Melting Of Lithosphere Research Articles

Heterogeneous mantle argon isotope composition in the subcontinental lithospheric mantle beneath the Red Sea region

We performed argon measurements on rocks of the subcontinental lithospheric mantle (SCLM) from the Red Sea region applying stepwise crushing extractions, and argon analyses using high-resolution stepwise heating techniques, the latter combined with 40Ar/ 39Ar dating. Maximum 40Ar/ 36Ar ratios of individual samples range from 4190 ± 240 to 25,590 ± 620, the latter (sample SA 87-2/12) being the highest measured value of any mantle peridotite or pyroxenite sample so far. For five samples, reasonably defined binary mixing correlations between atmospheric and mantle-derived argon and neon resulted in calculated sample-specific mantle- 40Ar/ 36Ar ratios of 5040 ± 50, 7500 ± 70, 9510 ± 220, 29,600 ± 2700, and 39,000 ± 2500 at an assumed mantle- 20Ne/ 22Ne value of 12.5. These observations indicate a highly heterogeneous argon isotopic composition of the trapped mantle fluids. Furthermore, most samples exhibit mantle- 36Ar/ 22Ne ratios higher than considered to represent an unfractionated mantle source. These characteristics could be explained by mixing of plume-related (low 40Ar/ 36Ar) and lithospheric (high 40Ar/ 36Ar) melts or fluids following elemental fractionation between Ar and Ne. Alternatively, we propose the presence of a severely fractionated atmospheric component highly enriched in argon, admixed to the mantle fluids before trapped by the ultramafic rocks. This primary, seriously fractionated atmospheric component cannot be monitored by Ne isotopes. Incorporation of such a primary atmospheric component could occur either during magma ascent at relatively shallow, upper crustal levels, or by mobilization of atmospheric fluids in a subduction-related metasomatized lithospheric mantle during Pan African times (c. 600–800 Ma ago).

First field evidence of southward ductile flow of Asian crust beneath southern Tibet

There is lively debate on whether Asian plate material was involved in southward flow of mid-lower crust in a ductile channel beneath southern Tibet. One argument against such involvement is the apparent absence of material derived from Asian lithosphere within the High Himalayan Series (Indian plate) that could represent the putative channel. A north-south–trending mid-Miocene dike swarm that intrudes the Tethyan sedimentary cover of the Sakya gneiss dome (Indian plate) yields new Sr-Nd isotopic data (87Sr/86Sr = 0.7071–0.7079; ϵNd −4 to −6) indicating that these melts share the same source as Miocene dacitic dikes from north of the Indus-Tsangpo suture. Moreover, dikes on both sides of this suture represent crustal melts derived largely from mid-lower crust of the Asian plate, exposed today as the Nyainqentanglha gneisses that underlie the Gangdese batholith. We infer that melting of the Asian lithosphere extended south of the surface trace of the suture, requiring southward propagation of anatectic Asian middle crustal material during the Miocene. The emplacement ages of the southern dike swarm (12–9 Ma) thus delimit the timing of active southward ductile flow of Asian material.

The tectonic significance of high-K2O volcanism in the Sierra Nevada, California

K 2 O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K 2 O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K 2 O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K 2 O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K 2 O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K 2 O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K 2 O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy.

Collision‐Driven Slab Breakoff Magmatism in Northwestern Anatolia, Turkey

Postcollisional Eocene magmatism in northwestern Anatolia produced two E‐W‐trending linear belts of plutons along and north of the Izmir‐Ankara‐Erzincan suture zone (IAESZ), whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution. The suture zone granitoids (SZGs) in the southern belt have ages around 54–48 Ma, are intrusive into blueschist rocks of the IAESZ, and are composed of diorite, quartz diorite, granodiorite, and syenite. The Marmara granitoids (MGs) in the northern belt are slightly younger (48–35 Ma), intrusive into the Paleozoic‐Mesozoic crystalline basement rocks of the Sakarya continent, and composed of monzogranite, granite, and granodiorite. Both SZGs and MGs have moderately to highly evolved medium‐ to high‐K calc‐alkaline compositions and are predominantly metaluminous I‐type granitoids. Nd‐Sr isotope systematics ($$^{87}\mathrm{Sr}\,/ {}^{86}\mathrm{Sr}\,_{( \mathrm{t}\,) }=0.70624{\mbox{--}} 0.70704$$; $$^{143}\mathrm{Nd}\,/ {}^{144}\mathrm{Nd}\,_{( \mathrm{t}\,) }=0.512430{\mbox{--}} 0.512439$$) and trace element compositions of the SZGs and MGs suggest a metasomatized lithospheric mantle source modified by the Late Cretaceous subduction event for their parental melts. Partial melting of this mantle lithosphere was facilitated by asthenospheric upwelling and associated thermal perturbation in response to a slab breakoff experienced by the Tethyan oceanic lithosphere subducted beneath the Sakarya continent. Mantle‐derived melts were modified by crustal contamination, assimilation, and fractional crystallization processes as they migrated through the overlying crust. Stronger depletion of the MG rocks in Eu, Ba, Sr, and P and their higher contents of Pb, K, Ni, and SiO2 in comparison to the SZG rocks suggest greater amounts of crustal contamination during the evolution of their magmas rising through the Sakarya continental crust. The geochemistry, petrogenesis, and geochronology of the subparallel SZG and MG belts along and north of the IAESZ provide a case study of the evolution of slab breakoff magmatism in an Alpine‐style collision zone.

Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China

Volcanic activity in Northeast China and adjacent regions is widespread, with eruption ages ranging from Late Cretaceous (80 Ma) to about 300 years ago. Volcanic rock types are principally basanite, alkali olivine basalt, and tholeiite, with minor evolved trachyte and rhyolite. Ultramafic xenoliths, mainly spinel lherzolite and harzburgite, are common in alkali olivine basalt and basanite. We present new major and trace element data from 11 volcanic fields, and helium isotope data from ultramafic xenoliths of three of those fields. 3He/ 4He ratio ranges from 5 to 7 times the atmospheric ratio, indicating that previously reported high 3He/ 4He ratios are likely due to the contamination by cosmogenic 3He. There is currently no evidence for a high- 3He/ 4He mantle plume component beneath NE China. Sr–Nd–Pb isotopic data from the literature are reinterpreted to result from mixing between a FOZO end-member and a LoMu end-member. The LoMu end-member appears to reside within continental lithosphere. Major and trace elements do not indicate significant contributions from a subducted slab. Differences in depth and the extent of partial melting in the asthenosphere, both locally within individual fields and regionally between the different volcanic fields, explain most of the correlations between major and trace elements observed in this study, with deeper melting usually associated with a smaller degree of melting. Highly potassic basalts are associated with the LoMu end-member and are interpreted to be the products of lithosphere melting.

Effect of melt structure on trace-element partitioning between clinopyroxene and silicic, alkaline, aluminous melts

We have investigated trace-element partitioning between clinopyroxene and silicic melts ranging from 62 to 69 wt% SiO 2 in piston-cylinder experiments at 1.1 GPa and 1145-1155 °C. Run products consist of diopsidic clinopyroxene (with 3-5 wt% Al 2 O 3 and ∼1 wt% Na 2 O) coexisting with silicic, aluminous, and alkaline melts. Clinopyroxene-melt partition coefficients for rare earth elements (REE) are significantly higher (e.g., D Sm = 0.76 to 3.1) than previous studies of clinopyroxene-basaltic melt partitioning. The maximum D REE is also shifted toward middle REE (e.g., Tb). Although the partition coefficients of Sr and Ti (D Sr = 0.23-0.39, D Ti = 0.28-1.0) are also elevated relative to clinopyroxene-basaltic melt, partition coefficients for Zr and Li (0.14-0.23 and 0.16-0.25, respectively) are within the range of literature values. The high partition coefficients for REE, Sr, and Ti in this study do not reflect crystal-compositional effects. For instance, no relationship is observed between IV Al and these partition coefficients. REE and Ti partition coefficients correlate poorly with jadeite component of the clinopyroxene; however, the elevated Na 2 O contents of both the melt and clinopyroxene make it difficult to discriminate fully the effect of jadeite component from the effect of changes in melt structure. The non-bridging oxygen to tetrahedral cation ratio (NBO/T; Mysen et al. 1985) varies from 0.08 to 0.17 and roughly correlates with epx/melt D REE , consistent with a recent study (Gaetani 2004) that concluded that melt structure significantly affects partitioning when NBO/T < 0.49. Notably, we find a strong correlation between the molar Ca 2+ /(M + + M 2+ ) of the melt and the optimum D (D 0 ) for the REE, indicating that melt structure, and not jadeite content, is the dominant control on clinopyroxene-melt partitioning in these experiments. The partition coefficients for the REE, Ti, Zr, and Sr are in good agreement with observed clino-pyroxene-silicic glass partitioning within mantle xenoliths (Chazot et al. 1996; Vannucci et al. 1998), indicating these results may apply to lithospheric melting. Furthermore, the origin of hump-shaped REE patterns observed in residual clinopyroxene from abyssal peridotites could be explained if shallow depleted melts of peridotite are silica-rich. A two-stage fractional melting model assuming clinopy-roxene-silicic melt partitioning at shallow mantle depths from 10 to 30 km beneath a mid-ocean ridge can reproduce the observed hump-shaped REE patterns. If so, hump-shaped REE patterns may provide evidence for the existence and extraction of silica-rich melt created in the shallow mantle.

