High Large Ion Lithophile Element Research Articles

A Cretaceous forearc ophiolite in the Shyok suture zone, Ladakh, NW India: Implications for the tectonic evolution of the Northwest Himalaya

The northwestern Himalaya in India contains critical evidence of the convergent margin and collision tectonics between the Ladakh arc and the Karakoram block. Here we present new petrochemical and geochronological data from a forearc ophiolite at the Shyok‐Nubra river confluence. Whole-rock geochemical data show relatively high large-ion lithophile elements (LILE), light rare earth elements (LREE), and Ba, U and Pb anomalies, and depletions in La, Ce and Zr, particularly, the high-field strength elements (e.g. Nb, Ta); these geochemical characteristics are similar to those in modern ophiolites that formed in arc-related environments. Meta-volcanic greenschists that contain spinel layers have high MgO, Ni, Co, and Cr contents. In contrast, they have low contents of TiO2, very low Nb and Zr that are diagnostic of high‐Ca boninitic magmas in modern forearc settings, as in Izu–Bonin–Mariana (IBM) and Tonga. The spinels have high Cr# [Cr/(Cr+Al)] and Mg#[Mg/(Mg+Fe2+)], which are characteristic of spinels in forearc boninite‐type of melts. The spinel-free meta-volcanic samples have incompatible trace element abundance patterns similar to those of MORB. However, their enrichments in Cs, Rb, Pb, U and depletion in HFSE may reflect an input of subduction fluids that are different from MORB; these MORB-like basalts are suggestive of a forearc complex that erupted prior to the formation of boninitic rocks.Our data from the Shyok ophiolite indicate the existence of supra-subduction rocks on the southern Karakoram margin. Albite porphyroblasts in greenschists yield a K–Ar age of 104.4±5.6Ma that represents the time of early exhumation of the greenschists. The boninite‐type melts formed prior to 104Ma, thereby indicating that the time of initiation of subduction on the southern margin of the Karakoram block was before 104Ma. These geochemical signatures together with the spatial and temporal distribution of the arc rocks on the southern margin of Karakoram block suggest northward subduction of NeoTethys in the Early Cretaceous. The subsequent collision between the Ladakh arc and the Karakoram block thrusted/obducted the forearc ophiolite onto the southern edge of the Karakoram block probably between 74Ma and 97Ma.

Permian-Carboniferous arc magmatism and basin evolution along the western margin of Pangea: Geochemical and geochronological evidence from the eastern Acatlan Complex, southern Mexico

In the Acatlan Complex of southern Mexico, a late Paleozoic assemblage, consisting of a gabbro-diorite-tonalite-trondhjemite suite (Totoltepec pluton) and clastic-calcareous metasedimentary rocks (Tecomate Formation), postdates collisional orogeny that resulted in the amalgamation of Pangea. This region offers a rare opportunity to examine assemblages developed at different crustal levels along the periphery of Pangea at the critical stage between amalgamation and breakup. The Totoltepec pluton consists of minor mafic-ultramafic rocks (306 ± 2 Ma; concordant U-Pb zircon analysis) that are marginal to the main mafic-felsic intrusion (289 ± 2 Ma). Geochemistry of the marginal rocks indicates an arc tholeiitic to calc-alkaline character with high large ion lithophile elements (LILEs)/high field strength elements (HFSEs), flat rare earth element (REE) patterns, and initial ɛ Nd values of +1.3 to +3.3. The younger Totoltepec phase exhibits a calc-alkaline trace-element geochemistry with flat to moderately fractionated light (L) REE–enriched patterns and initial ɛ Nd values of –0.8 to +2.6, which are also consistent with an arc environment. The Sm-Nd isotopic signature is more primitive compared to contemporaneous arc-related igneous rocks in southern Mexico, suggesting the pluton was emplaced in a less mature, outboard part of the arc, and/or along a fault conduit. The Tecomate Formation, as currently defined, is a composite of lithologically similar strata deposited in several fault-bounded basins ranging from Carboniferous to Early Permian in age. To the south of the Totoltepec pluton, the depositional age of the Tecomate Formation is tightly constrained in one section to ca. 300 Ma, but in another section, it is between ca. 288 and ca. 263 Ma. The Tecomate Formation rocks are interpreted to have been derived from a late Paleozoic arc based on (1) arc-related geochemistry, (2) ɛ Nd (t) values ranging from –5.6 to +0.3 (t = 288 Ma) that overlap those of the Totoltepec pluton, and (3) detrital zircons with predominantly Carboniferous–Permian ages. The Totoltepec and Tecomate units in the study area form part of a continental arc extending from Guatemala to California, which necessitates subduction of the paleo-Pacific oceanic lithosphere beneath the western margin of a Pangea-A configuration.

