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

Fresh basalts from the Ontong Java Plateau (OJP) and the Shatsky Rise show lithium enrichments comparable to those of mid-oceanic ridge basalts (MORBs) and ocean island basalts (OIBs), with Li contents being significantly higher at a given MgO content. The Li isotopic compositions of the Shatsky Rise basalts (δ 7 Li = +6‰ to +7‰) are at the higher end of the range exhibited by OIBs, whereas OJP basalts (δ 7 Li = +3‰ to +5‰) have Li isotopic compositions similar to MORBs. Among all the basalts from the two oceanic large igneous provinces (LIPs), one sample from the Shatsky Rise is isotopically enriched (e.g., low 143 Nd/ 144 Nd and 176 Hf/ 177 Hf) and has higher K/Ti and lower La/Nb than the other samples. Relationships between δ 7 Li and K/Ti, La/Nb, and Rb/Nb of this sample indicate that it may have been affected by mantle that was metasomatized by slab-derived fluids. Apart from this isotopically enriched sample, δ 7 Li values of basalts from the two oceanic LIPs are positively correlated with K/Ti and Rb/Nb. Obvious linear relationships exist between δ 7 Li and Yb/Li, Y/Li, and Dy/Li for samples from the Shatsky Rise. These geochemical relationships can be explained by magmatic assimilation of hydrothermally influenced crust. The high δ 7 Li values of the Shatsky Rise basalts imply that the degree of assimilation is high because shallow magma chambers allow greater assimilation of hydrothermally influenced crust. In contrast, the low δ 7 Li values of the OJP samples may indicate they have undergone little assimilation as compared with the Shatsky Rise basalts.

Abstract Crustal xenoliths are key subvolcanic material for studying processes that operate in a magma dyke during intrusion into its surrounding country rock. The Biot number is the appropriate parameter to advance knowledge of the rock–magma thermal interaction. We present an interactive approach that combines the petrological study of natural subvolcanic samples, through crustal xenoliths, which represents an essential pilot to explore and constrain the input parameters and boundary conditions in numerical simulations of fluid dynamics under a volcano. Our results – as a function of different dyke depths, aspect ratios and temperature gradients at the host and dyke – allow an accurate interpretation of the thermal history of magma flow in the dyke, revealing significant differences in heat transfer and thermal resistance between the crust and ascending magma. Conclusively, melt flow within rigid-walled channels depends directly on the depth (pressure gradient) and width of the dyke according to a balance between the rate of magma input and heat loss that determines how fast the magma may ascend. In addition, and with implications for crustal assimilation, there is evidence of a strong relationship between the preconditioning temperature and the integrity of the host rock over the time that xenoliths are immersed in the magma dyke.

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.

Newly generated and previously published strontium isotope signatures of plagioclase phenocrysts in Columbia River Basalt Group lavas exhibit heterogeneity largely imposed by mantle-derived magmas assimilating variable crustal rocks. Steens basalts assimilated accreted terrane crust with 87 Sr/ 86 Sr ratios of <0.7040. In contrast, Imnaha, Grande Ronde, Wanapum, and Saddle Mountains basalts likely assimilated crust with more radiogenic Sr (>0.7040) including a cratonic component, perhaps as a result of residence in magma chambers partly located east of the 87 Sr/ 86 Sr = 0.7060 line. Strontium isotope ratios in plagioclase phenocrysts from early- erupted Imnaha basalts anticipate whole-rock signatures of later-erupted Grande Ronde basalts consistent with a geochemical continuum between the two formations, which is also seen in whole-rock trace element abundances, undermining the notion of an abrupt change in the magma sources generating Imnaha and Grande Ronde basalts.

