The Hasan Dagi stratovolcano (Central Anatolia, Turkey): evolution from calc-alkaline to alkaline magmatism in a collision zone

The Hasan Dagi volcano is one of the two large Plio-Quaternary volcanoes in Cappadocia (Central Anatolia, Turkey). Three stages of edifice construction have been identified for this volcano: Paleovolcano, Mesovolcano and Neovolcano. Most samples from Hasan Dagi volcano are calc-alkaline and define an almost complete trend from basaltic andesite to rhyolite. However, the more recent (Neovolcano) mafic samples are alkaline basalts. The mineralogical and geochemical characteristics of the oldest lavas (Keçikalesi (13 Ma) and Paleo-Hasan Dagi (7 Ma)) are significantly different from those of the younger lavas (Meso- and Neo-Hasan Dagi (<1 Ma)). Calcic plagioclase and pigeonite are typically observed in these older lavas. The Paleovolcano basalts are depleted in alkalis and display a tholeiitic tendency whereas the differentiated lavas are depleted in Na₂O but enriched in K₂O compared to younger lavas. There is an evolution through time towards higher TiO₂, Fe₂O₃^*, MgO, Na₂O and K₂O and lower Al₂O₃ and SiO₂ which is reflected in the basalt compositions. All the basalts display multi-element patterns typical of continental margin magmas with a significant enrichment in LILE (K, Rb, Ba and Th) and LREE and strong (Paleovolcano) to moderate (Meso- and Neovolcano) negative Nb, Zr and Ti anomalies. However, the younger basalts are the most enriched in incompatible elements, in agreement with their alkaline affinities and do not systematically display negative HFSE anomalies. REE data suggest an hydrous amphibole-bearing crystallization history for both Meso- and Neovolcano lavas. The distinction between the older and younger lavas is also apparent in trace element ratios such as Nb/Y, Ti/Y and Th/Y. These ratios indicate the role of a subducted component±crustal contamination in the genesis of the Hasan Dagi lavas, particularly for the oldest lavas (Keçikalesi and Paleo-Hasan Dagi). The decreasing influence of this component through time, over the last 6–7 m.y., has been accompanied by an increasing contribution of melt-enriched lithosphere. Although the range of variation of Sr, Nd and Pb isotopic ratios is small (0.70457–0.70515; 0.51262–0.51273; 18.80–18.94; 15.64–15.69; 38.87–39.10), it also reflects the evolution of the magma sources through time. Indeed, the youngest (Neovolcano) and most primitive basalts display significantly lower ⁸⁷Sr/⁸⁶Sr than the Paleo- and Mesovolcano basalts, whereas the Mesovolcano basalts display more radiogenic Pb than Paleovolcano samples. Magma mixing processes between initially heterogeneous and/or variably contaminated magmas may account for the genesis of the less differentiated and intermediate lavas (48–57% SiO₂). Meso- and Neovolcano differentiated lavas (60–68% SiO₂) are either derived from the analyzed basalts or from more primitive and more depleted magmas by fractional crystallization±some crustal contamination (AFC). Furthermore, the highly differentiated samples (72–75% SiO₂) are not strongly contaminated. The strong calc-alkaline character of Hasan Dagi lavas, in the absence of contemporaneous subduction, must reflect the heritage of the early subduction of the Afro–Arabian plate under the Eurasian plate. The evolution towards alkaline compositions through time is clearly related to the development of extensional tectonics in Central Anatolia in the Late Miocene.     

