Abstract: The genetic connection between Large Igneous Province (LIP) and carbonatite is controversial. Here, we present new major and trace element data for carbonatites, nephelinites and Deccan basalts from Amba Dongar in western India, and probe the linkage between carbonatite and the Deccan LIP. Carbonatites are classified into calciocarbonatite (CaO, 39.5–55.9 wt%; BaO, 0.02–3.41 wt%; ΣREE, 1025–12 317 ppm) and ferrocarbonatite (CaO, 15.6–31 wt%; BaO, 0.3–7 wt%; ΣREE, 6839–31 117 ppm). Primitive-mantle-normalized trace element patterns of carbonatites show distinct negative Ti, Zr–Hf, Pb, K and U anomalies, similar to that observed in carbonatites globally. Chondrite-normalized REE patterns reveal high LREE/HREE fractionation; average (La/Yb) N values of 175 in carbonatites and approximately 50 in nephelinites suggest very-low-degree melting of the source. Trace element modelling indicates the possibility of primary carbonatite melt generated from a subcontinental lithospheric mantle (SCLM) source, although it does not explain the entire range of trace element enrichment observed in the Amba Dongar carbonatites. We suggest that CO 2 -rich fluids and heat from the Deccan plume contributed towards metasomatism of the SCLM source. Melting of this SCLM generated primary carbonated silicate magma that underwent liquid immiscibility at crustal depths, forming two compositionally distinct carbonatite and nephelinite magmas. Supplementary material: Detailed sampling locations and descriptions, major and trace element composition of apatite in carbonatite and nephelinite, analytical reproducibility for major and trace elemental analyses, and details of trace element modelling are available at https://doi.org/10.6084/m9.figshare.c.3819457

Abstract Determining the timescales of magma degassing is essential for understanding the mechanisms controlling the eruption style and the dynamics of magmatic systems. Towards this end, we measured 210 Pb– 226 Ra disequilibria in andesite lavas erupted from Volcán de Colima between 1998 and 2010. ( 210 Pb/ 226 Ra) 0 activity ratios range from 0.86 to 1.09, and are best explained in terms of 222 Rn degassing and accumulation. The range in 210 Pb deficits indicates that the timescales of 222 Rn degassing did not exceed 11 years. 210 Pb excesses are rare and small (<10%), which signifies that 222 Rn degassing is more effective than 210 Pb accumulation in this intermediate system despite the relatively low gas output at the surface. The absence of significant 210 Pb excesses strongly suggests that the volcanic activity results from episodic ascent of small magma batches through the vapour-saturated section of the magmatic system. Overall, the degassing models based on 210 Pb– 226 Ra disequilibrium suggest an open and complex subvolcanic magmatic system comprising several conduits in which multiple magma batches reside for up to 10 years. Shifts from effusive to explosive Vulcanian eruptive phases are not related to changes in degassing mode on timescales resolvable using 210 Pb– 226 Ra disequilibria.

We studied the formation of low-initial-Sr felsic plutons by using data from the Early Cretaceous western Peninsular Ranges batholith near Escondido, California. The systematically sampled Escondido plutons have a uniformly low initial 87 Sr/ 86 Sr isotope ratio of Sr i < 0.704, but a wide range of SiO 2 compositions, from 46 to 78 wt%, which fall in three distinct groups: 20% gabbros, 35% tonalites, and 45% granodiorites. These low-Sr i plutons are unique in having undergone one cycle of mantle melting to give basalt composition rocks, and a second cycle of arc basalt melting to give a range of SiO 2 plutons, but no third cycle of melting and contamination by old continental crust to yield high-Sr i rocks. After doing two-cycle partial melting and fractional crystallization calculations, it was recognized that mixing of gabbro and granodiorite magmas was necessary to yield the tonalites. The linear data pattern on Harker diagrams is interpreted as resulting from mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to form intermediate magma. These plutons provide a simplified two-cycle Phanerozoic example of the petrogenetic process for forming continental crust.