New major-element, trace-element, and isotopic (Nd, Sr) analyses of undersaturated alkaline lavas from the Kilimanjaro volcano (north Tanzania) are presented. These data concern 54 samples, ranging from basanites to phonolites, collected during a 1 mo field trip in March 2005. The three main cones of Kilimanjaro were sampled, Shira, Mawenzi, and Kibo, together with numerous parasitic cones located on a SE lineament on the main edifice. On the basis of both spatial distribution and major- and trace-element characteristics of analyzed samples, the previous classification of Kilimanjaro lavas is simplified into five groups: Shira, Mawenzi, Kibo 1, Kibo 2, and parasitic activity, each of which has distinct petrological and geochemical features. The major- and trace-element characteristics of the rare primitive lavas erupted on the volcano yield the ubiquitous signature of amphibole within the magma source. We propose that Kilimanjaro melts originated from the partial melting of lithospheric mantle. Combined modeling of trace-element behavior during partial melting + fractional crystallization and isotopic constraints allow us to propose a schematic model of melt genesis under the Kilimanjaro area. Thermal heating of the ancient continental lithosphere by an upwelling plume triggered partial melting in parts of the lithosphere where amphibole was present and led to the Shira volcanic episode. Then, during a time span of ~1 m.y., the depleted lithosphere was progressively infiltrated by plume melts that resulted in crystallization of a new generation of metasomatic amphibole. Finally, this rejuvenated lithosphere underwent partial melting, leading to the magmas that formed the main edifice (Mawenzi and Kibo), and leading to a progressive depletion of the source with time. An active contribution of true asthenospheric melts during the last magmatic events of the volcano cannot be excluded, but further detailed isotopic investigations are needed to test the model.

The Kodiak Border Ranges ultramafic complex, Afognak batholith, and Shuyak Formation on Kodiak and Afognak Islands together form the lower, middle, and upper portions, respectively, of a Jurassic–Triassic island-arc crustal section. The Kodiak section exhibits structural and geochemical trends similar, but not identical to, the Tonsina-Nelchina segment of the Talkeetna arc, located >500 km to the northeast. Exposed at the base of the Kodiak section is cumulate clinopyroxenite with associated dunite, wehrlite, and layered gabbro. In the inferred middle to upper crust, tonalite and quartz diorite of the Afognak batholith intrude Shuyak Formation basaltic flows, basaltic pillow lavas, and volcaniclastic sedimentary rocks. Despite the fault-bounded nature of the lower crustal and mantle rocks, continuous chemical trends in elements such as MgO, Ni, Cr, Nb, Sr, Y, and rare-earth elements exist across all three units. Modeling of these data suggest that Kodiak arc evolution occurred in two main stages: (1) a gabbroic initial melt underwent fractional crystallization that produced a pyroxenitic root and a gabbroic lower crust, and (2) melt in equilibrium with the gabbroic lower crust underwent assimilation-fractional crystallization to produce mid-crustal plutonic and upper-crustal volcanic rocks. Kodiak Island exposes the oldest and thinnest portion of the Talkeetna arc, with ages from the Afognak batholith ranging from ca. 215–185 Ma. In the eastern and western Talkeetna arc, magmatism migrated northward after ca. 180 Ma in response to inferred forearc erosion. Forearc erosion coupled with differential subduction-channel movement juxtaposed blueschist-facies rocks with middle and lower crustal arc rocks. These processes occurred earlier and to a greater degree in the western Talkeetna arc, causing the arc to split in half, separating the Kodiak and Alaskan Peninsula parts of the Talkeetna arc.

