Abstract: The Early Permian (290 Ma) Panjal Traps are the largest contiguous outcropping of volcanic (basaltic, andesitic and silicic) rocks within the Himalaya that are associated with the Late Palaeozoic break-up of Gondwana. The basaltic Panjal Traps have compositional characteristics that range from continental tholeiite to ocean-floor basalt but it is clear that crustal contamination has played a role in their genesis. The basalts that show limited evidence for contamination have Sr–Nd isotopes ( 87 Sr/ 86 Sr i = 0.7043–0.7073; ε Nd ( t ) = 0 ± 1) similar to a chondritic (subcontinental lithospheric mantle) source, whereas the remaining basaltic rocks have a wide range of Nd ( ε Nd ( t ) = −6.1 to +4.3) and Sr ( 87 Sr/ 86 Sr i = 0.7051–0.7185) isotopic values. The primary melt composition of the low-Ti Panjal Traps is picritic with mantle potential temperatures ( T P = 1400°C to 1450°C) similar to ambient mantle. The silicic volcanic rocks were derived by partial melting of the crust, whereas the andesitic rocks were derived by mingling between crustal and mantle melts. The Panjal Traps initially erupted within a continental rift setting. The rift eventually transitioned into a nascent ocean basin that led to seafloor spreading and the formation of the Neotethys Ocean and the ribbon-like continent Cimmeria.

Abstract: In this paper we illustrate the geochemical diversity of the basalts of Panjal ( c. 289 Ma), Rajmahal ( c. 117 Ma) and Deccan ( c. 65 Ma) continental large igneous provinces (continental LIPs) from India as controlled by multiple mantle sources. The Panjal primitive basalts are formed by 5–20%, Rajmahal basalts by 10–20% and Deccan picritic basalts by 1–20% melting of the mantle. The depth of melting beneath Deccan is greater than beneath Rajmahal, which greater than beneath Panjal. We have categorized the Panjal, Rajmahal and Deccan basalts into Type I and Type II. The Type I basalts with lower La/Nb, Th/Nb and Th/Yb ratios and higher ε Nd i values ( ε Nd i = −3 to +8) are derived from the sublithospheric sources; some of the Type I basalts represent lithosphere-contaminated melts. The Type II basalts with higher La/Nb, Th/Nb and Th/Yb ratios and lower ε Nd i values ( ε Nd i = −20 to −3) are produced from the subcontinental lithosphere. We estimate that lithosphere and sublithosphere (plume and/or E-MORB patches within asthenosphere) have contributed, respectively, approximately 67 and 33% to the Panjal, 52 and 48% to Rajmahal, and 28 and 72% to Deccan LIPs. Fertility of the Indian subcontinental lithosphere may have increased with decreasing age related to the break-up of Gondwana. Supplementary material: References for the geochemical data, extracted from the GEOROC database, on the basaltic rocks from Panjal, Rajmahal and Deccan Large Igneous Provinces are available at: https://doi.org/10.6084/m9.figshare.c.3821881

Within the duration of the U.S. Geological Survey (USGS)–based Southern California Areal Mapping Project (SCAMP), many samples from the northern Peninsular Ranges batholith were studied for their whole-rock radioisotopic systematics (rubidium-strontium [Rb-Sr], uranium-thorium-lead [U-Th-Pb], and samarium-neodymium [Sm-Nd]), as well as oxygen (O), a stable isotope. The results of three main studies are presented separately, but here we combine them (>400 analyses) to produce a very complete Pb-Sr-Nd-O isotopic profile of an arc-continent collisional zone—perhaps the most complete in the world. In addition, because many of these samples have U-Pb zircon as well as argon mineral age determinations, we have good control of the timing for Pb-Sr-Nd-O isotopic variations. The ages and isotopic variations help to delineate at least four zones across the batholith from west to east—an older western zone (126–108 Ma), a transitional zone (111–93 Ma), an eastern zone (94–91 Ma), and a much younger allochthonous thrust sheet (ca. 84 Ma), which is the upper plate of the Eastern Peninsular Ranges mylonite zone. Average initial 87 Sr/ 86 Sr (Sr i ), initial 206 Pb/ 204 Pb ( 206 Pb i ), initial 208 Pb/ 204 Pb (average 208 Pb i ), initial epsilon Nd (average ε Ndi ), and δ 18 O signatures range from 0.704, 18.787, 38.445, +3.1, and 4.0‰–9.0‰, respectively, in the westernmost zone, to 0.7071, 19.199, 38.777, −5, and 9‰–12‰, respectively, in the easternmost zone. The older western zone is therefore the more chemically and isotopically juvenile, characterized mostly by values that are slightly displaced from a mantle array at ca. 115 Ma, and similar to some modern island-arc signatures. In contrast, the isotopic signatures in the eastern zones indicate significant amounts of crustal involvement in the magmatic plumbing of those plutons. These isotopic signatures confirm previously published results that interpreted the Peninsular Ranges batholith as a progressively contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith. Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones. Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sr i , 206 Pb i , 208 Pb i , and ε Ndi signatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic values for rocks from the nearby Transverse Ranges, suggesting a genetic connection between the two.