Sr, Nd, Pb and Hf isotopic compositions of late Cenozoic alkali basalts in South Korea: Evidence for mixing between the two dominant asthenospheric mantle domains beneath East Asia

We have determined the Sr, Nd, Pb and Hf isotopic compositions of 13 samples from six late Cenozoic centers of basaltic volcanism in South Korea, including Baengnyeong Island, Jogokni, Ganseong, Jeju Island, Ulleung Island and Dok Island, in order to understand the nature of the mantle source and melt-generating processes. The basalts have OIB-like trace element abundance patterns, and also contain mantle-derived xenoliths of primarily spinel peridotite. Combining our new isotopic data with previously published data from areas with late Cenozoic basalts throughout East Asia shows that the mantle source has a DMM–EM1 array for northeast China and a DMM–EM2 array for Southeast Asia. The Korean Peninsula basalts we have studied are geographically located between these two broad domains, and interestingly, define an array between DMM and an intermediate end member between EM1 and EM2 on various isotopic correlation diagrams. Variations along the array on isotopic correlation diagrams are systematic regionally, with EM2 signatures predominant on Jeju Island and EM1 becoming increasingly more important toward Ulleung and Dok Islands, the Ganseong area, and Baengnyeong Island. This is without any corresponding changes in the basement and the lithospheric mantle beneath the region. These observations suggest that the Korean Peninsula late Cenozoic basalts we have studied and other East Asia late Cenozoic intraplate volcanism are sourced in the asthenospheric mantle. This mantle is characterized by two distinct, large-scale domains — one a mixture of DMM and EM1 components, and the other a mixture of DMM and EM2. Previous studies on East Asian Cenozoic volcanic rocks have advocated origins by either plume activity or decompressional melting of the mantle lithosphere in a rift environment. On the basis of our new trace element and isotopic compositions, which have OIB-like characteristics, we rule out an origin in the mantle lithosphere. Furthermore, because of the absence of typical hotspot tracks with clear age progressions, and so far, lack of unequivocal tomographic images delineating distinct, deep-seated thermal anomalies that could be interpreted as plumes, we suggest that the late Cenozoic alkalic magmatism in East Asia originated in the shallow asthenosphere, largely in response to dramatic changes in stress regimes. The interplay between the India–Eurasia collision and subduction of the Pacific Plate beneath the eastern margin of Eurasia resulted in a late Cenozoic tensile setting in East Asia capable of inducing melting in the asthenosphere by decompression. This is contrary to previous studies which advocated a plume or a mantle lithospheric origin for the East Asia late Cenozoic alkalic volcanism.

Fault-controlled emplacement of arc-related magmas along the Neoproterozoic northern Gondwanan margin: An example from the Antigonish Highlands, Nova Scotia

In the Antigonish Highlands, Nova Scotia, Neoproterozoic (ca. 620–605 Ma) magmatism is a local representative of regional arc-related tectonothermal activity that typifies Avalonia and related terranes located along the northern Gondwanan margin. Three distinct types of coeval magma are represented by volcanic rocks of the Georgeville Group and by syn- to post-tectonic plutons (e.g. Greendale Complex); (i) calc-alkalic subduction-related basaltic andesites and andesites, (ii) crustally-derived rhyodacites and rhyolites, and (iii) Fe–Ti rich continental tholeiites derived from the melting of subcontinental lithosphere. These magmas were probably generated in an extensional arc regime with deep-seated faults providing conduits for rising magmas. The Georgeville Group was polydeformed by thrusts, isoclinal folds and upright folds associated with movement along NE-trending faults very soon after it was deposited. A 85-km-long 1–2 km wide magnetic anomaly transects Georgeville Group strata and connects outcrops of Fe–Ti rich continental tholeiites and ultramafic cumulate rocks of the Greendale Complex. The N–S orientation of this anomaly is compatible with coeval sinistral shear along NE-trending faults, and its magnetic signature suggests that it is a shallow (<1 km) complex of narrow dykes that fed the tholeiites. This suggests that continental tholeiites were emplaced in a local sinistral transtensional basin within the arc along the northern Gondwanan margin. Deformation of the basin-fill volcanic and sedimentary rocks was accompanied by a switch to dextral shear during basin inversion. Regional studies indicate that the kinematic reversal and the cessation of magmatism is a local expression of the Neoproterozoic–Early Paleozoic termination of subduction and generation of a transform fault along the northern Gondwanan margin, possibly by ridge–trench collision, analogous to the Cenozoic evolution of the San Andreas Fault system in the western United States.