Geochemistry and petrogenesis of the Huashan granites and their implications for the Mesozoic tectonic settings in the Xiaoqinling gold mineralization belt, NW China

The Huashan complex granitic batholith, occurred in the core of the Xiaoqinling orogen and gold mineralization belt along the south margin of the North China Craton, consists of amphibole monzogranite and biotite monzogranite in the Wengyu and Fangshanyu valleys, respectively. Zircon LA-ICP-MS U–Pb dating yields ages of 205±2Ma and 132±1Ma for the Wengyu and Fangshanyu granites, respectively, which represent the two major episodes of Mesozoic magmatism in the region. These rocks are characterized by metaluminous, high silica and total alkalinity, and have high FeOtotal/(FeOtotal+MgO) ratios, high large-ion lithophile elements (LILEs, especially Sr and Ba) and light rare earth elements (LREEs), and low heavy rare earth elements (HREEs) and Y concentrations and insignificant negative Eu anomalies. The Wengyu granites have higher Ba and Sr contents, which can be classified as high Ba–Sr granites, whereas the Fangshanyu granites show much lower Ba and Sr contents and display an adakitic affinity. Elemental and isotopic compositions suggest that the main sources of both the Wengyu and Fangshanyu granites are likely the ancient basement materials of Taihua Group. However, the high Ba and Sr contents of the Wengyu granites require addition of small amounts of enriched lithospheric mantle metasomatised by fluid/melt derived from pelagic sediments-bearing subducted slab in the sources. The Wengyu granites were most likely generated earlier by partial melting of the lowest part of the crust due to the subducted slab break-off under the post-collision extensional stage of the continental collision orogeny. The Fangshanyu granites might be mainly derived later from the partial melting of thickened lower crustal materials, representing by the Taihua Group basement rocks, corresponding to the tectono-magmatism of the post-orogeny to rift extensional environment during the tectonic transition from the Paleo-Tethys subduction-collision system to the Paleo-Pacific regime.

A new petrogenetic model for low‐Ca boninites: Evidence from veined sub‐arc xenoliths on melt‐mantle interaction and melt fractionation

Low‐Ca boninites (LCB) are arc‐related magmatic rocks enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), Zr and Hf relative to medium to heavy REE (MREE‐HREE). These signatures are commonly attributed to a unique slab‐derived agent that metasomatized a depleted mantle source but their origin in such an agent remains enigmatic. We report andesite‐hosted refractory spinel harzburgite xenoliths from the Kamchatka arc, which contain a series of orthopyroxene‐rich veins; these veins range in thickness, the contents of clinopyroxene and amphibole and degrees of reaction with the host. Vein minerals in reaction zones with host harzburgites show progressive depletion in MREE‐HREE at constant Zr‐Hf and develop patterns (U‐shaped REE with Zr‐Hf spikes) that mimic those of LCB. Major and trace element modeling suggests that these veins (1) formed from a high‐temperature (≥1400°C), MgO‐rich (∼30 wt.%) and silicic (∼54 wt.% SiO2) initial melt, strongly depleted in Al2O3, TiO2and alkalis and (2) record fractionation of the initial melt to form hydrous liquids; the initial melt was equilibrated with metasomatized Kamchatka harzburgites and was similar in trace element abundances to island arc tholeiite (LREE‐depleted to flat LREE‐MREE patterns, low Nb and Ta but no significant Zr‐Hf anomalies, high LILE). We argue that primary magmas of LCB formed by fluid‐fluxed (LILE‐rich and [Nb, Ta]‐depleted fluid) melting of a cpx‐free, highly refractory (≥30% melt extraction) harzburgitic source similar to arc peridotites from Kamchatka and elsewhere. This peculiar source may explain the distinctive major element features of LCB primary magmas. We propose that the primary magmas of LCB develop their characteristic trace element patterns through fractionation and reaction with refractory peridotites in the mantle wedge. Slab‐related components alone may explain the high LILE in LCB but not their distinctive REE patterns and positive Zr‐Hf anomalies.

Triassic granitoids in the eastern Songpan Ganzi Fold Belt, SW China: Magmatic response to geodynamics of the deep lithosphere

The Songpan Ganzi Fold Belt (SGFB), SW China, was developed from a passive continental margin into an orogenic belt with the consumption of the Paleo-Tethys. During the evolution of the SGFB, numerous Late Triassic granitic plutons formed and exhibited a progressive development from adakite/I-type granite, high Ba–Sr granite, A-type granite and monzonite. Representative Late Triassic plutons were studied to unravel the bewildering evolution of the eastern SGFB. The Menggu Pluton (224 ± 3 Ma) consists of granites with high alkali (K2O+Na2O = 7.85–10.4 wt.%) and adakitic characteristics (Sr/Y = 19–38). The εNdT values (− 2.77 to − 5.03), initial 87Sr/86Sr ratios (0.7050–0.7063) and low Nb/Ta ratios (8–10) are indicative of an origin by partial melting of amphibolitic lower crust. Rocks from the Niuxingou Pluton (215 ± 3 Ma) are richer in K than Na (K2O/Na2O = 1.1–1.5) and contain high Sr (1006–1662 ppm) and Ba (1277–2009 ppm), typical of shoshonite and high Ba–Sr granite. They have less enriched εNdT values (+ 0.08 to − 2.04) and less radiogenic 87Sr/86Sri ratios (0.7047–0.7048), and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle. The Taiyanghe Pluton (205 ± 3 Ma) consists of monzonites, with high Al2O3 (> 20 wt.%), but low MgO (0.94–1.39 wt.%). The rocks are richer in Na than K (K2O/Na2O < 0.7), contain high large ion lithophile element (LILE) (681–834 ppm Sr and 2142–9453 ppm Ba) and display strongly fractionated REE patterns ((La/Yb)N = 35–63). These features, together with their enriched Nd–Sr isotopic compositions (εNdT = − 4.78 to − 6.50; 87Sr/86Sri = 0.7074–0.7090), suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle. Although a continuous compressional regime during the Mid- and Late Triassic has been invoked for the SGFB, the generation of crustally derived adakitic and shoshonitic plutons reflect thickening in response to an arc-continental collision accompanied by fracturing of the lithosphere and an extensional regime in the deep lithosphere in the Late Triassic. The 205 Ma Taiyanghe Pluton was emplaced simultaneously with a rapid uplift of the lithosphere, when surface deposits changed from deep-water turbidite to tidal flat sediments. It was therefore generated during decompression, probably related to the rapid removal of the overthickened lithospheric mantle. The Triassic magmatism in the eastern SGFB is therefore important for probing geodynamic processes in the deep lithosphere.