The Santiago Peak volcanics, in the northern Santa Ana Mountains, are the northernmost exposures of the Santiago Peak–Alisitos magmatic arc present along the western edge of the Peninsular Ranges batholith. Remnants of deeply eroded volcanic sequences in the Santa Ana Mountains consist of subaerial basaltic-andesite to rhyolite lavas, rare basalt, welded tuff, and pyroclastic rocks that were emplaced across deformed Middle Jurassic turbidites. Subalkaline lavas have mixed calc-alkaline and tholeiitic affinities. Relatively primitive ε Sr (−18 to +5) and ε Nd (+7.5 to +0.1) values for the lavas plot along the mantle array. Silicic lavas have higher ε Sr and ε Nd values in comparison to mafic lavas. Parental magmas were derived from hydrous melts of relatively depleted mantle wedge, followed by fractionation and the assimilation of up to 10% crustal materials. The whole-rock compositions, isotopic data, and U/Pb and 40 Ar/ 39 Ar ages (128–110 Ma) of the Early Cretaceous Santiago Peak volcanics and related Estelle Mountain volcanics overlap with emplacement ages of plutons of the western zone of the Peninsular Ranges batholith. The volcanic rocks are interpreted as the volcanic component of the arc plumbing system of the batholith. Arc rocks are in turn unconformably overlain by a forearc sequence of Upper Cretaceous through Tertiary strata that indicate deep erosion of the Santiago Peak volcanics by 95 Ma. Volcanic clasts of Turonian age within the forearc sequence yield U/Pb ages of 108–106 Ma. Age data and whole-rock geochemistry of the volcanic clasts indicate that they were eroded from supracrustal volcanic rocks located farther east within the Elsinore block.

In the Peninsular Ranges batholith of southern California, a central belt of Jurassic metagranites was intruded by a Cretaceous magmatic arc that migrated from west to east across the belt. The Cretaceous batholith has been divided into western and eastern zones, zones that correspond to age, lithologic, geochemical, and geophysical zonations. In this study, density and magnetic susceptibility measurements performed on ~960 hand samples show that, in the eastern zone of the Peninsular Ranges batholith, values of magnetic susceptibility are uniformly low (<0.5 × 10 −3 cgs [centimeter-gram-second] units), while density values are in general lower and have less scatter than those in the western zone. A relatively sharp break between western and eastern zones indicates the existence of two crustal types separated by a tectonic suture: on the west, oceanic crust (mainly Mesozoic and older mantle and mantle-derived rocks) and on the east, continental crust (Neoproterozoic, Paleozoic, and early Mesozoic rocks). Previous studies in the San Diego County segment of the Peninsular Ranges batholith revealed petrologic distinctions between two Jurassic metagranite suites (S-type and transitional I-S type) and nine Cretaceous granite suites (exclusively I-type). The results of electron microprobe (EM) analyses of mafic minerals from Jurassic and Cretaceous plutonic rocks in general confirm plutonic suite subdivision. On biotite and hornblende variation diagrams, Early Cretaceous plutons tend to plot in distinct fields/trends that are characteristic of their various plutonic suites. Hornblende from three Early Cretaceous tonalite suites is Mg enriched, as expected from melts of mafic-intermediate composition that were H 2 O rich and contained hornblende as an early-crystallizing phase. Hornblende from gabbro plutons (Cuyamaca Gabbro) shows the greatest Mg enrichment for a given whole-rock SiO 2 value, reflecting cumulate processes in the evolution of gabbroic magmas. Biotite and hornblende from highly evolved leucomonzogranite-leucogranodiorite plutons assigned to three leucogranite suites have the most Fe- and Mn-rich compositions. Hornblende compositions of two Late Cretaceous tonalite suites overlap those of the Early Cretaceous tonalite suites, but, in general, Late Cretaceous hornblende does not show the extreme fractionation shown by hornblende of Early Cretaceous suites with similar SiO 2 contents. Biotite of two Jurassic plutonic suites has the most aluminous compositions of all Peninsular Ranges batholith suites, with biotite of the S-type Harper Creek suite markedly more Al rich than that of the transitional I-S–type Cuyamaca Reservoir suite. Complete overlap of Harper Creek biotite compositions with those of metasedimentary rocks of the Triassic–Jurassic Julian Schist indicates that partial melting of the latter was an appropriate source for Harper Creek magma. The existence of two Cuyamaca Reservoir biotite trends suggests that its parental magma originated by fractionation and contamination of an I-type magma by aluminous metasedimentary material, thus producing transitional I-S characteristics. All but one sample of hornblende from the Cuyamaca Reservoir suite falls in the subaluminous compositional range.