The Hudson Volcano and surrounding monogenetic centres (Chilean Patagonia): An example of volcanism associated with ridge–trench collision environment

Hudson Volcano (45°54′S, 72°58′W) represents the southernmost central volcano of the South Andean Volcanic Zone (SVZ). It lies about 280 km east of Nazca–Antarctic–South American Plate Triple Junction, where the Chile spreading ridge enters the Chile Trench. Hudson is located on the border of a volcanic gap separating the Southern from the Austral Volcanic Zone (AVZ), which is commonly interpreted as linked to a slab window. The volcano is characterised by a volcano-tectonic depression about 10 km wide, set in a morphological high of the Patagonian Batholith. The scarce pre-caldera products are made up of mafic lavas interbedded with hyaloclastite. The majority of the volcanic products out from the caldera rims consist of thick deposits of lahars or hyaloclastitic breccias, subordinate lava flows and pyroclastic deposits linked to historical large explosive eruptions. Rough volume calculations on pre-caldera and post-caldera products suggest that the caldera formed in steps in the lifetime of the volcano and that it is mainly linked to a volcano-tectonic collapse during partial emptying of the magma chamber. The monogenetic cones surrounding Hudson (Río Ibanez and Murta) erupted primitive basalts that gave important clues on magma sources. The volcanic products of Hudson, with composition ranging from basalts to dacites, have a calc-alkaline signature. Nevertheless, they have distinctly higher K 2O, TiO 2, Na 2O, LILE, HFSE and REE than the calc-alkaline rocks of SVZ. The basaltic lavas from the monogenetic cones show La/Nb, Ba/Nb and Th/Ta significantly lower than the calc-alkaline lavas. Furthermore, they have some ratios as Th/Yb and La/Yb remarkably low, with values approaching those of MORB-like basalts. All the geochemical characteristics of Río Ibanez and Murta cones are very similar to those of the basalts dredged in the Chile Ridge. Geochemical modeling suggests that the Hudson basalts may derive from the basalts erupted at the Río Ibanez centre through an assimilation plus fractional crystallisation process. On the basis of these data, we propose a geodynamical model. In correspondence to the monogenetic centres, close to the border of the incipient opening of the slab window, the magma features are still those of the depleted asthenosphere under the subducting ridge. In this zone, remnant fragments of subducted oceanic crust undergo partial melting processes, while the mantle wedge is deeply modified by subduction-related components. In correspondence to Hudson and the nearby monogenetic centres, the MORB-type primary source component is modified by altered oceanic crust and variably mixed with andesitic magmas having a calc-alkaline signature. A contribution from partial melting of the continental lithosphere may be present in the complex source mixing process at the border of the slab window.

Petrogenesis of Hawaiian postshield lavas: Evidence from Nintoku Seamount, Emperor Seamount Chain

We report major and trace element and Sr and Nd isotope data for a 283 m thick sequence of postshield lavas drilled during Ocean Drilling Project Leg 197 from Site 1205, Nintoku Seamount, an approximately 56 m.y. old volcano from the Emperor Seamount Chain. At least 25 subaerially erupted lava flow units were sampled, of which all but one are alkali basalts. Clasts of hawaiite and mugearite are present in a conglomerate overlying the basement sequence. Similar rock types characterize the postshield stage of many young Hawaiian volcanoes. Major and trace element, and age‐corrected Sr and Nd isotope compositions of the Nintoku lavas are similar to those of young postshield lavas from the Hawaiian Islands. Concentrations of highly incompatible elements tend to increase, La/Yb and Nb/Zr ratios increase, and 87Sr/86Sr decreases with decreasing depth in the drill core. The more evolved rock types can be related to the underlying alkali basalts by fractional crystallization of olivine, clinopyroxene, plagioclase, and Fe‐Ti oxides, all of which occur as phenocryst or significant groundmass phases in these lavas. However, variations in the trace element and isotopic compositions of the Nintoku lavas indicate derivation from a heterogeneous mantle source. The isotopic and trace element compositions of the lavas can be modeled using a modified version of the Chen and Frey (1985) mixing model for Hawaiian lavas, in which small‐degree melts of the ∼60 m.y. old oceanic lithosphere beneath Nintoku, previously fertilized by small‐degree melts soon after its formation at a ridge axis, are mixed with melts from the Hawaiian plume. The contribution from the oceanic lithosphere became more important with time as Nintoku Seamount moved away from the Hawaiian plume axis. Together with other data from Leg 197, our results show that the 87Sr/86Sr ratios of not only shield but also postshield lavas from the Emperor Seamounts increased with decreasing age between ∼80 and 50 m.y. ago. For alkalic postshield lavas, this could result from either increased melting of the oceanic lithosphere beneath volcanoes situated on younger oceanic crust or lower degrees of incompatible trace element enrichment in the lowermost portions of younger oceanic lithosphere.

Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field ( JMVF), the site of the Valles caldera, lies at the intersection of the Jemez lineament, a Proterozoic suture, and the Cenozoic Rio Grande rift. Parental magmas are of two types: K-depleted silicaundersaturated, derived from the partial melting of lithospheric mantle with residual amphibole, and tholeiitic, derived from either asthenospheric or lithospheric mantle. Variability in silica-undersaturated basalts reflects contributions of melts derived from lherzolitic and pyroxenitic mantle, representing heterogeneous lithosphere associated with the suture. The K depletion is inherited by fractionated, crustally contaminated derivatives (hawaiites and mugearites), leading to distinctive incompatible trace element signatures, with Th/ (Nb,Ta) and La/(Nb,Ta) greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth. These compositions dominate the mafic and intermediate lavas, and the JMVF is therefore derived largely, and perhaps entirely, from melting of fertile continental Jemez lineament lithosphere during rift-related extension. Significant variations in Pb and Nd isotope ratios (Pb/Pb 1⁄4 17 20–18 93; Nd/ Nd 1⁄4 0 51244–0 51272) result from crustal contamination, whereas Sr/Sr is low and relatively uniform (0 7040–0 7048). We compare the effects of contamination by low-Sr/Sr crust with assimilation of high-Sr/Sr granitoid by partial melting, with Sr retained in a feldspathic residue. Both models satisfactorily reproduce the isotopic features of the rocks, but the lack of a measurable Eu anomaly in most JMVF mafic lavas is difficult to reconcile with a major role for residual plagioclase during petrogenesis.

Geochemistry and petrogenesis of mafic magmatic rocks of the Jharol Belt, India: geodynamic implication

The belt consisting of deep-water meta-sediments of the Aravalli Supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels. These are the basal Bagdunda volcanics and the Gopir magmatic rocks represented by Gopir dikes and flows associated with the ultramafic rocks along the Kaliguman lineament. The Bagdunda and Gopir mafic volcanic rocks are sub-alkaline, varying in composition from high Mg-tholeiite to basaltic komatiite. They are predominantly LREE depleted, but some flat REE patterns are also observed. Bulk geochemical data, especially the incompatible trace elements discount the possibility of crustal contamination. Gopir dikes are highly enriched in terms of their REE [(Ce/Sm) N and (Ce/Yb) N ] and other incompatible trace elements and closely resemble the basal Aravalli volcanics. Their ratio (rock)/ratio (PM) and normalized multi-element abundance patterns depict their overall enriched nature compared to N-MORB and primitive mantle. The Gopir and Bagdunda volcanics reflect trace element characteristics transitional between E-MORB and OIB. Gopir dikes show remarkable similarity with continental tholeiite and Continental Flood Basalts (CFBs) with negative Nb, P and Ti anomalies and low Nb/Ce ratio unlike the volcanics of both the suites. Petrogenetic modeling based on the compositionally corrected [Mg] and [Fe] abundances for Gopir dikes, Gopir and Bagdunda volcanics indicates (a) their derivation from non-pyrolitic sources (b) their derivation from sources that were variably enriched in [Fe/Mg] ratios with large variation in their [Fe] contents and (c) olivine was a major phase to fractionate, followed by a lesser amount of clinopyroxene in the case of Gopir volcanics, whereas in Bagdunda volcanics a combination of olivine and clinopyroxene fractionation is suggested. Based on the geochemical data, supported by field evidence, we propose a geodynamic model for the development of the Jharol Belt in which we suggest that the basement rock, i.e. the Banded Gneissic Complex (BGC) started rifting probably under influence of a mantle plume during the late Archaean–early Proterozoic period. During the opening of the basin, magma derived from asthenospheric mantle reached the surface contemporaneously with sedimentation. The first phase of volcanism is represented by Bagdunda volcanics. With continuous rifting, the crust became highly attenuated and facilitated asthenospheric upwelling, causing high degrees of melting (indicated by a large volume of mafic and ultramafic rocks) during the second phase of magmatism (Gopir volcanics) that occurred at later stages of Jharol sedimentation. Coeval melting of the sub-continental lithosphere under the adjoining BGC craton probably caused the emplacement of dikes along with the Gopir volcanics. At this stage of progressive rifting, an oceanic crust very similar to that in marginal basins developed in the Jharol Belt.