Petrogenesis of Volcanic Rocks in the Khabr‐Marvast Tectonized Ophiolite: Evidence for Subduction Processes in the South‐Western Margin of Central Iranian Microcontinent

Abstract:The Late Cretaceous Khabr–Marvast tectonized ophiolite is located in the middle part of the Nain–Baft ophiolite belt, at the south‐western edge of the central Iranian microcontinent. Although all the volcanic rocks in the study area indicate subduction‐related magmatism (e.g. high LILE (large ion lithophile elements) / HFSE (high field strenght elements) ratios and negative anomalies in Nb and Ta), geological and geochemical data clearly distinguish two distinct groups of volcanic rocks in the tectonized association: (1) group 1 is comprised of hyaloclastic breccias, basaltic pillow lavas, and andesite sheet flows. These rocks represent the Nain–Baft oceanic crust; and (2) group 2 is alkaline lavas from the top section of the ophiolite suite. These lavas show shoshonite affinity, but do not support the propensity of ophiolite.

Subduction-related Volatile Recycling and Magma Generation beneath Central Mexico: Insights from Melt Inclusions, Oxygen Isotopes and Geodynamic Models

The subduction-related Michoaca¤n^Guanajuato Volcanic Field (MGVF) in central Mexico contains 900 cinder cones and numerous larger shield volcanoes of Late Pliocene to Holocene age.We present data for major, trace and volatile (H2O, CO2, S, Cl) elements in olivine-hosted melt inclusions from eight calc-alkaline cinder cones with primitive magma characteristics and one more evolved alkali basalt tuff ring.The samples span a region extending from the volcanic front to 175 km behind the front. Relationships between H2O and incompatible trace elements are used to estimate magmatic H2O contents for 269 additional volcanic centers across the MGVF and central Mexico. The results show that magmatic H2O remains high (3^575 wt %) for large distances (150 km) behind the front. Chlorine and S concentrations are strongly correlated with melt H2O and are also high across most of the arc (700^1350 ppm Cl, 1500^2000 ppm S).The alkali basalt, located far behind the front (175 km), has much lower volatile contents (515wt%H2O, 200 ppm Cl, 500 ppm S), and is compositionally similar to other melts erupted in this region. Oxygen isotope ratios of olivine phenocrysts (56^6o) from the calc-alkaline samples are higher than for typical mantle-derived magmas but do not vary systematically across the arc. Calc-alkaline samples have high large ion lithophile element concentrations relative to Nb andTa, as is typical of subduction-related magmas, but alkali basalt samples far behind the front have high Nb and Ta and lack enrichments in fluidmobile elements. Modeling based on volatiles and trace elements suggests that the calc-alkaline magmas were generated by 6^15% partial melting of a variably depleted mantle wedge that was fluxed with H2O-rich components from the subducted slab. In contrast, the alkali basalts formed by small degrees of decompression melting of an ocean island basalt source that had not been fluxed by slabderived components. Based on high d18Oolivine values and trace element characteristics, the H2O-rich subduction components added to the mantle wedge beneath the MGVF are likely to be mixtures of oceanic crust derived fluids and sediment melts. Integrating these results with new 2-D thermo-mechanical models of the subduction zone beneath the MGVF, we demonstrate that the present-day plate configuration beneath theMGVFcauses fluids to be released beneath the forearc and volcanic front, and that sediment melts can be produced beneath the volcanic front by the waning stages of fluid released from the oceanic crust percolating through already dehydrated sediments. Down-dragging of serpentine- and chloritebearing peridotite in the lowermost mantle wedge probably plays a role in fluid transport from the forearc to beneath the arc. H2O-rich magmas located more than 50 km behind the volcanic front can be explained by mantle hydration related to a shallower slab geometry that existed at 3 Ma. Rollback of the slab over the last 2 Myr has resulted in strong mantle advection that forms low-H2O, high- Nb alkali basaltic magmas by decompression melting far behind the present-day volcanic front.

