospheric delamination. Combination of the laser ablation ICP-MS zircon Lu–Hf isotope data with the U–Pb ages of inherited cores suggests that Cretaceous granitoids formed by melting of heterogeneous crustal protoliths, which results in significant variation in eHf(t) data (from −12.9 to +2.2). These protoliths were probably composed of reworked Early Proterozoic crust, minor juvenile Late Proterozoic magmatic components, and Paleozoic to pre-Late Cretaceous recycled crustal material. Moreover, the Late Cretaceous zircon domains of the different granitoids are characterized by a crustal signature, with a relatively restricted zircon eHf(t) data ranging from −4.1 to −8.8. This variation is only about twice the reproducibility (ca. ±1 eHf) of the data, but much smaller than the isotope variability of inherited zircons. Our preferred interpretation is effective isotopic homogenization of the heterogeneous central Anatolian crust during the Late Cretaceous high-grade metamorphic and magmatic events, a process that we propose to be relevant for other active continental margins.

Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.

The origin of decoupled Hf–Nd isotope compositions in Eoarchean rocks from southern West Greenland

Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu–Hf and Sm–Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf–Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800Ma old tholeiitic metabasalts and gabbros from the ∼3700Ma and ∼3800Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial εHf–εNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (−0.7 to +6.3 and −0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm–Nd and Lu–Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium–Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm–Nd space, indicating early disturbance of the Sm–Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the εHf–εNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction components. However, the most depleted samples still exhibit decoupled Hf–Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial Hf–Nd compositions between southern West Greenland TTGs and the metabasalts analyzed here. This overlap suggests a genetic relationship between these lithologies, where the TTGs may have inherited their unusual Hf–Nd compositions from mafic precursors isotopically similar in composition to the Isua tholeiites.

Geoquímica e petrogênese de metabasitos do grupo Votuverava (Terreno Apiaí, Cinturão Ribeira Meridional): evidências de uma bacia retroarco calimiana

The Votuverava Group (ca. 1500 - 1400 Ma), which is part of the Apiai Terrane, a segment of the Southern Ribeira Belt, is composed of distal turbidites with significant basic magmatism represented by concordant lenses of metabasite. Twenty metabasite samples were investigated using geochemical analyses (major and trace elements). Three groups (G1, G2, G3) were identified based on Ti/V ratios, rare earth element patterns and MORB-normalized multi-element diagrams. All samples have tholeiitic signatures. The G1 group has a volcanic arc signature, with Ti/V ratios of 15 - 20, enrichment of large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE) relative to the MORB. The trace element pattern indicates an important contribution of the subduction component. The G2 group has a MORB-type composition, with Ti/V ratios of 22 - 28. Nevertheless, the trace element behavior indicates a small contribution of the subduction component. The G3 group has an enriched MORB signature, with Ti/V ratio of approximately 40 and enrichment of LILE and HFSE. The variability of the TiO2/Yb and Nb/Yb ratios suggests that the G1 and G2 rocks were generated by shallow melting (N-MORB and E-MORB mantle sources, respectively). The G3 rocks are enriched in Ti and Nb, suggesting interaction between MORB and plume mantle sources. The coexistence of volcanic arc basalt and mid-ocean ridge basalt compositions, the overall geochemical patterns, and the sedimentary environment of the Votuverava Group are consistent with a back-arc basin environment.

Geochemistry and geodynamic significance of the dike series of the Aluchin ophiolite complex, Verkhoyansk-Chukotka fold zone, Northeast Russia

The Aluchin ophiolites represent a tectonomagmatic complex, the upper crustal part of which is made up of two dike series. One series includes diabases and gabbrodiabases, which are exposed in the Late Triassic Atamanov Massif (226 Ma) and subdivided into low-potassium, low and moderate-titanium varieties. In terms of rare-earth element (REE) distribution pattern, these rocks correspond to the mid-ocean ridge basalts (N-MORB). At the same time, trace element composition of some samples indicates the enrichment in subduction component (Ba, Th), as well as variable depletion in Ta, Nb and other high-field strength elements (HFSE), which leads us to conclude that these rocks were formed from melts similar to back-arc basin basalts (BABB) at a sufficiently mature stage of back-arc spreading. The diabases of other series form separate dike bodies (dike swarms) that cut across a mantle ultrabasic body in the northern part of the Aluchin Massif. The most part of these rocks reveal prominent island-arc signatures, primarily, REE and trace-element distribution patterns. At the same time, they are characterized by slightly elevated contents of titanium, nickel, and chromium, and low content of aluminum. On the basis of these data, the diabases of the Aluchin Massif can be regarded as BABB basalts with distinct island-arc characteristics, which are usually termed as arc-like member. The joint geochemical evolution of the two diabase series is well consistent with that of the rocks from different structures of the Mariana trough, which, together with compositional data, indicate that the studied dike series mark the initial and mature stages of the opening of the Late Triassic suprasubduction basin.

Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains

Received 24 March 2011; revised 25 May 2011; accepted 31 May 2011; published 8 July 2011. [1] We present three‐dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the Northwest U.S. under the High Lava Plains (HLP). We show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast‐axis, style of subduction, and time evolving flow are important. Due to increased horizontal shear, systems with a component of rollback subduction have simple trench‐normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex. In the HLP, splitting observations are consistent with rollback‐driven laboratory results. Citation: Druken, K. A., M. D. Long, and C. Kincaid (2011), Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains, Geophys. Res. Lett., 38, L13310, doi:10.1029/2011GL047541.

Two Primary Basalt Magma Types from Northwest Rota-1 Volcano, Mariana Arc and its Mantle Diapir or Mantle Wedge Plume

Primitive basalts are rarely found in arcs.The active NWRota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51^52 wt % SiO2) with 7·5^9·5 wt % MgO and Mg-number of 61^67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo93 and contain 0·4 wt % NiO; the Cr/(CrþAl) values of spinels are mostly 0·5^0·8, indicating equilibrium with depleted mantle.There are three petrographic groups, based on phenocryst populations: (1) cpx^olivine basalt (COB); (2) plagioclase^olivine basalt (POB); (3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3·1^3·2 and 1·2^1·5, respectively) than in COB (Zr/Y1⁄4 2·8^3·0 and Nb/ Yb1⁄4 0·7^0·9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COB have Ba/Nb (70^80) and Th/Nb (0·4^ 0·5) that are higher than for POB (Ba/Nb1⁄4 30^35 and Th/ Nb1⁄4 0·1^0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched Sr/Sr and Nd/Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. Hf/Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr,Yand HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB.The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2^1·5 GPa (equivalent to 65^50 km depth).These magmas formed by 24^18% melting of mantle peridotite having Mg-number 89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at Sumisu andTorishima volcanoes in the Izu^Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents.

Isotopy of lead from the Sora porphyry Cu-Mo magmatic center ( Kuznetsk Alatau)

We studied the isotopic composition of lead in feldspars from the intrusive rocks of enclosing, ore-bearing porphyry, and post-ore complexes and in sulfides from the Sora magmatic center including the Sora porphyry Cu-Mo deposit. The arrangement of the isotopic composition points below the orogen evolution curve on the uranogenic lead diagram evidences a predominance of lead of mantle genesis in the igneous rocks and a minor content of crustal lead. On the 207Pb/ 204Pb– 206Pb/ 204Pb diagram, the lead of magmatites form a linear sequence (trend), which is interpreted as mixing of lead from different sources. According to the geochemical parameters, weakly depleted mantle metasomatized by subduction fluids was a source of magmatism. The linear correlation between the ratios of Pb isotopes in the igneous rocks of the Sora magmatic center is explained by the mixing of lead of the mantle and subduction fluids. Mantle lead is predominant in the basites of the enclosing, porphyry, and post-ore complexes and in the leucogranites of the enclosing complex. According to lead isotopy data, the differentiated magmatites (enclosing monzonitoids, ore-bearing porphyry, and post-ore syenite-porphyry) contain a subduction component, whose content increases in passing from early to late complex. The Pb-isotopic composition of sulfides suggests the mantle source of Pb in molybdenite of the breccia ores and the heterogeneous source (mantle and subduction fluids) of Pb in chalcopyrite of the veinlet ores.