The Steens Formation, or Steens Basalt, is formally recognized as the oldest lithostratigraphic unit of the Columbia River Basalt Group, with an estimated areal extent and volume of 53,000 km 2 and 31,800 km 3 , respectively. We integrate petrochemical, paleomagnetic, and 40 Ar- 39 Ar age data on 13 collected sections to help evaluate stratigraphic and petrogenetic relationships through the Steens succession. We estimate that the overall duration of Steens Basalt volcanism from lingering eruptions could be as much as 300,000 yr, centered at ca. 16.7 Ma, but that the far greater volume erupted in <50,000 yr at an effusion rate ~0.67 km 3 /yr. Lava flows of primitive, homogeneous tholeiite initially erupted over a wide expanse of eastern Oregon during a reversed polarity interval (R 0 ). Later eruptions became more focused at the presumed shield volcano at Steens Mountain, where dikes exploited a NNE-trending zone of crustal weakness related to the northeast extension of the mid-Cretaceous western Nevada shear zone. The Steens Mountain shield volcano generated increasingly more diverse flows of tholeiite, alkali basalt, and basaltic trachyandesite that erupted during a geomagnetic polarity transition culminating in upper flows of normal polarity (N 0 ). The Steens sequence is dominated by compound flows (~10–50 m thick) produced by the rapid eruption of thin (<2 m) pahoehoe flow lobes. Analysis of these stacked sequences in the Catlow Peak section reveals periodic recharge of the magma chamber and ubiquitous fractional crystallization of plagioclase and olivine in each compound flow, accommodated by plagioclase accumulation and selective crustal contamination. The overall flood basalt stratigraphy records a rapid and progressive change in eruption style, from the early, near-continuous eruptions of small-volume Steens Basalt flows to later, more episodic eruptions of large-volume, tabular flows comprising the Imnaha, Grande Ronde, and Picture Gorge Basalts.

The Grande Ronde Basalt lavas constitute ~63% of the Columbia River Basalt Group, a large igneous province in the NW United States. The lavas are aphyric or contain less than 5% phenocrysts of plagioclase, augite, pigeonite, and olivine (altered). Plagioclase hygrometry shows that the erupted lavas generally contained less than 0.3% dissolved H 2 O; however, the presence of rare disequilibrium An 96 plagioclase phenocrysts suggests that some magmas may have originally had 4.5 wt% dissolved H 2 O at depth, but they all degassed during ascent and eruption. The size of plagioclase phenocrysts suggests an average plagioclase phenocryst residence time in the magmas of 160 yr. Ignoring hiatuses between eruptions, we estimate that the ~110 flows of the Grande Ronde Basalt erupted over a cumulative time of 17,600 yr, with an average eruption rate of ~8.6 km 3 /yr. The average interval between eruptions is estimated to be 3658 yr. It is envisaged that a shallow intrusive dike-sill complex, rather than large kilometer-sized magma chamber(s), fed the Grande Ronde basalt lavas. We performed model simulations using the COMAGMAT software to retrace the pre-eruption histories of the Grande Ronde Basalt lavas. Based on such simulations and petrological reasoning, we find that the primary melts could have been generated from a spinel peridotite source at 1.5 GPa, perhaps under hydrous conditions. Extensive melting of lithospheric eclogite may have played an important role as well; however, this is not constrained by our simulations. All lavas were contaminated by the crust, and they were last processed (mixing, crystallization) during their short residence within shallow (6 km) intrusive rocks prior to eruption. Our petrologic conclusions lead us to present a petrotectonic model that supports the hypothesis that the Columbia River Basalt Group magma generation was greatly aided by a thinned lithosphere and H 2 O that may have come off the asthenospheric wedge.