Quaternary syn-collision magmatism from the Iran/Turkey borderlands

Quaternary basaltic and andesitic lavas from the NW Iran/eastern Turkey border area are related to the active Arabia–Eurasia collision. The lavas occur within the Turkish–Iranian plateau, which ceased crustal thickening before the establishment of a number of volcanic centres within Iran and eastern Turkey, beginning at ~ 10 Ma. Models for generating syn-collision magmatism in eastern Anatolia have invoked slab-break-off beneath the thick crust and thin mantle lithosphere of the Cenozoic East Anatolia Accretionary Complex and/or partial loss of the lower lithosphere. Here we report geochemical and Sm–Nd/Rb–Sr data from a ~ 200 km long, N–S traverse that samples volcanic flows in NW Iran, many of which originate from centres in Turkey such as Ararat and Tendürek. Samples are transitional alkali/tholeiitic basalts and andesites. Ararat samples have lower Nb, lower large ion lithophile element (LILE) concentrations with 143Nd/ 144Nd ~ 0.51290. Other, volumetrically smaller, centres have higher Nb, higher LILE, with 143Nd/ 144Nd ~ 0.51265. Abundances of LILE and Nb increase from north to south. The presumed degree of partial melting increases in the opposite direction, away from the Arabia–Eurasia suture. Melting is inferred to have taken place in the spinel lherzolite field, largely from a continental lithosphere source influenced by Mesozoic and early Cenozoic Neo-Tethyan subduction, but a separate source with long-term enrichment is needed to explain the high Nb, lower 143Nd/ 144Nd compositions.

Assimilation of Plutonic Roots, Formation of High-K ‘Exotic’ Melt Inclusions and Genesis of Andesitic Magmas at Volcán De Colima, Mexico

Phenocryst-hosted melt inclusions from the 1998^2005 andesite eruptions of Volcan de Colima (Mexico) show broad ranges of major and trace element contents that do not overlap with the bulk- rock compositions and indicate that melt inclusions can be formed by and record a range of processes involved in the genesis of andesites. The melt inclusions that demonstrably record the evolution of the melt feeding the eruption indicate low-pressure (130^10 MPa) crys- tallization of a dacitic melt despite the monotonous bulk andesitic composition of historical magmas at Volcan de Colima. Mingling of dacite melt with gabbroic fragments in the shallow sub-volcanic system is the process responsible for generating the bulk andesitic composition of the magmas. A significant proportion of the melt inclusions have distinctive high large ion lithophile element (LILE) signatures. These 'exotic' high-K melt inclusions in pyrox- enes are thought to result from incongruent melting of interstitial bio- tite during assimilation of gabbroic fragments in the dacitic melt. A second group of exotic high-K melt inclusions found in plagioclase are likely to result from dissolution of higher-pressure (4200 MPa) amphibole, plagioclase, magnetite and biotite cumulates during assimilation in the ascending dacitic melt. Although they are not volumetrically abundant, high-K melts formed during assimilation of plutonic fragments and crystal cumulates made a significant con- tribution to the LILE contents of the magmas and represent a poten- tial source for this group of elements. The range of melt inclusion compositions in Volcan de Colima magmas emphasizes the impor- tance of mixing between ascending evolved melts and crystal popula- tions formed during previous episodes of magmatism over a range of pressures, temperatures and volatile contents. Cannibalization of plutonic roots appears to be a fundamental process in the genesis of andesite magmas and melt inclusions at continental arc volcanoes.

Calc-Alkaline Magmatism at the Archean–Proterozoic Transition: the Caicó Complex Basement (NE Brazil)

The Paleoproterozoic metaplutonic rocks of the Caico* Complex Basement (Serido* region, NE Brazil) provide important and crucial insights into the petrogenetic processes governing crustal growth and may potentially be a proxy for understanding the Archean^ Proterozoic transition. These rocks consist of high-K calc-alkaline diorite to granite, with Rb^Sr, U^Pb, Pb^Pb and Sm^Nd ages of c. 225^215 Ga. They are metaluminous, with high YbN, K2O/Na2Oand Rb/Sr, low ISr ratios, and are large ion lithophile elements (LILE) enriched. Petrographic and geochemical data demonstrate that they belong to differentiated series that evolved by low-pressure fractionation, thus resulting in granodioritic liquids.We propose a model in which the petrogenesis of the Caico* Complex orthogneisses begins with partial melting of a metasomatically enriched spinel- to garnet-bearing lherzolite (with high-silica adakite melt as the metasomatic agent), generating a basic magma that subsequently evolved at depth through fractional crystallization of olivine, followed by low-pressure intracrustal fractionation. A subduction zone setting is proposed for this magmatism, to account for both negative anomalies in high field strength elements (HFSE) and LILE enrichment. Mantle-derived juvenile magmatism with the same age is also known in the SaÐ o Francisco andWest Africa cratons, as well as in French Guyana, and thus the Archean^Proterozoic transition marks a very important continental accretion event. It also represents a transition from slab-dominated (in the Archean) to wedge-dominated post-Archean magmatism.

Petrotectonic evolution and melt modeling of the Penon Blanco arc, central Sierra Nevada foothills, California

The Penon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Penon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ∼900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Penon Blanco Formation, which is made up of ∼700 m of crystal-lithic basaltic tuff, 1–3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Penon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Penon Blanco arc against the inboard Calaveras Complex. New geochemical data from the Penon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Penon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30–60 km depth by low to moderate amounts of partial melting (∼3%–5% for Jasper Point and 5%–7.5% for Penon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Penon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America.