Volcanism and structure of southwestern Bolivia

Cenozoic volcanic activity started in southwestern Bolivia during the Oligocene. From the Early Miocene up to the Holocene, volcanism produced about 8000 km³ of lavas and pyroclastic rocks. Five major cycles could be distinguished.North—south-trending fissures in the eastern part of southwestern Bolivia were sources of Miocene ignimbrites. The eruptive centers of Pliocene ignimbrites are scattered throughout the investigated area, but are concentrated in the western part. During Pleistocene times small ignimbrite flows were formed by collapse of composite volcanoes.The oldest lavas (Oligocene) are alkaline and differ considerably from younger rocks, which are rhyodacites and dacites with only a small limited range of compositions. Based on the K₂O/Na₂O ratios and the mineralogical composition, the lavas can be subdivided into a calc-alkaline and a high-K calc-alkaline (shoshonitic) association. Hornblende is absent from lavas with high K₂O/Na₂O ratios.In southwestern Bolivia a westward migration of volcanic activity is apparent. The potassium content of the lavas decreases from the Miocene to the Holocene, whereas the sodium content increases. The potassium content cannot, therefore, be correlated with the depth of the Benioff zone. It is suggested, that the lavas and ignimbrites were formed by partial melting of material from different crustal levels.     

The Negros de Aras Nuée Ardente deposits: a cataclysmic eruption of Socompa volcano (Andes of Atacama,Chile)

Studies of ERT Satellite photographic documents and of acrial photographs with complementary lield work reveal the presence of recent very large nuée ardente deposits north-west of Socompa Volcano (Andean Cordillera of Atacama, northern Chile). Three zones are distinguishable from the bottom of Socompa Volcano to the front of the nuée ardente deposits: 1) pumice blocks are covered with parallel ridges of debris (lava blocks) from the north-western flank of Socompa Volcano, 2) pumice blocks lie upon small cones and flows from El Negrillar volcanoes located inside the graben of Negros de Aras, 3) pumice flow threads its way between cones and flows from El Negrillar volcanoes and stops more than 40 km away from the base of Socompa Volcano. The calculated thermal energy of this cruption is 7.9 × 10²⁵ ergs, being in the range of of the most important recorded eruptions on earth. The pumice is almost aphyric (rare plagioclase, hypersthene and hornblende phenocrysts) and is of dacite composition lying pertectly on the K₂O-SiO₂ trend of the Socompa Volcano. Trace and major element data of the pumice are similar to those of two dacites from a pre-nuée lava flow and a post-nuée lava dome of Socompa Volcano and support a common magmatic origin with the Socompa Volcano lavas. A relative chronology is proposed.     

Composition of island arcs and continental growth

Island arc volcanism has contributed and is still contributing to continental growth, but the composition of island arcs differs from that of the upper continental crust in its lower abundance of Si, K, Rb, Ba, Sr and light rare earth elements. In their advanced stage of evolution, island arcs contain more than 80% of tholeiitic and 15% of ‘island arc’ calc-alkaline rocks with varied SiO₂ contents. The larger proportion of tholeiitic rocks is in the lower crustal levels. The high stratigraphical levels of the island arcs are composed of tholeiitic plus calc-alkaline and/or high potash (shoshonitic) associations with higher abundances of K, Rb, Sr, and Ba. Stratification of the island arc crust is accentuated by another type of calc-alkaline volcanism (Andean type) originating at a late stage of arc evolution, probably by partial melting at the base of the crust. This causes enrichment of the upper crust in K, Rb, Ba and REE and accounts for upper crustal abundances of these elements as well as of SiO₂.     

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia,South America

Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO₂; 4.3–5.9 wt.% Na₂O+K₂O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO₂; 4.0–4.4 wt.% Na₂O+K₂O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.     

Relation of lava compositions to volcano size and structure in El Salvador

Most of the lavas at the nine volcanic centers along the volcanic front of El Salvador are basalts, basaltic andesites and andesites. The compositional variation within and among these centers can be explained by fractionation processes within the crust. Cognate gabbroic inclusions found in the lavas have appropriate mineralogy (plagioclase, olivine, magnetite and augite) to be cumulates formed by fractional crystallization. Two main variation trends occur, depending on the proportion of plagioclase removal. The more common, or normal, trend has a high (> 55%) proportion of plagioclase being removed. A less common, Al-rich, trend has a low (40%) proportion of plagioclase being removed. The Al-rich trend is found only at volcanoes that lack large negative Bouguer gravity anomalies. These volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited.The incompatible element (Na₂O, K₂O, Rb, Ba) contents of lavas vary systematically with the volume of the volcanic centers. At the same level of SiO₂, large volcanic centers have higher incompatible element contents than small volcanic centers. This suggests that open system fractionation in a periodically refilled chamber is the controlling factor. The large difference in Ba contents of lavas between eastern (low) and western (high) El Salvador suggests a difference in the mantle source region.     