The Sierra Madre Occidental is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America and to the opening of the Gulf of California. The stratigraphy of the Sierra Madre Occidental consists of five main igneous complexes: (1) Late Cretaceous to Paleocene plutonic and volcanic rocks; (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called Lower Volcanic Complex; (3) silicic ignimbrites mainly emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), and grouped into the “Upper Volcanic Supergroup”; (4) transitional basaltic-andesitic lavas that erupted toward the end of, and after, each ignimbrite pulse, which have been correlated with the Southern Cordillera Basaltic Andesite Province of the southwestern United States; and (5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the Late Miocene, Pliocene, and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapping in space and time, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages beneath the rest of the Sierra Madre Occidental. The oldest intrusive rocks of the Lower Volcanic Complex (ca. 101 to ca. 89 Ma) in Sinaloa, and Maastrichtian volcanics of the Lower Volcanic Complex in central Chihuahua, were affected by moderate contractile deformation during the Laramide orogeny. In the final stages of this deformation cycle, during the Paleocene and Early Eocene, ∼E-W to ENE-WSW–trending extensional structures formed within the Lower Volcanic Complex, along which the world-class porphyry copper deposits of the Sierra Madre Occidental were emplaced. Extensional tectonics began as early as the Oligocene along the entire eastern half of the Sierra Madre Occidental, forming grabens bounded by high-angle normal faults, which have traditionally been referred to as the southern (or Mexican) Basin and Range Province. In the Early to Middle Miocene, extension migrated westward. In northern Sonora, the deformation was sufficiently intense to exhume lower crustal rocks, whereas in the rest of the Sierra Madre Occidental, crustal extension did not exceed 20%. By the Late Miocene, extension became focused in the westernmost part of the Sierra Madre Occidental, adjacent to the Gulf of California, where NNW-striking normal fault systems produced both ENE and WSW tilt domains separated by transverse accommodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon plate was still active off Baja California. Geochemical data show that the Sierra Madre Occidental rocks form a typical calcalkaline rhyolite suite with intermediate to high K and relatively low Fe contents. Late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are volumetrically dominant. Initial 87 Sr/ 86 Sr ratios mostly range between 0.7041 and 0.7070, and initial ϵNd values are generally intermediate between crust and mantle values (+2.3 and -3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from a few sites in the Sierra Madre Occidental, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron and others considered the mid-Tertiary Sierra Madre Occidental silicic magmas to have formed by fractional crystallization of mantle-derived mafic magmas with little (<15%) or no crustal involvement. In contrast, other workers have suggested the rhyolites, taken to the extreme case, could be entirely the result of partial melting of the crust in response to thermal and material input from basaltic underplating. Several lines of evidence suggest that Sierra Madre Occidental ignimbrite petrogenesis involved large-scale mixing and assimilation-fractional crystallization processes of crustal and mantle-derived melts. Geophysical data indicate that the crust in the unextended core of the northern Sierra Madre Occidental is ∼55 km thick, but thins to ∼40 km to the east. The anomalous thickness in the core of the Sierra Madre Occidental suggests that the lower crust was largely intruded by mafic magmas. In the westernmost Sierra Madre Occidental adjacent to the Gulf of California, crustal thickness is ∼25 km, implying over 100% of extension. However, structures at the surface indicate no more than ∼50% extension. The upper mantle beneath the Sierra Madre Occidental is characterized by a low-velocity anomaly, typical of the asthenosphere, which also occurs beneath the Basin and Range Province of the western United States. The review of the magmatic and tectonic history presented in this work suggests that the Sierra Madre Occidental has been strongly influenced by the Cretaceous-Cenozoic evolution of the western North America subduction system. In particular, the Oligo-Miocene Sierra Madre Occidental is viewed as a silicic large igneous province formed as the precursor to the opening of the Gulf of California during and immediately following the final stages of the subduction of the Farallon plate. The mechanism responsible for the generation of the ignimbrite pulses seems related to the removal of the Farallon plate from the base of the North American plate after the end of the Laramide orogeny. The rapid increase in the subduction angle due to slab roll-back and, possibly, the detachment of the deeper part of the subducted slab as younger and buoyant oceanic lithosphere arrived at the paleotrench, resulted in extension of the continental margin, eventually leading to direct interaction between the Pacific and North American plates.

Recent advances in the knowledge of the Cenozoic structure and stratigraphy of southern México reveal a geological evolution characterized by Upper Cretaceous orogenic deformation, followed by truncation of the continental margin and gradual extinction of arc magmatism in the Sierra Madre del Sur, prior to the onset of magmatism in the Trans-Mexican Volcanic Belt. Orogenic deformation began in the Late Cretaceous and was coeval with the Laramide orogeny with structures of similar orientation. Deformation consisted of E-W shortening that migrated to the east with time and with a general easterly vergence. Models that relate the Laramide deformation to a decrease in the angle of subduction of the Farallon plate, which was converging in western México, cannot be applied in southern México because Paleocene to upper Eocene arc magmatism occurs near the inferred paleotrench. An alternative possible origin due to collision of an insular arc against the western margin of México suffers from an absence of features and petrogenetic associations indicating the closure of an oceanic basin. In light of recent geochronological data, the general pattern of magmatic extinction from Upper Cretaceous–Paleocene in Colima and Jalisco to the middle Miocene in central and southeastern Oaxaca presents some variations inconsistent with a simple pattern of extinction toward the E-SE. Maastrichtian to lower Paleocene plutonism recognized in the Jalisco block and Manzanillo areas is contemporaneous with a magmatic episode that has some documented adakitic affinities in the central part of the Sierra Madre del Sur. Magmatism from the Paleocene to middle Eocene seems to be concentrated in the Presa del Infiernillo area, although isolated centers existed in areas such as Taxco or the eastern Jalisco block. Finally, the main axis of magmatism between the middle Eocene and Oligocene developed along what is the present-day continental margin and extends 200 km inland as a broad band. In general, the geochemical characteristics of this magmatism indicate a low degree of continental crustal assimilation. Two episodes of principally sinistral lateral faulting that activated NW-SE– and later N-S–oriented faults, with variations in time and space, have been documented during the Eocene and lower Oligocene. The N-S set of faults was active only in the north of the Sierra Madre del Sur, whereas the activity of the NW-SE set continued during the Oligocene along the Oaxaca continental margin. The recognition of these deformational episodes suggests that extensional directions related to lateral faulting changed from NNW-SSE to NE-SW, and locally produced normal displacements on preexisting discontinuities. Fundamental problems still exist in the interpretation of the plate tectonic processes that produced the stress regimes acting on the different sets of faults, as well as in the determination of the factors influencing the migration of magmatism. Some of the arguments used to postulate the presence of the Chortis block off the southwestern Mexican continental margin during the early Cenozoic are uncertain. On the other hand, models that explain restricted displacements of the Chortis block with respect to the Maya block—without juxtaposition with the southwestern margin of México—suggest that continental truncation was essentially caused by subduction erosion and leave open the interpretation of the observed magmatic migration.