Melting and Multi-stage Metasomatism in the Mantle Wedge beneath a Frontal Arc Inferred from Highly Depleted Peridotite Xenoliths from the Avacha Volcano, Southern Kamchatka

Peridotite xenoliths from the Avacha volcano, Kamchatka, derived from the mantle beneath the volcanic front of the Kamchatka arc, are mainly highly depleted clinopyroxene-poor harzburgites with highly forsteritic olivine (Fo 90^92) and high Cr-number spinel (0 5^0 7). The Avacha peridotites have experienced metasomatism to various extents, with the formation of metasomatic orthopyroxene replacing primary olivine, by infiltration of SiO2-rich fluids. The metasomatic orthopyroxenes can be subdivided into two textural types; (1) radially aggregated prismatic grains (opx II-1); (2) stout grains associated with interstitial glass and metasomatic minerals (opx II-2).The Avacha peridotites exhibit light rare earth element (LREE) enrichment relative to heavy REE (HREE), even in primary lithologies with52 vol.% of metasomatic orthopyroxene. However, the metasomatism has not altered the chemical characteristics of the Avacha peridotites significantly, and the Fo contents of olivine do not increase with an increase in the Cr-number of spinel. The clinopyroxene in the most depleted primary peridotite, with the highest spinel Cr-number, has the highest CeN (where subscript N indicates normalized to primitive mantle) and relatively lowYbN contents.We conclude that the Avacha peridotites are residues of high-degree melting induced by infiltration of an aqueous fluid with relatively high Fe/Mg and LREE/HREE ratios. In situ analysis of olivine grain boundaries by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) showed significant concentrations of some trace elements (Ce and Ba), even on glassfree boundaries; such grain boundaries can be a repository for largeion lithophile elements (LILE). Orthopyroxene and Ca-amphibole contribute to the HREE and middle REE (MREE) budget, respectively, of the bulk peridotites.Three types of mantle metasomatism are recorded in the Avacha peridotites. The agent that formed HREE-poor opx II-1 was a slab-derived aqueous fluid rich in SiO2. It was a successor of the fluid influx involved in the initial partial melting event. On the other hand, the metasomatic minerals in highly metasomatized peridotites have high LILE (Th, U and Sr) and LREE contents relative to HREE, indicating involvement of a hydrous melt of adakitic affinity derived from the subducted slab. Glasses associated with opx II-2 are similar in their trace-element characteristics to the host Avacha volcanic rocks, indicating the likelihood that the metasomatic agent was a forerunner of the recent Avacha arc magmatism.

Geochemical evidence in clinopyroxenes from gabbroic sequence for two distinct magmatisms in the Oman ophiolite

In the Oman ophiolite, one of the best preserved and most studied ophiolites in the world, two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the Wadi Haymiliyah section is established using trace element chemistry of Ca-rich clinopyroxenes. The earlier GB1 suite is characterized by plagioclase with lower An (Ca/(Ca + Na)) content and clinopyroxene with low large-ion lithophile elements (LILE) concentrations. The later DWGB2 suite contains plagioclase with rather high An content and clinopyroxene with high LILE. This difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section: lower, (earlier) HV1 suite with low LILE clinopyroxene and upper (later) HV2 suite with high LILE clinopyroxene. Difference in LILE concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas. The GB1/HV1 suites formed at fast-spreading MOR setting and DWGB2/HV2 suites at SSZ setting, supporting a model of transition from mid-oceanic ridge to supra-subduction zone settings of the Oman ophiolite. Our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites.

Reply to: Comments on “Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan volcanic province” by Ramananda Chakrabarti and Asish R. Basu

The ∼1.88 km diameter Lonar impact crater formed ∼570 ka ago and is an almost circular depression hosted entirely in the Poladpur suite of the ∼65 Ma old basalts of the Deccan Traps. To understand the effects of impact cratering on basaltic targets, commonly found on the surfaces of inner Solar System planetary bodies, major and trace element concentrations as well as Nd and Sr isotopic compositions were determined on a suite of selected samples composed of: basalts, a red bole sample, which is a product of basalt alteration, impact breccia, and impact glasses, either in the form of spherules (<1 mm in diameter) or non-spherical impact glasses (>1 mm and <1 cm). These data include the first highly siderophile element (HSE) concentrations for the Lonar spherules. The chemical index of alteration (CIA) values for the basalts and impact breccia (36.4–42.7) are low while the red bole sample shows a high CIA value (55.6 in the acid-leached sample), consistent with its origin by aqueous alteration of the basalts. The Lonar spherules are classified into two main groups based on their CIA values. Most spherules show low CIA values (Group 1: 34.7–40.5) overlapping with the basalts and impact breccia, while seven spherules show significantly higher CIA values (Group 2: >43.0). The Group 1 spherules are further subdivided into Groups 1a and 1b, with Group 1a spherules showing higher Ni and mostly higher Cr compared to the Group 1b spherules. Iridium and Cr concentrations of the spherules are consistent with the admixture of 1–8 wt% of a chondritic impactor to the basaltic target rocks. The impactor contribution is most prominent in the Group 1a and Group 2 spherules, which show higher Ni/Co, Ni/Cr and Cr/Co ratios compared to the target basalts. In contrast, the Group 1b spherules show major and trace element compositions that overlap with those of the impact breccia and are characterized by high EFTh (Enrichment Factor for Th defined as the Nb-normalized concentration of Th relative to that of the average basalt) as well as fractionated La/Sm(N), and higher large ion lithophile element (LILE) concentrations compared to the basalts. The relatively more radiogenic Sr and less radiogenic Nd isotopic composition of the impact breccia and non-spherical impact glasses compared to the target basalts are consistent with melting and mixing of the Precambrian basement beneath the Deccan basalt with up to 15 wt% contribution of the basement to these samples. Variations in the moderately siderophile element (MSE) concentration ratios of the impact breccia as well as all the spherules are best explained by contributions from three components – a chondritic impactor, the basaltic target rocks at Lonar and the basement underlying the Deccan basalts. The large variations in concentrations of volatile elements like Zn and Cu and correlated variations of EFCu-EFZn, EFPb-EFZn, EFK-EFZn and EFNa-EFZn, particularly in the Group 1a spherules, are best explained by evaporation-condensation effects during impact. While most spherules, irrespective of their general major and trace element composition, show a loss in volatile elements (e.g., Zn and Cu) relative to the target basalts, some spherules, mainly of Group 1, display enrichments in these elements that are interpreted to reflect the unique preservation of volatile-rich vapour condensates resulting from geochemical fractionation in a vertical direction within the vapour cloud.