Isotope geochemistry and origin of calc-alkaline lavas from a caledonian continental margin volcanic arc

Analyses for major and trace elements, including REE, and Sr, Nd and Pb isotopes are reported from a suite of Siluro-Devonian lavas from Fife, Scotland. The rocks form part of a major calc-alkaline igneous province developed on the Scottish continental margin above a WNW-dipping subduction zone. Within the small area (ca. 15 km²) considered, rock types range from primitive basalts and andesites (high Mg, Ni and Cr) to lavas more typical of modern calc-alkaline suites with less than 30 ppm Ni and Cr. There is a marked silica gap between these rocks (< 62%) and the rare rhyolites (> 74%), yet the latter can be generated by fractional crystallization from the more mafic lavas. In contrast, variation in incompatible element concentrations and ratios in the mafic lavas can not be generated by fractional crystallization processes. Increasing SiO₂ is accompanied by increasing Rb, K, Pb, U and Ba relative to Sr and high field strength elements, increasing LREE enrichment and increasing _Sr calculated at 410 Ma, and by decreasing HREE, Eu/Eu*, Sm/Nd and _Nd (410). _Nd and _Sr are roughly anticorrelated and have more radiogenic compositions than the mantle array, in common with data reported elsewhere from this part of the arc. The correlation extrapolates up to cross the mantle array within the composition field of the contemporary MORB source, and extrapolates down towards the probable compositional range of Lower Palaeozoic greywackes, which may form the uppermost 8 km of the crust, or may be supplied to the source by subduction. One sample, however, lies within the mantle array, and closely resembles lavas from northwestern parts of the arc, where a mantle source with mild time-integrated Rb/Sr and LREE enrichment has been inferred. The lavas have relatively high initial ²⁰⁷Pb/²⁰⁴Pb for their ²⁰⁶Pb/²⁰⁴Pb, a feature which has been interpreted elsewhere as the result of incorporation of a sediment component into arc magmas. The systematic changes with increasing SiO₂ in isotopic and chemical parameters can be explained by mixing of a greywacke-derived component with depleted mantle. The various possible mixing mechanisms are discussed, and it is considered most likely that mixing occurred in the mantle source through greywacke subduction. The bulk of the Rb, K, Ba and Pb in the lavas is probably recycled from the crust, whereas less than some 40% of the Sr and Nd is recycled. The calc-alkaline chemical trends are solely a function of mixing with the sediment component.     

Silica standardization: a discriminant technique applied to a volcanic arc system

A classic tholeiitic, calc-alkaline, and shoshonitic island-arc sequence has developed in western Luzon, the Philippines, as a result of subduction defined by an eastward-dipping Benioff zone. A simple technique of standardizing the data allows chemical concentrations to be recast to “andesite equivalent” compositions. Based on the excellent linear correlations that exist between SiO₂ and other elements, individual analyses were standardized to 55% SiO₂ by projecting parallel to least-squares regression lines. Raw and SiO₂-standardized data were then treated by an univariate one-factor analysis of variance and a multivariate discriminant function analysis.Results of the analysis of variance indicate that at the 99% confidence level; 20 of the 27 chemical variates discriminate between at least two of the three geographically distinct magma series; SiO₂-standardized data generally discriminate better than do raw data; and K₂O, Ba, Rb, Sr, and Zr provide the best discrimination. Better classification of samples into the appropriate series is obtained from results of the discriminant function analysis using the SiO₂-standardized data rather than results using the raw data. With this standardizing technique it was possible to detect offsets of SiO₂-standardized K₂O isopleths along the Manila Bay fracture zone that correspond to offsets in contours of depths for earthquake foci.     

