Base Of Crust Research Articles

Plio-Pleistocene intra-plate magmatism from the southern Sulu Arc, Semporna peninsula, Sabah, Borneo: Implications for high-Nb basalt in subduction zones

New analyses of major and trace element concentrations and Sr, Nd and Pb isotopic ratios are presented for Plio-Pleistocene basalts and basaltic andesites from the Semporna peninsula in Sabah, Borneo, at the southern end of the Sulu Arc. Depletion of high field strength elements (HFSE), which is characteristic of many subduction-related magmatic suites, is present in more evolved Semporna rocks but is associated with radiogenic Sr and Pb, and less radiogenic Nd isotopic ratios and results from contamination of mafic melt by, possibly ancient, crustal basement. The most mafic lavas from Semporna, and elsewhere in the Sulu Arc, display no HFSE depletion relative to other elements with similar compatibility. High-Nb basalt from Semporna formed when mantle resembling the source of Ocean Island Basalt (OIB) upwelled into lithospheric thin spots created during earlier subduction. This mantle did not experience enrichment by fluids or melt derived from subducted crust. The presence of similar lavas throughout the Sulu Arc and around the South China Sea suggests that the OIB-like component resides in the convecting upper mantle. Depletion of light rare earth elements, with respect to other incompatible elements, throughout the Sulu Arc could result from melt–mantle interaction during magma transport through the lithosphere. Such depletion is absent in suites from the South China Sea, where magma probably migrated along large, lithosphere-penetrating structures. Semporna high-Nb basalts are not associated with adakitic magmatism which is a frequent, but not ubiquitous, association in some active subduction zones. Both geochemical signatures are developed early in the history of a melt pulse, either in the source (high-Nb basalt) or during deep differentiation (adakite). Preservation of these distinctive geochemical signatures is favoured in settings that minimise (1) interaction with other, more copious melt types, and/or (2) subsequent differentiation in the shallow crust. Where found, the high-Nb basalt–adakite association is a result of transport through favourable lithospheric conditions and not due to any link between their mantle sources.

Geology and geochronology of neoarchean anorogenic magmatism of the Keivy structure, Kola Peninsula

Anorogenic magmatic complexes were formed during protoplatformal evolution of the Keivy structure. This evolution ended with development of aluminous schists, which were derived by deep disintegration and redeposition of the rocks from the lower parts of the sequence and surrounding of the structure. The anorogenic rocks of the region are represented by the following magmatic complexes: gabbro-labradorite-latite-monzonite-granites; ophitic gabbro and gabbrodiabases; quartz syenite-alkaline granites; alkaline and nepheline syenites. The magmatic activity of this period, starting from the emplacement of gabbrolabradorite massifs and ending with alkaline and nepheline syenite bodies, was caused by ascent of mantle asthenolith, which destructed the Earth’s crust basement in this area. The anorogenic magmatism of the Keivy structure lasted for no more than few or few tens of million years. The granitoid subcomplex of the gabbro-labradorite-latite-monzonite-granite complex is dated at 2674 ± 6 Ma, which is comparable with an age of alkaline granites of the Ponoy and Beliye Tundry massifs (2673 ± 6 Ma). The considered complexes are separated in time by intrusion of amphibole-biotite plagiomicrocline granites with an age of 2667 ± 8 Ma. Gabbrolabradorites of the Shchuch’e Ozero and Tsaga massifs have close ages (2663 ± 7 and 2668 ± 10 Ma, respectively, Bayanova, 2004), but were formed earlier than granitoids (Bayanova, 2004). Formation of alkaline syenites of the Sakharijok I Massif, which finalized the Neoarchean anorogenic magmatism of the region, falls in the same interval. During Paleoproterozoic transformations, the rocks of the Keivy structure were sheared and uranium was introduced in the contact zones of the alkaline granite massifs, which caused formation of palingenetic melts and subsequent formation of pegmatites in the outer contact zones of the granite bodies.