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.

Most of the largest volcanoes in México are located at the frontal part of the Trans-Mexican Volcanic Belt and in other isolated areas. This chapter considers some of these volcanoes: Colima, Nevado de Toluca, Popocatépetl, Pico de Orizaba (Citlaltépetl), and Tacaná. El Chichón volcano is also considered within this group because of its catastrophic eruption in 1982. The volcanic edifice of these volcanoes, or part of it, was constructed during the late Pleistocene or even during the Holocene: Colima 2500 yr ago, Pico de Orizaba (16,000 yr), Popocatépetl (23,000 yr), Tacaná (∼26,000 yr), and Nevado de Toluca (>45,000). The modern cones of Colima, Popocatépetl, Pico de Orizaba, and Tacaná are built inside or beside the remains of older caldera structures left by the collapse of ancestral cones. Colima, Popocatépetl, and Pico de Orizaba represent the youngest volcanoes of nearly N-S volcanic chains. Despite the repetitive history of cone collapse of these volcanoes, only Pico de Orizaba has been subjected to hydrothermal alteration and slope stability studies crucial to understand future potential events of this nature. The magmas that feed these volcanoes have a general chemical composition that varies from andesitic (Colima and Tacaná), andesitic-dacitic (Nevado de Toluca, Popocatépetl, and Pico de Orizaba) to trachyandesitic (Chichón). These magmas are the result of several magmatic processes that include partial melting of the mantle, crustal assimilation, magma mixing, and fractional crystallization. So far, we know very little about the deep processes that occurred between the upper mantle source and the lower crust. However, new data have been acquired on shallower processes between the upper crust and the surface. There is clear evidence that most of these magmas stagnated at shallow magma reservoirs prior to eruption; these depths vary from 3 to 4 km at Colima volcano, 4.5–6 km at Nevado de Toluca, and ∼6–8 km at Chichón volcano. Over the past 15 years, there has been a surge of studies dealing with the volcanic stratigraphy and eruptive history of these volcanoes. Up to the present, no efforts have achieved integration of the geological, geophysical, chemical, and petro logical information to produce conceptual models of these volcanoes. Therefore, we still have to assume the size and location of the magma chambers, magma ascent paths, and time intervals prior to an eruption. Today, only Colima and Popocatépetl have permanent monitoring networks, while Pico de Orizaba, Tacaná, and Chichón have a few seismic stations. Of these, Popocatépetl, Colima, and Pico de Orizaba have volcanic hazard maps that provide the basic information needed by the civil defense authorities to establish information programs for the population as well as evacuation plans in case of a future eruption.

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.