Petrogenesis of the Mesozoic intrusive rocks in the Tongling area,Anhui Province,China and their constraint on geodynamic process

TheTonglingarea,whichiscalledtheChineseCopperCapital,isoneofthemostimportantnon-ferrousmetalproducersinChina(e.g.Cu,AuandAg,especiallyCu).ManyresearchershavenotedthatthemetaldepositsarecloselyrelatedtotheMesozoicintrusiverocksinthisarea.Therefore,theTongl…     

The Zealandia Volcanic Complex: Further evidence of a lower crustal “hot zone” beneath the Mariana Intra‐oceanic Arc,Western Pacific

This paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO₂; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.     

Petrogenesis of the late Cretaceous K‐rich volcanic rocks from the Central Pontide orogenic belt,North Turkey

Subduction‐related volcanic rocks are widespread in the Central Pontides of Turkey, and represented by the Hamsaros volcanic succession in the Sinop area to the north. The volcanic rocks display high‐K calc‐alkaline, shoshonitic and ultra‐K affinities. ⁴⁰Ar/³⁹Ar age data indicate that the rocks occurred during the Late Cretaceous (ca 82 Ma), and the volcanic suites were coeval. Primitive mantle‐normalized trace element patterns of all the lavas are characterized by strong enrichments in large ion lithophile elements (LILE) (Rb, Ba, K, and Sr), Th, U, Pb, and light rare earth elements (LREE; La, Ce) and prominent negative Nb, Ta, and Ti anomalies, all typical of subduction‐related lavas. There is a systematic increase in the enrichment of incompatible trace elements from the high‐K calc‐alkaline lavas through the shoshonitic to the ultra‐K lavas. In addition, the shoshonitic and ultra‐K lavas have significantly higher ⁸⁷Sr/⁸⁶Sr (0.70666–0.70834) and lower ¹⁴³Nd/¹⁴⁴Nd (0.51227–0.51236) initial ratios than coexisting high‐K calc‐alkaline lavas (⁸⁷Sr/⁸⁶Sr 0.70576–0.70613, ¹⁴³Nd/¹⁴⁴Nd 0.51245–0.51253). Geochemical and isotopic data show that the shoshonitic and ultra‐K rocks cannot be derived from the high‐K calc‐alkaline suite by any shallow level differentiation process, and point to a derivation from distinct mantle sources. The shoshonitic and ultra‐K rocks were derived from metasomatic veins related to melting of recycled subducted sediments, but the high‐K calc‐alkaline rocks from a lithospheric source metasomatized by fluids from subduction zone.     

Lithium isotope fractionation in the southern Cascadia subduction zone

We present lithium (Li) abundances and isotope compositions for a suite of anhydrous olivine tholeiites (HAOTs) and hydrous basalt-andesitic (BA) lavas from the Mt. Shasta and Medicine Lake regions, California. The values of δ⁷Li vary from + 0.9‰ to + 6.4‰ and correlate inversely with distance from the trench. These data are consistent with continuous isotope fractionation of Li during dehydration of the subducted oceanic lithosphere, an interpretation corroborated by uniformly high pre-eruptive H₂O contents in basaltic andesites accompanied by high Li, Rb, Sr, Ba and Pb abundances. The subduction-derived component that was added to these hydrous magmas is shown to be very similar beneath both Mt. Shasta and Medicine Lake volcanoes despite characteristically distinct Li isotope compositions in the magmas themselves. More evolved andesites and dacites from Mt. Shasta have δ⁷Li from + 2.8 to + 6.9‰ which is identical with the range obtained for HAOTs and BA lavas from Mt. Shasta. Therefore, Li isotopes do not provide evidence for any other crustal component admixed to Mt. Shasta andesites or dacites during magmatic differentiation and magma mixing in the crust.     