Mapping crustal stress and strain in southwest British Columbia

This paper investigates the orientation and sources of stress in the forearc of the Cascadia subduction zone in southwest British Columbia, using Bayesian inversion results from focal mechanism data and comparing results with GPS derived short-term strain rates. The subduction margin in this region includes a change in orientation from N-S in Washington State to NW-SE in British Columbia. Over 1000 focal mechanisms from North American crustal earthquakes have been calculated to identify the dominant style of faulting, and ∼600 were inverted to estimate the principal stress orientations and the stress ratio. Our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench, from approximately margin-normal along the coast to approximately margin-parallel 100-150 km inland from the coast. Comparing stress orientations with GPS data, we relate the margin-normal stress direction to subduction-related strain rates due to the locked interface between the North American and Juan de Fuca plates just west of Vancouver Island. Further from the margin the plates are coupled less strongly, and the margin-parallel maximum horizontal compressive stress in the North American Plate relates to the northward push of the Oregon Block, which is also observed in the horizontal shortening direction of the residual strain rates, after the subduction component is removed.

The Earth’s tungsten budget during mantle melting and crust formation

During silicate melting on Earth, W is one of the most incompatible trace elements, similar to Th, Ba or U. As W is also moderately siderophile during metal segregation, ratios of W and the lithophile Th and U in silicate rocks have therefore been used to constrain the W abundance of the Earth’s mantle and the Hf–W age of core formation. This study presents high-precision W concentration data obtained by isotope dilution for samples covering important silicate reservoirs on Earth. The data reveal significant fractionations of W from other highly incompatible lithophile elements such as Th, U, and Ta. Many arc lavas exhibit a selective enrichment of W relative to Th, U, and Nb–Ta, reflecting W enrichment in the sub-arc mantle via fluid-like components derived from subducting plates. In contrast, during enrichment by melt-like subduction components, W is generally slightly depleted relative to Th and U, but is still enriched relative to Ta. Hence, all arc rocks and the continental crust exhibit uniformly low Ta/W (ca. 1), whereas W/Th and W/U may show opposite fractionation trends, depending on the role of fluid- and melt-like subduction components. Further high-precision W data for OIBs and MORBs reveal a systematic depletion of W in both rock types relative to other HFSE, resulting in high Ta/W that are complementary to the low Ta/W observed in arc rocks and the continental crust. Similar to previous interpretations based on Nb/U and Ce/Pb systematics, our Ta/W data confirm a depletion of the depleted upper mantle (DM) in fluid mobile elements relative to the primitive mantle (PRIMA). The abundance of W in the depleted upper mantle relative to other immobile and highly incompatible elements such as Nb and Ta is therefore not representative of the bulk silicate Earth. Based on mass balance calculations using Ta–W systematics in the major silicate reservoirs, the W abundance of the Earth’s primitive mantle can be constrained to 12 ppb, resulting in revised ratios of W–U and W–Th of 0.53 and 0.14, respectively. The newly constrained Hf–W ratio of the silicate Earth is 25.8, significantly higher than previously estimated (18.7) and overlaps within error the Hf–W ratio proposed for the Moon (ca. 24.9). The 182Hf– 182W model age for the formation of the Earth’s core that is inferred from the 182W abundance and the Hf/W of the silicate Earth is therefore younger than previously calculated, by up to 5 Myrs after solar system formation depending on the accretion models used. The similar Hf/W ratios and 182W compositions of the Earth and the silicate Moon suggest a strong link between the Moon forming giant impact and final metal–silicate equilibration on the Earth.

Subduction components in Pleistocene to recent Kurile arc magmas in NE Hokkaido, Japan