A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle

There is no consensus on the processes responsible for near-coeval formation of Archaean continental crust (dominantly tonalite–trondhjemite–granodiorite: TTG), greenstone belts dominated by komatiitic to tholeiitic lavas (KT), and sub-continental lithospheric mantle (SCLM). The Douglas Harbour domain (2.7–2.9 Ga) of the Minto Block, northeastern Superior Province, has two TTG suites, the western and eastern Faribault–Thury (WFT and EFT), with embedded KT greenstones. Tonalites of both suites have high light/heavy rare-earth element ratios (L/HREE), high large ion lithophile element (U–Th–Rb–Cs–La: LILE) contents, positive Sr–Pb anomalies, and negative Nb–Ta–Ti anomalies. Such typical Archaean TTG signatures are commonly explained by melting of subducted oceanic crust, but could also originate by melting the base of thick basaltic plateaux formed above mantle upwellings (plumes), leaving behind restites containing pyroxene, garnet, and rutile. Field relationships (in situ segregation veins), phase equilibria (hornblende stabilized at lower crustal pressure), petrography (corroded epidote and muscovite phenocrysts, rare plagioclase phenocrysts), and trace element models, all imply that FT tonalite to trondhjemite evolution reflects hornblende-dominated fractional crystallization, not partial melting of subducted crust. The geochemistry of parental FT tonalites can be modeled by 15–30% melting of FT tholeiitic metabasalts, with residues of eclogite, garnet-websterite, or hornblende-garnet websterite. A minor residual Ti-phase such as rutile is also needed to generate negative Ti–Nb–Ta troughs in the TTGs. However, large volumes of eclogitic restites complementary to TTG are not observed either at the base of Archaean crustal sections, or in the SCLM. Additional problems with slab-melting models include: (a) the rarity of lithologies and associations characteristic of active margins (ophiolites, andesites, blueschists, accretionary mélanges, molasse, flysch, high-pressure belts, and thrust-and-fold belts); (b) the need to deliver plume-derived KT melt through the slab; and (c) extracting enough TTG melt from a subducting slab in the time available (200–300 my). In the plateau-melting model, heat for crustal anatexis is supplied by ongoing KT magma derived from mantle upwellings. However, SCLM rocks differ from predicted 1-stage mantle melting residua; and the voluminous residual eclogites complementary to TTG generation somehow need to be removed. These two problems might solve one another if the dense crustal restites disaggregated and mixed into the underlying depleted mantle. Mantle melting slows upon exhaustion of Ca–Al-rich phases, with large temperature increases needed to extract more melt from harzburgite residua. Physical addition of delaminated crustal restites would refertilize the refractory mantle, allowing extraction of additional melt increments, and might explain the ultra-depleted and orthopyroxene-rich nature of the SCLM. A hybrid source composed of 10% eclogitic restite of EFT tonalite generation, mixed with harzburgitic residues from 25% melting of primitive mantle, yields model melts with trace element signatures resembling typical Munro komatiites. Variations in the mineralogy and geochemistry of the delaminated component might account for the diversity of komatiite types. Degassing of hornblende-rich delaminated restites would transfer LILE to surrounding depleted mantle and could generate boninites. Fusion of undepleted metabasalt sandwiched among denser restites could generate sanukitoids. Mantle melt pulses generated by catastrophic delamination events would underplate nascent TTG crust and trigger renewed crustal melting, followed by delamination of newly formed eclogitic restites, triggering additional mantle melting, and so on. I posit that delamination of crustal restites catalyzed multi-stage melting of the SCLM and maturation of the Archaean continental crust. Thus, Archaean crust and SCLM are genetically inter-linked, and both form above major mantle upwellings.