Miocene volcanism in the Lhasa block, Tibet: spatial trends and geodynamic implications

Miocene ultrapotassic (UP), shoshonitic (P) and calc-alkaline (CA) volcanism is recognized widely over the Tibetan plateau. This volcanism occurred after the end of India’s oceanic lithosphere subduction beneath the plateau. Ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data. We have sampled two volcanic fields in the Lhasa block: (1) the eastern edge of the Zabuye Salt Lake (central part of the block), (2) the southwestern edge of the Yangbajin Graben. Major and trace elements display high K 2O (6.06–6.54%) over a wide range of SiO 2 (75.03–55.30%), Na 2O (1.32–3.30%) and LREE/HREE enrichment with a significant rare earth element fractionation (26<(La/Yb) n <42) as well as negative Nb, Ta, and Ti anomalies. These features indicate that our samples are chemically identical to the post-collision Miocene UP to P volcanic rocks already identified all over the Tibetan plateau and the southern plateau specifically but distinct from the CA rocks found in the Lhasa block. The Miocene volcanics found in the Lhasa block present greater chemical variation than the rest of the Tibetan plateau. 40Ar/ 39Ar ages on both sanidine and biotite gave ages ranging from 16.16±0.12 Ma to 16.01±0.12 Ma and 10.60±0.14 Ma to 10.88±0.17 Ma (2σ, excluding systematic errors, relative to Fish Canyon sanidine at 28.02 Ma) for Zabuye Salt Lake and Yangying geothermal field volcanics respectively. These results are consistent with the range of previously published ages from this part of the Tibetan plateau. The presence of N–S-trending dikes and the location of the volcanic fields suggest that local E–W extension in the Lhasa block was coeval. We propose that the synchronous UP, P and CA volcanism in the Lhasa block between 23 and 8 Ma is the result of progressively deepening northward subduction of Indian continental lithosphere producing asthenospheric upwelling. Convective thinning (delamination) of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub-continental mantle lithosphere as well as eclogitic lower crust. This mechanism could also explain the local E–W extension observed in the same period (18–10 Ma). We also propose that lateral migration of the deepening Indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the Lhasa terrane.

Are laccolith complexes characteristic of transtensional basin systems? Examples from the Permo-Carboniferous of Central Europe

Abstract By comparing felsic laccolith complexes prominent in the Late Palaeozoic Ilfeld-, Saar-Nahe-, and Saale basins in Germany, a characteristic pattern related to transtensional tectonics is revealed. In contrast to the central magma feeding systems recognized so far for laccolith complexes, individual units of the Late Palaeozoic Central European complexes apparently were fed synchronously by numerous feeder systems arranged laterally in a systematic pattern. The Ilfeld Basin is a small strike-slip pull-apart basin in the SE of the Hartz Mountains cogenetic with a neighbouring rhomb horst — the Kyffhäuser. The Ilfeld Basin represents a ‘frozen-in’ early stage of laccolith complex evolution, with small isolated intrusions and domes emplaced within a common level at the intersections of intra-basinal Riedel shears. In the Saar-Nahe basin, numerous medium-sized felsic subvolcanic to subaerial complexes emplaced at a common level have been recognized (Donnersberg-type laccolith complex). The magmatic evolution of the Halle Volcanic Complex in the Saale Basin culminated in the more or less synchronous emplacement of voluminous porphyritic laccoliths within different levels of a thick pile of Late Carboniferous sediments (Halle-type laccolith complex). Laccoliths in the Halle area might consist of several laccoliths typical of the Saar-Nahe Basin according to outcrop pattern and host sediment distribution. The above-mentioned three post-Variscan Central European basins are characterized by a dextral transtensional tectonic regime, leading to a model for laccolith-complex evolution: 1 Initial lithosphere-wide faulting forms pathways for magma ascent. 2 Supracrustal pull-apart leads to the formation of a transtensional basin. 3 Continued transtension gives way to decompressional melting of the mantle lithosphere, especially if fertilized by previous magmatic activity, as in the Variscan orogen. The mantle melts rise into the lower crust to differentiate, mingle or cause anatexis. 4 The melts homogenize and start crystallizing in a mid- to upper-crustal magma chamber tapped during tectonic episodes. 5 The resulting SiO 2 -rich magmas ascend along major transtensional faults into thick sedimentary basin fill. The amount of transtension and the amount of melt rising from the lithospheric mantle have a major influence on the type and size of the laccolith complex to be formed. Additionally, the presence of a mid- to upper crustal magma chamber is a prerequisite for the formation of the Donnerberg and Halle type laccolith complexes.