Samples of Kurile arc lavas erupted between 1.6 Ma and ca. 30,000 years were collected from the Kutcharo, Mashu and Akan caldera area in NE Hokkaido, about 150 km west of the Kurile trench. The samples include rhyolitic pumice, rhyolite, dacite, andesite and, rare, tholeiitic basalt, and show ‘medium’ potassic calc-alkaline affinity. Except for relatively high concentrations of large ionic lithophile elements (LILE), Th and especially Pb, other trace elements, including the rare earths (REE) and high field strength elements (HFSE), show relatively low abundances when compared with those of normal mid-ocean ridge basalts (N-MORB). Their Sr, Nd isotopic compositions are relatively depleted, with 87Sr/ 86Sr ranging from 0.7033 to 0.7034 and 143Nd/ 144Nd from 0.51295 to 0.51230. Pb isotopic compositions are also relatively unradiogenic, with 206Pb/ 204Pb at about 18.4 and 208Pb/ 204Pb ranging from 38.3 to 38.4, significantly more depleted than other Quaternary lavas in NE Japan. The Kurile lavas show typical subduction-type element distributions, with high ratios of fluid-mobile incompatible elements over fluid-immobile HFSE, Ba/Nb, for example, ranging between ca. 200 and 450. The lack of covariance between (e.g.) Ba/Nb and Ba/Th with 87Sr/ 86Sr, and Nd/Pb with Pb isotopic ratios suggests minimal involvement of sediment-derived metasomatism of the magmatic source. Geochemical character of the latter probably reflects contamination by hydrous fluids derived from altered oceanic crust (AOC). This is indicated by the coupling of relatively depleted, MORB-like Sr and Pb isotopic compositions and high Sr and Pb contents. Thus, given their N-MORB-type isotopic compositions, the LILE, and HFSE-like character, evidenced by high ratios of Ba/Nb, and variable Nd/Pb and Th/Nd, suggests NE Hokkaido arc magma genesis is best explained in terms of a binary mixing model involving: a dominantly N-MORB-like (i.e. depleted) convecting mantle ‘wedge’, contaminated by hydrous AOC-derived fluid. Thus magmatic sources for the NE Hokkaido sector appear to have been minimally affected by sedimentary melt-derived metasomatism, consistent with their association with the Kurile arc system.

Devonian volcanics in the New England Orogen: tectonic setting and polarity

The tectonic setting of the Devonian rocks in the New England Orogen has been the subject of considerable debate and controversy for many years. Our studies reveal that they have formed in intra-oceanic island arc and back arc basin (BAB) settings based on Th/Yb, Nb/Yb, Ba/La and Zr/Y ratios. Further, many of the samples that formed in a BAB have a mixture of MORB and arc-like characteristics, a few are almost entirely MORB-like. The arc-like features are believed to be due to the presence of a subduction component in the basaltic magma, the amount of which is controlled by the distance from the arc. Those samples with MORB-like compositions are thought to have originated at spreading centres. The compositions of Late Devonian basalts become more arc-like to the west suggesting a west facing polarity. Based on the tectonic setting and spatial relationship of Late Devonian sequences, we propose that two subduction zones existed during the Late Devonian, one dipping west beneath the Lachlan Orogen, the other dipping east beneath a rifted intra oceanic arc. Obduction of this intra oceanic arc over the continental margin of the Lachlan Orogen in the latest Devonian at approximately 375 Ma led to the development of a new west dipping subduction zone oceanward and commencement of continental, arc magmatism.

Deformation model for the Tongling ore cluster region,east-central China

The Tongling region, an important Cu + Au ore district, consists of a series of S-folds and is defined by an irregular set of boundaries. The regional structural framework was traditionally considered to be the result of coaxial compression, followed by simple shear. By analysing the spatial distribution of fold geometries and fault densities, we propose a new deformation model. Two tectonic stages, that is, early NW–SE compression, and later NE–SW compression, have been newly identified in this region. In the early stage, shortening produced by the collision between the Lower Yangtze block and the North China block mainly acted on the south-eastern and north-western boundaries; NE–SW-trending anticlines adjacent to the regional boundaries are tight to isoclinal and overturned, and the density of NW–SE-trending strike-slip faults is high as revealed by the large box dimensions of the spatial fault distribution. In addition, NW–SE compressions of the two stress boundaries are non-coaxial and asymmetric. With regard to the disposition of the irregular boundaries, non-coaxial and asymmetric compression led to the formation of S-folds and to obvious displacement between the north-eastern and south-western parts of the region. In the later stage, weak NE–SW compression, derived from the south-westward component of subduction of the Izanagi plate beneath the Eurasian plate, acted on the north-eastern boundary and redeformed the local structures to a minor extent. High fault density near the region boundaries promoted the formation of an interconnected fault system, which facilitated magma migration and emplacement, as well as the development of deposits genetically related to the igneous activity.

Plume–subduction interaction in southern Central America: Mantle upwelling and slab melting

The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~1380–1410°C) in the Pliocene to ~1450°C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0–2.7GPa). The average final melting pressures range between 2.7 and 2.5GPa, which is interpreted as the lithosphere–asthenosphere boundary at ~85–90km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~10–8Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the hotter asthenosphere, and the presence of a slab melting signature in the volcanic front caused by recycling of Galapagos Hotspot tracks.