Reconsidering the origins of isotopic variations in Ocean Island Basalts: Insights from fine-scale study of São Jorge Island, Azores archipelago

New major and trace element and Sr–Nd–Pb isotope data have been determined for 21 basaltic samples from São Jorge Island, Azores archipelago. Samples can be separated into two groups best identified in a plot of 208Pb/ 204Pb versus 206Pb/ 204Pb where they form two sub-parallel mixing arrays. Lavas from the old (Topo) formation have lower 208Pb/ 204Pb for a given 206Pb/ 204Pb and more radiogenic Sr than samples from intermediate (Rosais) and young (Manadas) formations. Topo samples also tend to have higher MgO contents and lower incompatible trace element concentrations. Both Pb mixing arrays can be related to mixing of plume melts having a HIMU-like Pb isotope signature with two depleted components. One depleted component is best seen in analyses of Topo samples and is interpreted to represent upper mantle material from the nearby Mid-Atlantic ridge. The second depleted component has Sr–Nd–Pb isotopic characteristics similar to E-MORB and resides in the oceanic crust basement under São Jorge. Pb isotope analyses of lavas from São Jorge make it possible to re-define the Terceira end-member of the Azores archipelago, moving its composition to more radiogenic Pb with 206Pb/ 204Pb ~ 20.51, 207Pb/ 204Pb ~ 15.67 and 208Pb/ 204Pb ~ 39.56, at 143Nd/ 144Nd ~ 0.51295 and 87Sr/ 86Sr ~ 0.70375. Extending the interpretations made for the origins of the São Jorge isotopic mixing arrays to other islands from the Azores archipelago (Pico, Faial, Terceira and São Miguel), we show that most of the isotopic variability recorded by Azores magmas can be related to mixing of plume melts with two distinct and homogeneous signatures as well as several other components dispersed in the shallow mantle/lithosphere under the Azores. This illustrates how caution is required when interpreting ocean island basalt isotopic data as reflecting their deep mantle plume source, and subsequently for constraining mantle topology.

Genesis of the Assif El Mal Zn–Pb (Cu, Ag) vein deposit. An extension-related Mesozoic vein system in the High Atlas of Morocco. Structural, mineralogical, and geochemical evidence

The Assif El Mal Zn–Pb (Cu–Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, amphibole, chloritoid, and garnet. The Assif El Mal Zn–Pb (Cu–Ag) mineralization forms subvertical veins with ribbon, fault breccia, cockade, comb, and crack and seal textures. Two-phase liquid–vapor fluid inclusions that were trapped during several stages occur in quartz and sphalerite. Primary inclusion fluids exhibit T h mean values ranging from 104°C to 198°C. Final ice-melting temperatures range from −8.1°C to −12.8°C, corresponding to salinities of ∼15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater. Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions and homogenization temperatures ranging from 75°C to 150°C. The δ18O and δD fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived organic waters. Low δ13CVPDB values ranging from −7.5‰ to −7.7‰ indicate a homogeneous carbon source, possibly organic matter disseminated in black shale hosting the Zn–Pb (Cu–Ag) veins. The calculated δ34SH2S values for reduced sulfur (22.5‰ to 24.3‰) are most likely from reduction of SO4 2− in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical sulfate reduction in which organic matter was oxidized to produce CO2 which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent with fluid–rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif El Mal formed during the Jurassic opening of the central Atlantic Ocean.

Helium isotopes in early Iceland plume picrites: Constraints on the composition of high 3He/ 4He mantle

A detailed study of the geochemistry of a new suite of early Iceland plume picrites shows that extremely high 3He/ 4He ratios (up to 50 R a) are found in picrites from Baffin Island and West Greenland. High 3He/ 4He picrites display a wide range in 87Sr/ 86Sr (0.70288–0.70403), 143Nd/ 144Nd (0.51288–0.51308) and incompatible trace element ratios (e.g. La/Sm n = 0.5–1.6). These overlap the complete range of compositions of mid-ocean ridge basalts and most northern hemisphere ocean island basalts, including Iceland. Crustal contamination modelling in which high-grade Proterozoic crustal basement rocks for the region are mixed with a depleted parent cannot account for the trend displayed by the Baffin Island and West Greenland picrites. This rules out the possibility that the incompatible trace element, Sr and Nd isotope range of the high 3He/ 4He picrites is due to crustal contamination. The compositional range at high 3He/ 4He is also inconsistent with derivation from a primordial-He-rich reservoir that is a residue of ancient mantle depletion. This implies that the composition of the high 3He/ 4He mantle cannot be determined simply by extrapolating ocean island basalt He–Sr–Nd–Pb–Os isotope data. The apparent decoupling of He from trace element and lithophile radiogenic isotope tracers is difficult to attain by simple mixing of a high-[He], high 3He/ 4He reservoir with various depleted and enriched He-poor mantle reservoirs. The possibility that primordial He has diffused into a reservoir with a composition typical of convecting upper mantle cannot be ruled out. If so, the process must have occurred after the development of existing mantle heterogeneity, and requires the existence of a deep, primordial He-rich reservoir.

Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions

P–T pseudosections were calculated in the system Na–Ca–K–Fe–Mg–Al–Si–Ti–H–O with the PERPLE_X software package for the pressure–temperature range 1–25 kbar and 150–450 °C to gain a better understanding of the phase relations and the dehydration behaviour of psammopelite and mid-ocean ridge basalt (MORB) during prograde metamorphism at very-low-grade and low-grade. For this purpose, the thermodynamic data set of Holland & Powell was enlarged by end-member data for Fe2+- and Fe3+-pumpellyite, Fe2+- and Mg-stilpnomelane, actinolite, and magnesioriebeckite. In addition, a three-component solid-solution model for pumpellyite, a two-component model for stilpnomelane, and four-component models for amphibole and sodic pyroxene were created. Studied metamorphosed MORB and psammopelite contain around 6 wt.% structural H2O bound in minerals at 150 °C and pressures up to 5 kbar. Prograde metamorphism causes dehydration patterns, which are, for example, important for an understanding of the formation of accretionary-wedge systems: along a relatively high geotherm of 15 °C/km, significant dehydration (1.5 wt.% H2O release) of metapsammopelite can be noted in the temperature range 220–240 °C. We believe that this process leads to softening of the sedimentary cover of oceanic crust during early subduction so that this material can be scraped off the basic crust, which then would dehydrate at higher T , to form frontal accretionary prisms. Basal accretionary prisms are generated at lower geotherms ( e.g ., 12 °C/km) by dehydration of metapsammopelite between 260 and 300 °C. Again, metabasic material would dehydrate at significantly higher T and is, therefore, only subordinately involved in accretionary wedges. Along geotherms lower than 7 °C/km, almost no water is released up to temperatures of 400 °C and more. Thus, the corresponding material is subducted to mantle depths as in the subduction/exhumation channels of collision zones. We also hypothesize that accretionary-wedge complexes of the hot subduction zones in Precambrian times should mainly have formed by frontal accretion.

Phase transition in the subducted oceanic lithosphere and generation of the subduction zone magma

Two metamorphic processes, i.e. subsolidus dehydration and partial melting occurring in MORB, metasediments and peridotite of subducted oceanic lithosphere are discussed on the basis of available experimental work and phase equilibrium modeling. Phase diagrams of hydrous MORB show that in most cold subduction P-T (pressure-temperature) regimes a large portion of water in the basic layer has released below the onset of blueschist facies (< 20 km), and at a depth (60–70 km) of transition from lawsonite blueschist to lawsonite eclogite facies through glaucophane dehydration; only a smaller portion of water will escape from the slab through dehydration of lawsonite and chloritoid in the depth range suitable for arc magma formation; and a very small portion of water stored in lawsonite and phengite will fade into the deeper mantle. The role of amphibole for arc magma formation is still arguable. In cold subduction P-T regimes, the dehydration of chlorite and talc in Al-poor metasediments, and chloritoid and carpholite in Al-rich metapelites at a depth around 80–100 km will make some contributions to the formation of arc magma. Comparatively, dehydration of serpentine in hydrated peridotite occurs at depths of 120–180 km, playing an important role in the arc magmatism. Subduction of oceanic crust along warm P-T regimes will cross the solidi at a depth over 80 km, resulting in partial melting under fluid-saturated and fluid-absent conditions in the metasediments involving biotite and phengite, and in the basic rocks involving epidote and amphibole. The melt compositions of the basic crust are adakitic at pressures < 3.0 GPa, but become peraluminous granitic at higher pressures.