Kimolos and Polyegos compound volcanoes, which belong to the Milos volcanic field in the South Aegean arc, Greece, mainly consist of late Pliocene volcanic units. Their history can be divided into two periods of activity separated by a long quiescence phase, as testified by thick epiclastic deposits. Basaltic to rhyolitic lavas and voluminous pyroclastic rocks were erupted during the first period, whereas rhyolitic pyroclastic deposits and perlitic lava domes characterize the second period of activity. During the general period of stasis, activity only occurred NE of Kimolos, where the Geronikolas tuff ring and lavas were emplaced. The rock composition belongs to the calc-alkaline and high-K calc-alkaline series. Olivine (Fo% = 63–68) is mainly found in the central Kimolos (Sklavos) basalts, and orthopyroxene (Mg# = 0.57–0.68) is stable in andesites at NE Kimolos (Geronikolas), whereas hornblende is abundant in some dacites (Vromolimni tuff). Clinopyroxene (Mg# = 0.62–0.85) and plagioclase (An% = 25–94) show large compositional ranges and complex zoning. Kimolos and Polyegos volcanics also show a large variability in trace-element contents and Sr and Nd isotope ratios (0.70456–0.70638 and 0.512484–0.512703, respectively), where the youngest rocks are more depleted in incompatible trace elements. Rhyolitic magmas are derived from the basalts by fractional crystallization and associated crustal assimilation and mixing processes, which occur at different crustal levels. The bimodal composition of the NE Kimolos (Vromolimni) tuff points to the presence of a zoned magma chamber refilled by mafic magmas. Magma source processes that imply different fluid to sediment-melt proportions of the subducted-mantle metasomatism components are responsible for the geochemical variability of mafic magmas. The genesis of the youngest magma is characterized by the lowest sediment input and the highest aqueous fluids and degrees of partial melting of the mantle source. Kimolos and Polyegos data have been interpreted in the context of the currently active South Aegean arc, where volcanic activity began in the western sector of the arc during the early Pliocene (4.7 Ma). Volcanism in the central sector of the arc began later. Partial melting of a mid-ocean-ridge basalt (MORB)–like asthenospheric mantle, metasomatized by subducted sediments and aqueous fluids, is thought to have produced all the South Aegean arc parental magmas. According to west-east isotopic trends, the amount of subducted sediments involved in the magma genesis decreases from west to east, together with a possible change in the composition of the subducted sediments. The lowest fluids to sediment-melts proportions are involved in the genesis of Santorini magmas, whereas the highest are involved in the magma genesis at Milos and Nisyros volcanic fields. The aqueous fluids should derive from subducted oceanic crust, at least in the central and eastern sectors of the arc. Most of the geochemical characteristics of the Santorini magmas are explained by higher partial melting degrees of the mantle source, probably triggered by the greater lithosphere extension, which induces adiabatic upwelling of the mantle. The higher lithosphere extension also causes Santorini magmas to reside at shallower levels, thus preventing amphibole crystallization and allowing a higher amount of mafic magmas to reach the surface. The Nisyros parental magmas are, on the contrary, generated by the least amounts of partial melting of the mantle. Partial melting degree of the mantle source seems to increase with time and passing from the external toward the central sectors of the arc.

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.

The Late Cretaceous calc-alkaline to alkaline plutons in the Central Anatolian crystalline complex are intrusive into metamorphic and ophiolitic rocks and mark a major magmatic pulse in the late Mesozoic evolution of the eastern Mediterranean region. We grouped the plutonic rocks with similar mineral and chemical compositions and inferred genetic relations into the granite, monzonite, and syenite supersuites. Granitic plutons mainly occur along the western edge of the Central Anatolian crystalline complex, whereas syenitic plutons form smaller bodies that crop out in the inner part. The granite supersuite is composed of granitic to granodioritic rocks, and the monzonite supersuite consists mainly of quartz monzonite and monzonitic rocks; both supersuites show enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE). The granitic-granodioritic rocks have compositions typical of high-K calc-alkaline and high-K shoshonitic series, whereas the monzonitic rocks display compositions characteristic of shoshonitic series. The syenite supersuite consists of quartz syenite, syenite, and nepheline- and pseudoleucite-bearing alkali rocks and shows more enrichment in LILE and a slight enrichment in HFSE compared to the other two supersuites. All three supersuite rocks have high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd ratios. These geochemical features suggest that the granite and monzonite supersuite magmas were derived from a subduction-modified and metasomatized mantle source and that the syenite supersuite magmas were derived from an enriched mantle source with considerable crustal contribution. New 40 Ar/ 39 Ar ages at 77.7 ± 0.3 Ma for the granite, 70.0 ± 1.0 Ma for the monzonite, and 69.8 ± 0.3 Ma for the syenite supersuite rocks indicate progressive evolution of the Central Anatolian crystalline complex magmatism from calc-alkaline to alkaline compositions with time. The latest Cretaceous emplacement of the complex's plutons and the chemical evolution of their magmas were syn-collisional in nature, after the leading edge of the Tauride platform collided with a trench in the Inner Tauride ocean and became partially subducted. Subsequent slab break-off resulted in asthenospheric upwelling and perturbation of the subduction-metasomatized lithospheric mantle that collectively produced the primary melts for the Central Anatolian crystalline complex plutons. These melts evolved through combined assimilation–fractional crystallization and mixing-mingling processes during their ascent and were affected by the interactions between mantle-derived and crustal-derived magmas. The late phases of this intracontinental magmatism were accompanied by tectonic extension of the thermally weakened orogenic crust in the Central Anatolian crystalline complex.