Provenance of late Ordovician to early Cretaceous sedimentary rocks from southern Ghana, as inferred from Nd isotopes and trace elements

Geochemical and Nd-isotopic data are reported for 24 shale and sandstone samples comprising the Upper Ordovician to Lower Cretaceous Sekondian Group, southern Ghana. The data are interpreted in terms of the provenance of these siliclastic sediments and the Paleozoic and Mesozoic geologic development of southern Ghana. The sandstones and shales generally have trace element characteristics typical of material eroded from the upper continental crust. Cr and Ni concentrations are low suggesting their derivation from dominantly felsic sources. There appear to be chemical and Nd-isotopic differences between the lower formations (i.e., Ajua Shale and Elmina Sandstone) and the uppermost formation (i.e., Essikado Sandstone) on one hand, and the upper formations (i.e., Takoradi Sandstone, Takoradi Shale, Effia Nkwanta Beds and Sekondi Sandstone) on the other. The upper formations are characterized by evolved major element compositions, high large-ion lithophile element abundances, large negative Eu-anomalies (0.58–0.73), and low ε Nd values (−10.8 to −4.1), indicating stable craton settings. On the other hand, the Ajua Shale, Elmina Sandstone, and Essikado Sandstone formations have less evolved major element compositions, lower large-ion lithophile abundances, and higher Eu-anomalies (0.69–0.88), and this may reflect an active margin setting of the source (juvenile-type Birimian felsic volcanics and/or granitoids) rather than Phanerozoic active continental settings. The upper formations have T DM model ages of 1.8–2.3 Ga indicating that the Paleoproterozoic Birimian rocks are the ultimate sources, whereas the lower formations and uppermost formation have slightly younger T DM values of 1.6–1.9 Ga suggesting of a component from the Pan-African mobile belt. These differences in model ages may, however, be related to lithology (rather than stratigraphy) where the shales (1.6–2.3 Ga; mean, 1.9 Ga) typically give older ages than the sandstones (1.6–1.9 Ga; mean 1.7 Ga).

Eocene shoshonitic mafic dykes intruding the Monashee Complex, British Columbia: a petrogenetic relationship with the Kamloops Group volcanic sequence?

The newly named Three Valley Suite (TVS) kersantite lamprophyre to shoshonitic mafic dykes of the Monashee Complex are inferred to be hypabyssal feeder dykes to an alkaline to calc-alkaline volcanic suite related to the Kamloops Group. These dykes were emplaced in a subvertical north-trending orientation coincident with inferred Eocene crustal extension (~50.0 Ma), based on the flat 40Ar/39Ar step-heating plateau of contact-metamorphic muscovite on the margin of a TVS dyke. These weakly altered mafic dykes are fine grained with phenocrysts (0.5–2.0 mm) of phlogopite, augite, amphibole, and olivine (pseudomorphed by clays), rare labradorite, and both primary and secondary carbonates set in a fine-grained groundmass of similar mineralogy consistent with their classification as plagioclase-bearing potassic diorite to kersantite lamprophyre. The dykes are weakly silica-undersaturated and alkalic (2.8 wt.% K2O, 7.7 wt.% MgO), with high large ion lithophile element contents (~300 times primitive mantle) and elevated high-field-strength element contents, with a prominent negative Nb (Ta) anomaly, and have radiogenic Nd and Sr isotopic signatures; these geochemical attributes are consistent with a calc-alkaline shoshonitic affinity. Therefore, it is inferred that the subducting oceanic plate influenced subcrustal mantle wedge metasomatism in the region. Decompression partial melting of this metasomatised lithospheric mantle was initiated by coupled rapid unroofing, regional trans pression, slab rollback, and slab window development to the south. The TVS is similar to the mafic volcanic rocks within the nearby Eocene volcanic rocks, suggesting that these dykes represent the feeder system to a volcanic field that is now eroded, i.e., a broad-terrane association.

New 40 Ar/ 39 Ar ages and geochemistry of late Carboniferous-early Permian lamprophyres and related volcanic rocks in the Saxothuringian Zone of the Variscan Orogen (Germany)

Abstract 40 Ar/ 39 Ar step-heating dating of mineral separates from a series of lamprophyre dykes in the Saxothuringian Zone of the Variscan Orogen yielded Viséan-Namurian (334–323 Ma) and Stephanian-early Permian (297–295 Ma) crystallization ages indicating magma generation over a period of 30 Ma. In many cases, dyke emplacement was controlled by faults. Many are composite or show evidence for mingling of primitive and evolved magmas, and, to a certain degree, contamination with crustal melts. The high MgO (6–7 wt%), Ni (75–270 ppm) and Cr (140–1250 ppm) contents and mafic phenocryst assemblage are evidence for derivation from a mantle source. Kersantites and minettes have similar incompatible trace-element and rare earth element (REE) patterns (light REE (LREE)- and medium REE (MREE)-enriched and heavy REE (HREE)-depleted) and high, but varying Th, Zr and Hf contents. Positive Ni v. Mg# (FeO=FeO tot ) correlations suggest early fractionation of olivine, and the general absence of negative Eu anomalies makes feldspar fractionation improbable. For the lamprophyres of the Spessart, the variations of Ba, Rb and TiO 2 indicate phlogopite fractionation. Negative Ta, Nb and Ti anomalies are common, and may be an artefact of the high large ion lithophile element (LILE) and REE contents, but are more likely to reflect derivation from a mantle source that was metasomatized during a previous (Devonian?) subduction event. The generation of the parent melts was possibly triggered by partial melting of metasomatized mantle due to lithosphere detachment, removal and replacement of metasomatized lithospheric mantle by upwelling hot asthenospheric mantle. Compared to the spessartites, the minettes and kersantites appear to have originated by partial melting of deeper-mantle sources. Lithospheric mantle detachment may have caused post-collisional Namurian uplift and cooling of the crust, and facilitated emplacement of lamprophyre dykes along fault zones at high crustal levels.