Mesoarchean assembly and stabilization of the eastern Kaapvaal craton: A structural‐thermochronological perspective

Understanding the construction and stabilization of Archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early Earth, as well as for the subsequent evolution of continents. We provide new constraints for the amalgamation and stabilization of the eastern Kaapvaal craton in the vicinity of the Barberton greenstone belt circa 3.3–3.1 Ga. Isotope dilution thermal ionization mass spectrometry U‐Pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high‐grade orthogneiss terranes from low‐grade supracrustal rocks and to relate this movement to the timing of differential exhumation. New and existing data are consistent with accretion at ∼3.23 Ga within an oblique subduction plate boundary manifested in the BGB as an asymmetric flower‐like structural geometry. For &gt;100 Ma following terrane assembly, transform boundary tectonics, manifested partially as transtension on reactivated faults, led to large‐scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths. Stabilization of this portion of Archean lithosphere involved at least a three‐stage process: (1) creation of a thick and rigid mantle lithosphere during crustal growth, subduction, and terrane assembly from circa 3.30 to 3.23 Ga, (2) generation of a rigid crust during strike‐slip and transtensional tectonics coupled with migration of granites and heat‐producing elements into the upper crust between circa 3.2 and 3.1 Ga, and (3) erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2.9 Ga.

Geochemistry of basal Cambrian black shales and cherts from the Northern Tarim Basin, Northwest China: Implications for depositional setting and tectonic history

Black shales and thin-bedded cherts in the basal Cambrian are widespread worldwide and they carry important information on the formation of sedimentary basins and on the tectonic history. We studied the geochemical signatures of the early Cambrian black shales and bedded cherts from the Northern Tarim Basin, China, with the objectives of understanding the depositional setting of these rocks and inferring the tectonic history in the region. Twenty two black shales, ten cherts, and two nodular phosphorites were collected from two outcrops at Xiaoerbulake and Sugaitebulake in the Northern Tarim Basin, spanning vertical sections of 8.8 and 7.5 m, respectively. A suite of techniques, including field investigations, X-ray diffraction, electron microscopy, trace element, rare earth element (REE), and isotope geochemistry, were employed to characterize the geochemical signatures of these rocks. Field evidence indicates that the black shales and bedded cherts are over- and underlain by dolomites, suggesting a shallow marine depositional environment. Mineralogical and trace element data suggest that the Tarim black shales and cherts were deposited in a suboxic continental shelf environment, and hydrothermal activity may have extracted certain trace elements from mafic continental crust and concentrated them in the sedimentary basin. REE characteristics for the cherts are very similar to those that are known to be deposited in pelagic ocean floor settings, suggesting that the hydrothermal fluids may be derived from the infant southern Tianshan Ocean in the north of the Tarim Basin. Os isotope signatures at the time of deposition ( 187Os/ 188Os i = 1.1–2.7) are typical of crustal signatures, and the radiogenic Os isotope signatures rule out the mantle as a possible source of Os and other metals. A positive correlation between 187Os/ 188Os and ε Nd is consistent with upper crust-derived basin sediments that contain a variable contribution of hydrothermal fluids possibly derived from ancient mafic continental crust. These trace element, REE, and isotope systematics collectively suggest that incorporation of hydrothermal fluids derived from ancient, mafic continental crust combined with deposition in relatively reducing conditions may have controlled the chemical and isotopic compositions of these rocks. We infer that the hydrothermal fluid was carried to the continental shelf by upwelling during the initial stages of formation of the southern Tianshan Ocean, where the fluid interacted with thinned, mafic crustal basement lithologies and was subsequently incorporated into the black shales and bedded cherts in the Northern Tarim Basin. This study provides important geochemical evidence for the creation of the Tianshan Ocean, which is a result of break-up of the Rodinia Supercontinent during the early Cambrian.

Neoproterozoic–Early Devonian magmatism in the Antigonish Highlands, Avalon terrane, Nova Scotia: Tracking the evolution of the mantle and crustal sources during the evolution of the Rheic Ocean