In Northeastern Anatolia, the Erzurum-Kars plateau comprises the northernmost part of a volcanic province related to the collision between the Eurasian and Arabian continents. It contains an almost complete record of the volcanism from 11 Ma to 1.5 Ma. Volcanic units on the plateau are calc-alkaline in character and contain a distinct subduction signature. There was a systematic temporal variation in volcanic activity all over the plateau that can be seen in terms of three stages: it initiated with bimodal volcanic products between 11 and 6 Ma (the early stage), turned abruptly into a unimodal intermediate volcanism dominated by andesitic lavas between 6 and 5 Ma (the middle stage), and finally reverted to bimodal activity between 5 and 1.5 Ma (the late stage). These three stages were diachronous as the volcanic succession got progressively younger from the west to the east. The temporal variations were strongly dependent upon the depth of the magma chambers from which the volcanic products were derived. Lavas of the early and late stages were derived from relatively shallow chambers (<10–13 km) that fractionated anhydrous phases and assimilated a minor amount of crustal material or none. In contrast, those of the middle stage were sourced by large, deeper (>13 km), compositionally zoned chambers where amphibole was a fractionating phase and assimilation and fractional crystallization was an important process. The isotopic compositions of the volcanic units do not exhibit a systematic temporal variation on the Erzurum-Kars plateau; instead they exhibit spatial changes. Lavas from the western part of the plateau are much more unradiogenic interms of their Pb isotopic ratios than those from the eastern part. These variations are possibly related to the composition and the amount of crustal material assimilated by the magmas, and hence indicate the existence of two different and isotopically distinct crustal domains beneath the plateau: (1) the Rhodope-Pontide fragment in the west and (2) the Northwest Iranian fragment in the east.

The Ironside Mountain batholith consists of the 170-Ma Ironside Mountain pluton, the quartz diorite of Happy Camp Mountain, the 167-Ma Denny intrusive complex, and the West China Peak intrusive complex. The contact aureole of the Ironside Mountain pluton is present in both the western and eastern Hayfork terranes, which indicates that emplacement followed regional thrusting along the Wilson Point thrust. The batholith marks the beginning of a 15-m.y.-long episode of arc magmatism. Al-in-hornblende barometry suggests emplacement at ∼4 kb. The Denny and West China Peak complexes are predominantly gabbro to olivine pyroxenite characterized by calcic plagioclase. Along with the quartz diorite of Happy Camp Mountain, these plutons have calcic bulk compositions, abundant amphibole, and low FeO/(FeO + MgO). Their chemical and petrographic features suggest oxidized, H 2 O-rich parental magmas. In contrast, the Ironside Mountain pluton has higher FeO/(FeO + MgO), shows K 2 O enrichment with increasing SiO 2 to ultrahigh-K compositions, and is typified by 2- and 3-pyroxene assemblages in which hydrous mafic silicates are sparse or absent, except in the most evolved rocks. The elements Rb, Zr, and Ba all behave incompatibly. Initial Nd and Sr isotope ratios (170 Ma) are uniform at 87 Sr/ 86 Sr ϵ Nd of ∼5.2 and of ∼0.7037. These data suggest that the pluton lacks significant crustal input, unlike younger Middle Jurassic plutons, and that it evolved primarily by fractional crystallization of a reduced, H 2 O-poor arc tholeiite. Enrichment of K 2 O is interpreted to result from differentiation at relatively high pressure (≥8 kb), under conditions in which augite fractionation predominated over olivine fractionation. High-P fractionation without appreciable crustal input is probably related to the thermal structure of the crust. Emplacement immediately after regional thrusting placed parental magmas into cool, nonreactive host rocks. Succeeding Middle Jurassic magmatism encountered progressively hotter crust, with consequent crustal melting and assimilation.