Melt and source mantle compositions in the Late Archaean: A study of strontium and neodymium isotope and trace elements in clinopyroxenes from shoshonitic alkaline rocks

Ca clinopyroxenes (Cpx) in minette lamprophyres and syenites in the southern Abitibi belt show initial 87Sr/ 86Sr and εNd at 2680 Ma ranging from 0.7008 to 0.7016 and from +0.8 to +3.0, respectively, indicating the derivation of these magmas from a mildly depleted mantle. Strontium isotope values lie between the bulk earth and the depleted mantle values at 2.68 Ga, interpolated from present-day values and bulk earth initial values. The Cpx is nevertheless high in large ion lithophile elements (LILEs). The melts calculated from Cpx show fractionated REEs ([La] N/[Yb] N= 21–47 for lamprophyres and 34–80 for syenites) and high LILEs relative to high field strength elements (HFSEs). Niobium and titanium are particularly low in the calculated melts (Ti/Ti* = 0.05 to 0.21 for lamprophyres and 0.05 to 0.26 for syenites). The REEs in the bulk lamprophyres are similar to those in the calculated melts, but those of the bulk syenites are much lower than those of the calculated melts. This disparity is explained by the differentiation of slowly solidifying melts for the coarse-grained syenites, leaving bulk-rock LILE contents much lower than those in “quenched” lamprophyres.Isotopic and chemical data are consistent with a model of producing partial melts in a depleted mantle (low contents of LREEs and strontium, 87Sr/ 86Sr of 0.7008, and high ratios of Rb/Sr and 143Nd/ 144Nd), which was enriched by dehydration of subducting slabs. The slab component contained high contents of LREEs and strontium, high ratios of 87Sr/ 86Sr (∼0.7016) and Sm/Nd, low contents HFSEs, and low ratios of Rb/Sr and 143Nd/ 144Nd (εNd = ∼ +1). Low Rb/Sr ratios of this slab component are attributed to low Rb/Sr of Archaean sediments and slabs. Isotopic data indicate that the subducting slabs did not contain old cratonic material and the mantle metasomatism took place shortly before the alkaline magmatism, probably during the last volcanic episode of the greenstone belt (∼2700 Ma), which produced calc-alkaline andesites.

Contrasting compositions of Mariana Trough fallout tephra and Mariana Island arc volcanics: a fractional crystallization link

Discrete Quaternary (<400 ka) tephra fallout layers (mostly <1 cm thick) within the siliceous oozes of the central Mariana Trough at 18°N are characterized by medium-K to high-K subalkalic volcanic glasses (K2O=0.8–3.2 wt.%) with high large-ion lithophile elements (LILE)/high-field-strength elements (HFSE) ratios and Nb depletion (Ba/La≈35; Ba/Zr≈3.5; La/Nb≈4) typical for convergent margin volcanic rocks. Compositional zoning within layers ranges from basaltic to dacitic (SiO2=48–71 wt.%; MgO=0.7–6.5 wt.%); all layers contain basaltic andesites. The tephra layers are interpreted as single explosive eruptive events tapping chemically zoned reservoirs, the sources being the Mariana arc volcanoes (MAV) due to their proximity (100–400 km) and similar element ratios (MAV: Ba/La=36±7; Ba/Zr=3.5±0.9). The glasses investigated, however, contrast with the contemporaneous basaltic to dacitic lavas of the MAV by being more enriched in TiO2 (≈1.2 wt.%; MAV≈0.8 wt.%), FeO* (≈10 wt.%, MAV≈8–9 wt.%), K2O (≈1.1 wt.%; MAV≈0.8 wt.%) and P2O5 (≈0.4 wt.%; MAV≈0.2 wt.%). (Semi-)Incompatible trace elements (including Rare Earth Elements (REE)) of the basaltic-andesitic and dacitic glasses match those of the dacitic MAV lavas, which became enriched by fractional crystallization. Moreover, the glasses follow a tholeiitic trend of fractionation in contrast to MAV transitional trends and have a characteristic P2O5 trend that reaches a maximum of 0.6 wt.% P2O5 at ≈57 wt.% SiO2, whereas MAV lavas increase linearly in P2O5 from 0.1 to 0.3 wt.% with increasing silica. Both explosive and effusive series are interpreted to have evolved in common magma reservoirs by convective fractionation. Similar parental magmas are suggested to have separated into coexisting Si-andesitic to dacitic and basaltic melts by in situ crystallization. The differentiated melt is interstitial in an apatite-saturated crystalline mush of plag+px±ox±ol at the cooler chamber margins in contrast to the less differentiated basaltic to basaltic-andesitic magmas, which are not yet saturated in apatite and occupy the chamber interior. Reinjection of interstitial melt into the chamber interior and mixing with larger melt fractions of the interior liquid (mixing ratios about ≈1: 8–9) can explain the paradoxical behavior of apatite-controlled P and MREE variation in the basaltic andesite glasses and their MAV dacite-like fractionation patterns. The process may also account for the exclusively tholeiitic trend of fractionation of the glass shard series, but in situ crystallization alone cannot cause their absolute enrichment in (semi-)incompatible elements. The newly mixed melt is suggested to form the basaltic end member of the glass shard series. However, it must have become physically separated from the main MAV magma body (possibly by density-driven convective fractionation) in order to allow for further evolution of the contrasting geochemical paths as well as differentiation.