The Antigonish Highlands lies in the Avalon terrane (Avalonia) in the Canadian Appalachians and records four episodes of continental rift-related bimodal magmatism ranging in age from Late Neoproterozoic to Middle Devonian. The oldest episode (ca. 615 Ma Clydesdale Formation) was emplaced in a local rift setting in an Andean-style arc when Avalonia resided along the northern Gondwanan margin. Early Cambrian magmatism (Arbuckle Brook Formation) occurred in a local transtensional setting after arc magmatism had given way to a San Andreas-type transform environment. Middle Ordovician (Dunn Point and Bears Brook formations) magmatism occurred in a local rift in an ensialic island arc setting, analogous to the modern Taupo Volcanic Zone in northern New Zealand, after Avalonia had separated from Gondwana. Middle Devonian (McArras Brook Formation and correlatives) occurred after Avalonia had collided with Laurentia. In each episode, mafic rocks are characterized by high FeO t, FeO t/MgO, TiO 2, Zr/Y and Ti/Y typical of differentiated within-plate mafic magmas. Multi-element spider-diagrams indicate that all four episodes were generated in an enriched sub-continental lithospheric mantle. LIL, HFS and REE patterns indicate that the Clydesdale, Dunn Point/Bears Brook and McArras Brook formations were sourced in the shallow (spinel lherzolite) mantle, and that the Arbuckle Brook Formation was generated in the deeper (garnet lherzolite) mantle. In each episode, coeval felsic rocks were generated by crustal anatexis. Taken together, the Sm–Nd isotopic data for the mafic rocks form an envelope that defines the evolution of the mantle source beneath the Antigonish Highlands that was enriched between 0.8 and 1.1 Ga and has an average Sm/Nd ratio of ca. 0.24 (a value that is typical of an enriched mantle source). Sm–Nd isotopic data for the felsic rocks are characterized by a recurrence of T DM ages between 0.95 and 1.0 Ga, interpreted to reflect repeated melting of the lower crust that was itself derived from a depleted-mantle source between 0.95 and 1.1 Ga. The remarkable geochemical and isotopic similarity of the Neoproterozoic, to Middle Devonian mafic and felsic magmas indicate that the crust and sub-continental lithospheric mantle beneath the Antigonish Highlands were coupled during the rift and drift of Avalonia from the Gondwanan margin during the formation of the Rheic Ocean in the Ordovician, its accretion to Baltica and to Laurentia in the Silurian, and post-accretionary strike-slip movement of Avalon along the Laurentian margin. Although the Sm–Nd isotopic data suggest that Avalonian basement and lithospheric mantle formed over the same time interval (0.8 to 1.1 Ga), the divergence of their respective envelopes with time reflects their differing ε Nd values and Sm/Nd ratios. We suggest that Avalonian crustal basement and its enriched mantle were both formed between 0.8 and 1.1 Ga in a Panthalassa-type ocean that surrounded the supercontinent Rodinia and were accreted to the northern Gondwanan margin at about 650 Ma, prior to the oldest magmatic event in the Antigonish Highlands.

Reflection attributes of paragneiss in the upper crust

The integrated study of the geological and seismic reflection data from the drilling area of CCSD has discovered that the density and the P-wave velocity of orthogneiss are almost the same as that of the paragneiss in the area; but the orthogneiss and the paragneiss hold different reflection attributes. The strong seismic reflector packes coinciding spatially with the paragneiss suites have implied that the paragneiss buried in the metamorphic crust itself can cause bone-like seismic reflector sets. The P-wave velocity of paragneiss shows little apparent difference with that of the orthogneiss; but its transverse wave velocity is lower, with the Vp/Vs ratios being high. The paragneiss has partially inherited the layering structures and textures of the protolithe of sedimentary rocks, hence shows strong heterogeneity and anisotropy, that is why the paragneiss are able to produce the bone-like reflectors in the upper crust. The low transverse wave velocity of paragneiss often means weak shear resistance, which will further cause cracks or fractures in the rock, consequentially increase its porosity and permeability during tectonic movements, and form the paragneiss reservoirs of low-permeability zones for gases uplifted from the deeper crust. Because the paragneiss in the crustal metamorphic basement can cause the seismic reflectors, seismic reflection sections are able to provide information about the paragneiss under certain prerequisites.

Quantitative P–T–X constraints on orthopyroxene‐bearing high‐pressure granulites in felsic–metapelitic rocks: evidence from the Huangtuling granulite, Dabieshan Orogen

AbstractHigh‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system, a series of P–T–X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low XAl [Al2O3/(Na2O + CaO + K2O + FeO + MgO + Al2O3) &lt; 0.35, mole proportion] or high XMg [MgO/(MgO + FeO) &gt; 0.85] felsic–metapelitic rock types. This study also reveals that the XAl values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.