We present a reappraisal of Columbia River basalt petrogenesis based on an internally consistent X-ray fluorescence and inductively coupled plasma–mass spectrometry data set for major and trace elements plus new and existing isotopic analyses of the Imnaha, Steens, Picture Gorge, and Grande Ronde Basalts. Source materials for the main-phase Columbia River Basalt Group are upwelling ocean-island basalt source–like mantle, depleted mantle variably fluxed by slab-derived fluids, Phanerozoic arc crust, and ancient North American cratonic crust. The mantle upwelling may be a deep-seated plume or material displaced and mobilized by fragmented sinking slabs. We endorse the conclusions of earlier workers that the Imnaha, Steens, and Picture Gorge Basalts represent different mixtures of upwelling mantle, depleted mantle, slab-derived fluids, and crust. Cratonic crust of the Idaho batholith has a minor role as a contaminant of Imnaha basaltic magma and a major role in the petrogenesis of the Grande Ronde basaltic andesites, which we model as contaminated Imnaha basalt. We emphasize the geochemical continuity of the Imnaha and Grande Ronde Basalts and propose that they derive from a single central crustal magma system (or chamber) that lasted from ca. 16.7 Ma to 16.0 Ma. The Imnaha–Grande Ronde magma system was centered beneath the location where the western Snake River Plain, Oregon-Idaho graben, and Chief Joseph dike swarm converge, and probably transgressed the cratonic boundary to the east. Magma from this system was injected into dikes and traveled hundreds of kilometers northward to erupt and feed the gigantic Grande Ronde lava flows. In contrast to previous studies, we question the idea that the dike swarm provides a geographic map of the magma source regions.

The ca. 570 Ma Catoctin volcanics, exposed in the Blue Ridge of northern Virginia, include metamorphosed rift-related basalts extruded during breakup of the supercontinent Rodinia. Field relationships, petrography, and geochemistry are used to decipher the stratigraphy for two areas of the volcanics, one at the base of the formation, and the other near its top. Geochemical characteristics of sequential flows can be explained by fractional crystallization of minerals commonly occurring in basalts. Intervening flows with slightly different geochemical features that cannot be explained by fractional crystallization from magma corresponding to an underlying flow, or by crustal contamination, most likely represent new pulses of magma. Positive Pb and negative Nb anomalies, coupled with ε Nd values of +1.5 to +4.5 and model ages that exceed crystallization ages by over 500 m.y., suggest that an enriched component was added to the mantle prior to melting. This component resulted from devolatilization of subducting sediments prior to assembly of Rodinia, likely during Grenville orogenic events. Stratigraphic control shows that lower lavas in the sequence contain more of the enriched component than upper flows.

The innermost domain of the autochthon of the Iberian Massif (Central Iberian Zone) consists of Upper Proterozoic–Lower Cambrian metasedimentary rocks and Early Paleozoic augen gneisses. The former were intruded by mafic magmas as small bodies that later became amphibolites under Variscan metamorphism, gabbro-diabasic textures being sometimes well preserved. Three main groups of amphibolites can be established: (A) the light rare earth element (LREE)-depleted group, characterized by an extremely low rare earth element (REE) fractionation factor [(La/Lu) CN = 0.27–0.34] and low Ti content; (B) the flat REE pattern group, characterized by a small REE fractionation factor [(La/Lu) CN small REE fractionation factor [(La/Lu) CN = 0.95–1.25]; and (C) the LREE-enriched group, characterized by a strong fractionation factor [(La/Lu) CN = 3.53–15.04] and high Ti content. Regular major element variations for the depleted amphibolites point to a low-pressure fractional crystallization as the major process, the strongly depleted amphibolites considered to be parent magmas, although crustal contamination accounts for their broad Nd and Sr isotope ranges as well as trace element variations. Part of the enriched amphibolite samples have an ocean island basalt–like signature and plot within the mantle array, suggesting a mantle mixed source. Other samples show decoupling of Nd and Sr isotopic systems as a response to a probable mixing process involving fluid-rock interaction during subsolidus evolution, which is supported by their high δ 18 O values. Geochemical characterization of amphibolite groups is consistent with an extensional within-plate tectonic setting, whereas processes involved may account for a similar mantle-crust interaction that occurred throughout the entire Central Iberian Zone and probably all over the northern margin of Gondwana during late Neoproterozoic–Early Paleozoic times.

Ignimbrites, as widespread sheets tens of meters thick, form the Central American Tertiary Ignimbrite Province. Geochemical data were collected from 99 Cenozoic marine ash layers within Caribbean Sea sediments, 76 vitrophyres, and 21 mafic lavas from Nicaragua and Honduras. Two major eruptive periods, one in the Eocene and one in the Miocene, have been broadly identified. 40 Ar/ 39 Ar laser fusion ages, determined from sanidine or plagioclase in 10 of the vitrophyre samples, have been interpreted to indicate that the bulk of the younger group of ignimbrites formed largely in the middle Miocene during a 3.5-m.y. period between 16.9 and 13.4 Ma. Modeling indicates that initial melts were from a normal mid-oceanic-ridge basalt (N-MORB)–type source, rather than the enriched mid-oceanic-ridge basalt (E-MORB)–type source postulated for the modern arc. All of the ignimbrites analyzed have 87 Sr/ 86 Sr isotope values ( 87 Sr/ 86 Sr = 0.7040–0.7069) within the range of continental crust. Trace element trends are similar to those estimated for lower continental crust. Assimilation– fractional crystallization and melt mixing models produce trends that are consistent with ignimbrite compositions. This evidence is consistent with a large influence of continental crust in the ignimbrite formation. In addition, the ignimbrite magmas, like those of the modern arc, have also been determined to have been contaminated by sediment-derived fluids. Abnormally rapid subduction of the Farallon-Cocos plate, which coincides with the formation of the ignimbrites, may have resulted in their generation. A slab gap that currently exists beneath the modern arc may be the cause of a change in source from N-MORB to E-MORB by allowing rising asthenospheric material to “recharge” the mantle wedge in trace elements that had been depleted prior to the gap formation.