U-series disequilibria in volcanic rocks from the Canary Islands: Plume versus lithospheric melting

U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and ( 238U)/( 232Th), ( 230Th)/( 232Th), ( 230Th)/( 238U) and ( 234U)/( 238U) for 43 samples, most of which are minimally differentiated, along with ( 226Ra)/( 230Th) and ( 231Pa)/( 235U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial ( 230Th)/( 238U) ranges from 1.1 to 1.6. ( 226Ra)/( 230Th) o ranges between 0.9 and 1.8 while ( 231Pa)/( 235U) o ranges between 1.0 and 2.0. Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume. Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia.

Dynamic processes controlling evolution of rifted basins

The extension of the lithosphere, controlling the development of rifted basins, is driven by a combination of plate-boundary forces, frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system. Although mantle plumes are not a primary driving force of rifting, they play an important secondary role by weakening the lithosphere and by controlling the level of rift-related volcanic activity. A distinction between “active” and “passive” rifting is only conditionally justified. The extension of the lithosphere, depending on its rate and magnitude, and the potential temperature of the asthenosphere, can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere. In rift systems, the level and timing of volcanic activity is highly variable. The lack of volcanic activity implies “passive” rifting. An initial “passive” rifting stage can be followed by a more “active” one during which magmatism plays an increasingly important role. Magmatic destabilization of the Moho may account for the frequently observed discrepancy between upper and lower crustal extension factors. Combined with evidence for thermal thinning of the mantle–lithosphere, this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models. The structural style of rifts is controlled by the rheological structure of the lithosphere, the availability of crustal discontinuities that can be tensionally reactivated, the mode (orthogonal or oblique) and amount of extension, and the lithological composition of pre- and syn-rift sediments. Simple-shear extension prevails in rifts that subparallel the structural grain of the basement. Pure-shear extension is typical for rifts cross-cutting the basement grain. Pre-existing crustal and mantle–lithospheric discontinuities contribute to the localization of rift systems. The duration of the rifting stage of extensional basins is highly variable. Stress field changes can cause abrupt termination of rifting. In major rift systems, progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts. Depending on constraints on lateral block movements, crustal separation can be achieved after as little as 9 My and as much as 280 My of rifting activity. Syn-rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere, its magmatic inflation and thermal attenuation of the mantle–lithosphere. Post-rift basin subsidence is governed by thermal reequilibration of the lithosphere–asthenosphere system. Deep-seated thermal anomalies related to syn-rift pull-up of the asthenosphere–lithosphere boundary have decayed after 60 My by about 65% and after 180 My by about 95%. The magnitude of post-rift subsidence is a function of the rift-induced thermal anomaly and crustal density changes, the potential temperature of the asthenosphere and initial water depths. Intraplate stresses can have an overprinting effect on post-rift subsidence. Stretching factors derived from post-rift subsidence analyses must be corrected for such effects.

15N-enriched Gondwana lamproites, eastern India: crustal N in the mantle source

A comagmatic suite of lamproites, ultramafic lamprophyres, and minettes was intruded at ∼100 Ma into thick siliciclastic sequences deposited in five intracratonic basins which developed during Gondwana rifting in eastern India. Lamproites are dominated by microphenocrysts of Ti-rich phlogopite, and olivine pseudomorphed by serpentine and talc, with variable modal abundances of aegirine, amphibole, apatite, carbonates, Cr-spinel, fluorite, perovskite, priderite, and titanomagnetite. Compositionally, lamproites are characterized by extremely high contents of TiO2 (4.2–10 wt%), K2O, and P2O5, over a range of Mg# from 80 to 75, in keeping with lamproites worldwide. They have high abundances of Ba, Th, U, and REE (1019–5400 ppm), with pronounced fractionation of REE, where La/Smcn=3.5–5.6, and Gd/Ybcn=7.3–18, consistent with residual garnet in the mantle source. Nitrogen isotope compositions range from +1.6 to +8.7‰ with an average of 3.8‰, and N contents average 107 ppm. Mantle δ15N averages −5‰, with a N content of 2 ppm, whereas continental crust varies from 2 to 6‰ with 30–90 ppm N. Atmospheric N2 is sequestered by N-fixing microorganisms, and stored as kerogen with an average δ15N of +3‰ in sedimentary rocks. Some of the organic N is converted to NH4+ which substitutes for K in crustal K-silicates. The 15N-enriched lamproites are interpreted to have formed from partial melting of enriched harzburgitic mantle lithosphere by decompressional melting accompanying extension. Enrichment resulted from subduction–erosion of continental crust, or subduction of sediments, low-degree partial melting, hybridization of the melts with mantle lithosphere and incubation, prior to decompressional melting. Consequently, some crustal N is recycled to the mantle.