The Mount Pavagadh volcanic suite, Deccan Traps: Geochemical stratigraphy and magmatic evolution

The patterns of eruption and dispersal of flood basalt lavas on the surface, or as magmas in dykes and sills within the crust, determine the volcanological and stratigraphic development of flood basalt provinces. This is a geochemical and Sr-isotopic study of lavas of varied compositions that outcrop around Mount Pavagadh (829 m), Deccan Traps, an important outlier north of the main basalt outcrop. Most of the ∼550-m thick exposed section at Pavagadh is made up of subalkalic basalts rich in the incompatible elements (particularly Nb, Ba, and Sr). Picrite and rhyolite–dacite flows also occur, the latter capping the sequence. The relatively high initial 87Sr/ 86Sr ratios (up to 0.7083) and chemical characteristics of the rhyolitic rocks of Pavagadh are consistent with a small but significant involvement of the granitic basement crust in their genesis. An assimilation–fractional crystallization (AFC) model involving the picrite lava and either a southern Indian or a western Indian granite as the contaminant explains the geochemical and Sr-isotopic variation in the basalts and the rhyolites quite well. A systematic comparison of the basaltic lavas (with binary plots, normalized multielement patterns, and discriminant function analysis) to the well-established lava stratigraphy of the Western Ghats, 400–500 km to the south, precludes any chemical–genetic relationships between the two. Basalts exposed in sections closer to Pavagadh (∼150–200 km), in the Toranmal, Navagam, and Barwani–Mhow areas, have several flows with some similar chemical characteristics. However, the Pavagadh sequence is significantly different from all of these sequences geochemically, petrogenetically, and in magnetic polarity, to be considered independently built. This result is significant in terms of eruptive models for the Deccan Traps, as it is increasingly apparent that there were separate but possibly coeval eruptive centers with their own distinctive chemistries developed in various areas of this vast province.

Petrogenesis and time-progressive evolution of the Cenozoic continental volcanism in the Biga Peninsula, NW Anatolia (Turkey)

The post-collisional Cenozoic magmatic activity in the Biga Peninsula (NW Anatolia) started in the Middle Eocene (45.3 ± 0.9 Ma) and lasted continuously until the Late Miocene (8.32 ± 0.19 Ma) with minor punctuations, producing widespread volcanoplutonic complexes. These magmatic suites are characterized by calc-alkaline, high-K calc-alkaline, shoshonitic, mildly alkaline, and alkaline series. The Middle Eocene to Middle Miocene lavas have high LILE/LREE, LILE/HFSE and 87Sr/ 86Sr( i) (0.7046–0.7087) and low ɛNd( i) (+ 1.2 to − 6.4) values. By contrast, the Late Miocene lavas have the lowest 87Sr/ 86Sr( i) (0.7030–0.7033) and the highest ɛNd( i) (+ 2.6 to + 6.7) values. Geochemical features of these rocks indicate two episodes of enrichment processes during the evolution of the Eocene–Early Miocene volcanism in the Biga Peninsula. The first stage, source enrichment metasomatism, occurred within the continental lithospheric mantle as a result of the previous subduction event(s), and produced a heterogeneously enriched source. The second stage, crustal contamination, resulted from interaction between the metasomatized mantle-derived melts and those magmas derived from the Sakarya crustal basement during their ascent to the surface. Slab breakoff-induced asthenospheric upwelling caused the melting of metasomatized mantle lithosphere beneath the Sakarya continent and produced the Middle Eocene calc-alkaline lavas. Geochemical, isotopic and age relationships suggest increasing amounts of crustal contamination and a decreasing subduction signature during the evolution of magmas from the Eocene through the Early Miocene. Crustal input was peaked during the Early Miocene volcanism, coinciding with the exhumation and rapid uplift of the Kazdag metamorphic massif. Asthenospheric upwelling beneath the Kazdag core complex contributed to the generation of hybrid magmas formed from mixing of mantle and crustal melts during this stage. The Middle Miocene (15.2 Ma) mildly alkaline rocks show less pronounced enrichment in LILE and LREE and display intermediate major and trace element compositions between the Early Miocene and Late Miocene lavas. The Cenozoic volcanism in Biga Peninsula waned briefly during 15–11 Ma, and was subsequently rejuvenated (Late Miocene, 11.0 ± 0.4–8.32 ± 0.19 Ma) producing mainly OIB-type alkaline lavas. Advanced extensional tectonics throughout NW Anatolia in the Late Miocene led to upwelling of the asthenospheric mantle and its decompressional melting that collectively resulted in the eruption of short-lived episodes of mainly OIB-type alkaline lavas. This tectonomagmatic evolution of the Cenozoic continental volcanism in western Anatolia presents a well-documented case study of an idealized post-collisional magmatism in most orogenic belts around the world.