Geophysical and geological observations document that beneath the submerged forearc, processes of sediment subduction and subduction erosion move large volumes of material toward the mantle. The conveying system is the subduction channel separating the upper plate from the underthrusting ocean plate. Globally, the zero-porosity or solid-volume rate at which continental debris is shuttled toward the mantle is estimated to be ∼2.5 km 3 /yr. To deliver this volume, the average thickness of the subduction channel is ∼1.0 km. Some deeply subducted material is returned to the surface of Earth as a component of arc magma or as tracks of high- P/T crustal underplates. But over long periods of time (>50 m.y.), most of the removed material is evidently recycled to the mantle. Applying Cenozoic recycling rates to the past astonishingly implies that since 2.5 Ga a volume of continental crust equal to the standing inventory of ∼6 × 10 9 km 3 has been removed from the surface of Earth. This minimum estimate does not include crustal material recycled at continental collision zones nor reliable estimates of recycling where large accretionary bodies form. The volume of demolished crust is so large that recycling must have been a major factor determining the areal pattern and age distribution of continental crust. The small areal exposure of Archean rock is thus probably more a consequence of long-term crustal survival than the volume originally produced. Reconstruction of older supercontinents is made difficult if not unachievable by the progressive truncation of continental edges effected by subduction zone recycling, in particular by subduction erosion.

Sr-Nd-Pb-B isotope and incompatible trace element investigations were carried out on volcanics of the Phlegrean Volcanic District, which includes Campi Flegrei and the islands of Ischia and Procida (Campanian region, southern Italy). The results of these investigations allowed us to define several components in the genesis of magmas, including transitional mid-ocean-ridge basalt (MORB)–type asthenospheric mantle, aqueous fluids, oceanic sediment melts derived from the subducted Ionian slab, and continental crust. A source contamination process that introduces variable proportions of slab-derived components into the mantle can explain the observed isotopic variations among the products of Ischia and Procida, and of Campi Flegrei older than 39 ka. Furthermore, comparison between Phlegrean Volcanic District and Aeolian arc volcanic rocks shows a different role of aqueous fluids and sediment melts from southeast to northwest. Energy-constrained assimilation and fractional crystallization modeling suggests that younger than 39 ka Campi Flegrei potassic trachybasalt magmas could have assimilated up to 2% of crustal melt, while evolving to trachyte during stagnation at ∼8 km depth. Crustal assimilation and mixing/mingling processes among isotopically distinct magma batches at shallower depth account for geochemical and isotopic variations of younger than 39 ka Campi Flegrei volcanic rocks.

In the Central American arc, southeastern Guatemala hosts the most diverse volcanism. Large stratovolcanoes at the volcanic front (VF) form as a result of subduction of the oceanic Cocos plate beneath the continental Caribbean plate. Behind the volcanic front (BVF) volcanism, however, has undergone a fundamental change in eruptive style during the Quaternary from older, polygenetic central volcanism to younger, monogenetic cinder cone volcanism. Magmas that traverse the 40–45-km-thick crust in southeastern Guatemala are highly susceptible to crustal contamination. Consequently, mineral chemical data, whole-rock oxygen isotope, and light element geochemistry are used to investigate the relationship between edifice type and the magnitude of crustal contamination. The lack of systematic variation between compositions of phenocryst phases and host rocks strongly suggests that open system processes were operating. Moreover, phenocryst core compositions are generally out of equilibrium with host rock compositions. Olivine from BVF cinder cones deviate only slightly from the equilibrium line in comparison to the older behind the volcanic front (OBVF) central volcanoes and VF stratovolcanoes, suggesting less assimilation of crustal lithologies. Steep arrays on the δ 18 O-SiO 2 diagram cannot be explained by crystal fractionation and favor the incorporation of 18 O-enriched crustal rocks. Higher δ 18 O values in the OBVF central volcanoes and VF stratovolcanoes support the idea that larger, shallow magma bodies experienced greater amounts of crustal contamination. Regional extension in the Ipala Graben of southeastern Guatemala likely promoted short residence times in crustal reservoirs and small degrees of crustal assimilation for the BVF cinder cone magmas.