A tectonostratigraphic synthesis of the Sub-Andean basins: inferences on the position of South American intraplate accommodation zones and their control on South Atlantic opening

It has been shown in the accompanying paper that the Sub-Andean foreland can be subdivided longitudinally into a number of tectonostratigraphic domains. To test the hypothesis that changes in palaeo-depositional setting rely on the presence of a series of transverse zones of structural accommodation, data have been digitally compiled from across the South American continent. Spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries (structural accommodation zones), and patterns of subsidence and intraplate deformation. The results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which, on a regional scale, interlink to form a transcontinental belt or zone that can accommodate intraplate deformation during episodes of plate reorganization. Their strong spatial relationship with Mesozoic, intraplate, alkaline igneous activity suggests that they exerted an important control on lithospheric melt siting during Gondwana breakup. These localized zones of high heat flow have important implications for source rock maturity in the interior, Phanerozoic intracratonic basins of South America. On the South Atlantic margin, the majority of these crustal lineaments correlate with failed arms of triple-junction rifts and define the boundaries to tectonostratigraphic domains recognized along the South Atlantic Rift System.

How Laramide-Age Hydration of North American Lithosphere by the Farallon Slab Controlled Subsequent Activity in the Western United States

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existing explanations for uplift or magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly beneath the Rocky Mountains of Colorado and New Mexico, and high in velocity to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the lowvelocity volume is partially molten Precambrian North America mantle. We suggest, as others have, that the Farallon slab was in contact with the lithosphere beneath most of the western U.S. during the Laramide orogeny. We further suggest that slab de-watering under the increasingly cool conditions of slab contact with North America hydrated the base of the continental lithosphere, causing a steady regional uplift of the western U.S. during the Laramide orogeny. Imaged low-velocity upper mantle is attributed to hydration-induced lithospheric melting beneath much of the southern Rocky Mountains. Laramide-age magmatic ascent heated and weakened the lithosphere, which in turn allowed horizontal shortening to occur in the mantle beneath the region of Laramide thrusting in the southern Rocky Mountains. Subsequent Farallon slab removal resulted in additional uplift through unloading. It also triggered vigorous magmatism, especially where asthenosphere made contact with the hydrated and relatively thin and fertile lithosphere of what now is the Basin and Range. This mantle now is dry, depleted of basaltic components, hot, buoyant, and weak.

Post‐collisional Adakitic Porphyries in Tibet: Geochemical and Sr‐Nd‐Pb Isotopic Constraints on Partial Melting of Oceanic Lithosphere and Crust‐Mantle Interaction

Abstract The distribution of Neogene felsic porphyries intruding in earlier granitic batholiths was mainly controlled by north‐south‐tending rifting zones and normal faults. The main rock types of the felsic porphyries include granodiorite‐porphyry, monzonitic granite‐porphyry and quartz monzonitic porphyry. The porphyries are characterized by high SiO2 (≥64.26%) and Al2O3 (&gt;15% at 70% SiO2), low Y and HREE (Yb) contents, strong enrichment of LILE and LERR, especially K and Sr. Geochemical features of the porphyries show distinct adakitic magma affinity. Nd, Sr and Pb isotopic compositions of the porphyries form a linear alignment from MORB to EM2, suggesting a mixing of the MORB reservoir with the metasomatized mantle reservoir. Considering also the geochemical characteristics of the porphyries and the sequence of observable structural‐thermal‐magmatic events at Gangdise, it is thought that the Neogene porphyries were formed by partial melting of dead subducted oceanic crust in a post‐collision setting. K‐enrichment in the porphyries is attributed to the interaction of slab‐derived melts, i.e., adakites, with the metasomatized mantle during the ascent There might be a delamination of residual eclogites or amphibole eclogites before the eruption of potassic lava on the Tibetan plateau since 13 Ma.