Spatial and temporal constraints on sources of seismic anisotropy: Evidence from the Scottish highlands

Routine SKS splitting analyses of seismic anisotropy usually suffer from limitations in station density that hinder attempts to place spatial constraints on anisotropic fabrics. Data from a ∼20 km station spacing network in Scotland show that splitting parameters vary considerably (δt = 0.45–1.5 s; ϕ = 49–128°) over short (10–20 km) length scales. Improved spatial constraints then lead to tighter temporal constraints on the anisotropic sources. Asthenospheric fabrics due to Tertiary rifting and present day plate motions do not strongly influence our results that instead correlate with lithospheric scale trends inferred from surface geology. Splitting observations track Scotland's tectonic history from the Precambrian emplacement of crustal basement and through the activation of large‐scale faulting and thrusting during the Caledonian Orogeny. The shallow lithosphere beneath Scotland has preserved a fossil anisotropic signature up to hundreds of millions of years after it was formed.

Geochemical and Pb-Sr-Nd isotopic compositions of granitoids from western Qinling belt: Constraints on basement nature and tectonic affinity

Geochemical and Pb-Sr-Nd isotopic compositions of five Indosinian granitoid intrusions from the western Qinling belt provide insights into basement nature and tectonic affinity. The results show that the western Qinling granitoids incline towards basic in their bulk chemical composition. The granitoids belong to high-K to shoshonitic series with K2O/Na2O=1.04–1.86 and are dominantly metaluminous with A/CNK=0.90–1.05 (most samples have A/CNK of <1.0). They have similar trace elemental compositional patterns. In Sr-Nd isotopic compositions, they display some extent heterogeneity with ISr=0.70682–0.70845, ɛNd(t)=−4.85 to −9.17 and TDM=1.26–1.66 Ga. They are characterized by high radiogenic Pb isotopic compositions. Their initial Pb isotopic ratios are 206Pb/204Pb=17.996–18.468, 207Pb/204Pb=15.565–15.677 and 208Pb/204Pb=38.082–38.587. Geochemical and Sr-Nd isotopic compositions reveal that magma for the granitoids was derived from partial melting of high-K (Rb) basaltic rocks, which might be formed in 900–1400 Ma. It is suggested that a large amount of the Proterozoic high-K (Rb) basaltic rocks, which underlie the Phanerozoic sedimentary cover, constitute the crustal basement of the western Qinling belt. Pb-Sr-Nd isotopic compositional comparison between the eastern Qinling and the western Qinling Indosinian granitoids indicates that the crustal basement of the western Qinling is distinct from that of the eastern Qinling. The Baoji-Chengdu railway close to south-north orientation can be taken as an approximate boundary between both basements. The Pb-Nd isotopic compositional characteristics of the western Qinling granitoids suggest that the basement of the western Qinling belt has an affinity with the Yangtze block.

Mineral chemistry and U-Pb SHRIMP geochronology of xenoliths and xenocrysts from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton, Western Australia

The Neoproterozoic (~815 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and contains a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing, and rare, metasomatised, phlogopite-biotite and rutile-bearing types, along with minor granite xenoliths. This suite of xenoliths provides new data on the composition and age of the central Kimberley Craton. Proton probe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a Proterozoic geotherm for the central Kimberley Craton, of ~35-40 mW/m2. Lherzolitic chromian pyrope, that is highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from reduced, depleted garnet peridotite mantle at ~150 km depth. We interpret the highly depleted chromian pyrope xenocrysts to have formed by exsolution from high temperature orthopyroxene in former harzburgite. Sampling of shallower levels of the lithospheric mantle by magmas in the north, and north extension lobes of the Aries kimberlite entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe-Mg exchange with olivine during slow cooling of the peridotite host rocks. Episodes of Fe-Ti-rich metasomatism in the spinel-facies Kimberley mantle are the likely source of Ti-phlogopite-biotite + rutile and Ti-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U-Pb SHRIMP 207Pb/206Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are Palaeoproterozoic in age.