Abstract Our goals today are several-fold. We have now spent five days examining different parts of the Mesozoic Sierran arc, and hopefully discussions are already under way attempting to integrate both the shared and distinct characteristics of these individual magma plumbing systems and synchronous tectonics. We will briefly continue these discussions below. Our main focus will be to consider the arc as a whole and introduce a number of new regional data sets related to the tectonic and magmatic components of this arc. By the end of the day, we hope that our discussions have evolved to a consideration of the overall petrologic evolution of the arc, the tectonic and magmatic arc tempos, and their potential links. Without an airplane, or satellite, or Hollywood earth coring machine, it is difficult to take you to field locations where we can observe large sections of the arc. Instead, as we travel west back across the arc, we have selected a number of scenic overview stops, where we will introduce and discuss these new data sets while looking at gorgeous views of the arc…

Intraplate-type volcanism (late Oligocene to Quaternary) occurs in México on both the North American and the Pacific plates. Oceanic localities are voluminous shield volcanoes on or near fossil-spreading ridges. Subaerial rocks of these volcanoes form either geochemically continuous and coherent rock series or bimodal suites. Low-pressure crystal fractionation of alkali basalt and assimilation of hydrothermally altered rocks from the volcanic pile determined the compositions of the mafic-intermediate rocks at Socorro. Trachytes at Socorro were formed by partial melting of alkali basalt, and rhyolites through crystal fractionation of parental trachytes. These felsic rocks also assimilated hydrothermally altered rocks. Continental localities (late Oligocene to Quaternary) occur scattered north of the Trans-Mexican Volcanic Belt; their location is independent of older volcanic provinces and boundaries between today's geologic/tectonic provinces. Intraplate-type rocks have high TiO 2 , Nb, and Ta contents, and host mantle and/or lower crust xenoliths ± megacrysts. Many fields are in the southern Basin and Range and the most extensive and voluminous were contemporaneous with normal faulting. The locations of some fields suggest that magma ascent was influenced by regional structures. However, normal faulting is minor or absent in many other Mexican fields. The petrogenetic processes in late Oligocene to Miocene magmas differ from those in Plio-Quaternary magmas. Slow ascent of magmas formed during early stages of extension-favored assimilation-fractional crystallization and gravitational settling of xenoliths. Plio-Quaternary xenolith-bearing magmas traveled faster through cooler crust where brittle structures caused by extension were able to propagate deeper. Geochemical evidence of assimilation is more subtle in the younger volcanic rocks.

Geophysical, geological, and petrologic data in southwestern China have been integrated in order to characterize magmatic underplating associated with the Late Permian Emeishan large igneous province (LIP; ca. 260 Ma). Seismic reflection and refraction reveals a heterogeneous crustal structure with high-velocity layers or bodies in the upper crust (6.0–6.6 km/s), lower crust (7.1–7.8 km/s), and upper mantle (8.3–8.6 km/s). These seismically anomalous bodies are all confined in the inner zone of the prevolcanic domal structure, but are generally absent in the intermediate and outer zones. There is a decreasing trend in crustal thickness from the inner zone (>60 km, with a ∼20-km-thick high-velocity lower crust, or HVLC) via the intermediate zone (∼45 km) to the outer zone (<40 km). Because the domal uplift immediately preceding eruption of the Emeishan basalts was unambiguously related to a mantle plume, such a configuration highlights a genetic relationship between the formation of the high-velocity crust and the mantle plume that led to the eruption of the Emeishan basalts. It is proposed that the HVLC may have resulted from magmatic underplating associated with the Emeishan volcanism, whereby the fast mantle represents the residues left after extensive melt extraction from the plume head. Magmatic underplating can also account for the prolonged crustal uplift that formed the Chuandian “old land” in southwestern China. Petrologic modeling further suggests that the HVLC may represent fractionated cumulates from picritic melts and that the Emeishan basalts represent residual melts after polybaric fractionations. This relationship allows a reestimation of the volume of Emeishan magmas, which is as much as 3.8 × 10 6 km 3 , typical of plume-generated LIPs in the world.

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.