Alteration and remelting of nascent oceanic crust during continental rupture: Evidence from zircon geochemistry of rhyolites and xenoliths from the Salton Trough, California

Rhyolite lavas and xenoliths from the Salton Sea geothermal field (Southern California) provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust. Salton Buttes rhyolite lavas contain xenoliths that include granophyres, fine-grained altered rhyolites (“felsite”), and amphibole-bearing basalts. Zircon is present in lavas and xenoliths, surprisingly even in the basaltic xenoliths, where it occurs in plagioclase-rich regions interpreted as pockets of crystallized partial melt. Zircons in the xenoliths are exclusively Late Pleistocene–Holocene in age and lack evidence for inheritance. U–Th isochron ages are: 20.5 − 1.2 + 1.2 ka (granophyres), 18.3 − 3.5 + 3.6 ka (felsite), 30.1 − 12.4 + 14.1 ka and 9.2 − 6.6 + 7.0 ka (basalts; all errors 1 σ). The dominant zircon population in the rhyolite lavas yielded U–Th ages between ∼ 18 and 10 ka, with few pre-Quaternary xenocrysts present. δ 18O zircon values are lower than typical crustal basement values, thus ruling out rhyolite genesis by melting of continental crust. Moreover, δ 18O zircon values are ∼ 0.5–1.0‰ lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid-ocean ridge basalt, suggesting that basaltic crust and silicic plutons in the subsurface of the Salton Sea geothermal field isotopically exchanged with meteoric waters. This is evidence for deep-reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma.

Geochronological and isotopical review of pre-Devonian crustal basement of the Colombian Andes

Under the flatlands east of the Andes, the crustal basement is exposed in a few places, composed mainly of the Mitú migmatitic complex and the Parguaza granite, whose ages range between 1.78 and 1.45 Ga. Extensive outcrops of high-grade metamorphic rocks are found in several places. Two metamorphisms are dated between 1.2–1.1 and 1.0–0.9 Ga. They are considered blocks that formed during the Grenville orogeny and have Sm–Nd T DM model ages of 1.87–1.47. The Andaquí terrane is formed mainly by the Garzón Massif, composed of granulites, migmatites, and granites, and the metamorphic rocks of the Sierra de la Macarena, which are covered by undeformed Cambrian sediments. It is believed that after the Grenville orogeny, this unit remained attached to the Amazonic Craton. All the other areas grouped in the Chibcha terrane, though they formed during the Grenville orogeny, are believed to have remained either as part of another continental block or dispersed islands to be amalgamated to the Amazonic Craton during the Lower Paleozoic orogeny, which in the Quetame Massif is dated between the Silurian and Devonian and is named the Quetame orogenic event.

Oxygen isotope evidence for chemical interaction of Kilauea magmas with basement rocks

David Green has shown us that the geochemistry of basaltic volcanoes is largely governed by mantle processes. However, Kilauea volcano, one of the most active volcanoes on Earth, also records interaction with its crustal rocks. Kilauea is built on a ~ 6 km thick crustal basement of ~100 Ma oceanic crust [1] overlain by 8-10 km of young lavas (<1 Ma) from the adjacent growing shield volcano, Mauna Loa. The contribution of these basaltic basement rocks on the geochemistry of Kilauea magma was thought to be negligible based on major, trace element and Pb, Nd and Sr isotope analyses of historical lavas from this volcano [2]. New oxygen isotope results provide evidence for extensive exchange between Kilauea magmas and the volcano’s basement. This exchange is documented by analyses of basalt matrix material and olivine-matrix pairs for historical lavas. The matrix oxygen isotope ratios are lower and the disequilibrium between olivine and matrix is greatest following summit explosions and collapses in 1790 and 1924. The source of this contamination is not likely to be shallow because blocks from the 1924 explosions (derived from depths <400 m) have relatively high O isotope ratios (5.5 per mil). In contrast, lavas from the volcano’s deeper (2-3 km) and hotter interior (at least 300°C) have much lower O isotope ratios (1.7-2.3 per mil). These deeper rocks are more appropriate for lowering O matrix ratios following summit collapses via assimilation of hydrothermally altered rock. The positive correlation between Pb and O isotopes is consistent with Mauna Loa basement rocks being contaminants for recent Kilauea lavas. Our results show that oxygen isotopes are a sensitive indicator of crustal interaction in basaltic lavas and can help eliminate an exotic mantle source component as the cause for isotopic variations.