Eruptive history and petrology of Mount Drum volcano,Wrangell Mountains,Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km³ of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO₂, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in ⁸⁷Sr/⁸⁶Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993     

Occurrence of oceanic plagiogranites in the older tectonic zone,Southwest Japan

K, Rb and Sr concentrations and Sr isotopic compositions were determined for the Dai granitic rocks of trondhjemitic composition occurring in a serpentinite mass in the Nagato tectonic zone formed in the Late Paleozoic era, and for the granitic rocks of quartz dioritic composition recently dredged from the seamount of the Kyushu-Palao Ridge. Both granitic rocks are characterized by low abundances of K and Rb, low K₂O/Na₂O ratios, high K/Rb ratios, low Rb/Sr ratios and low initial⁸⁷Sr/⁸⁶Sr ratios. These characteristics suggest that strong similarities may exist between the Dai granitic rocks and the dredged granitic rocks, and that the Dai granitic rocks may be classified as oceanic plagiogranite. These oceanic plagiogranites may plausibly represent single-stage mantle-derived granites, possibly from the suboceanic mantle.     

Transverse geochemical variations across the Antarctic Peninsula: Implications for the genesis of calc-alkaline magmas

Magnetic activity throughout the Antarctic Peninsula and the South Shetland Islands has been dominantly of a calc-alkaline nature for the last 200 Ma. Chemically, the plutonic and volcanic products are typical of a continental margin magmatic arc, similar to those from western South America. Within any one area, volcanic and plutonic rocks are compositionally indistinguishable, and all magmatic products show increasing SiO₂, and increasing K/Si, Rb/Si, Th/Si and to a lesser extent Ce/Si and La/Si ratios away from the proposed trench axis. The calc-alkaline basaltic compositions also have high large ion lithophile (LIL; e.g. K, Rb, Th)/high field strength (HFS; e.g. Zr, Nb, Ti) ratios relative to non-orogenic counterparts, and increasing LIL/HFS element ratios with increasing fractionation. It is proposed that the high LIL/HFS element ratios in basaltic and andesitic melts are primary features due to dehydration processes with the subducted slab and to fractionation of minor mineral phases from the melt. The increasing LIL/HFS element ratios in more acid rocks are probably due to removal of minor mineral phases from the melt. Although zone refining may contribute to the spatial variations across the peninsula, we have proposed that an enriched subcontinental mantle provides a viable alternative source for the observed K-h variations and for the increased LIL-element contents found in continental margin calc-alkaline magmas.     

Trace element distribution in the cainozoic lavas of Nevado Coropuna and Andagua Valley,Central Andes of Southern Peru

Major and minor elements have been determined on 26 samples of andesitie to rhyolitic lavas from Nevado Coropuna and Andagua valley in Southern Peru. Nevado Coropuna dating back since late Miocene is the highest stratovolcano of Peru. It is located at 150 km NW of Arequipa and at 110 km E of the Pacific coast. Andagua valley is situated at about 30 km E of Coropuna. The magmatic activity there, as shown by the presence of several cones, is more recent than that of Corpouna and is related to the tectonic graben characterizing this valley. The geological position of the valley is very important because it is near the transverse line separating the zone of rather flat subduction of the Nazca plate from another one dipping more steeply to the SE. The lavas from Andagua show higher Ti, P, Sr and alkali contents than those from Coropuna, and several display some alkaline tendency with Na affinity. No shoshonitic rocks have been found in the area. According to their geochemistry, Corpuna and Andagua andesites do not seem to have been originated by a single process. In particular, the distribution of Ni, Cr, Ti, Zr, Y, P, Nb, and Sr would exclude either adirect origin from pyrolitic materials, or aprogressive crustal contamination as the most important factors for their origin. Calculations of mineral/melt equilibria for Coropana andesites suggests crystallization processes at depth less than 35 km and H₂O-understurated conditons at the time of the phenocrysts precipitation, indicating a possible high undersaturation at depth of the source zone. As lar as the rhyolites are concerned, their geochemical characteristics do not preclude a crustal origin. A statistical study of the chemical zonation of the Plio-Quaternary lavas of southern Peru has shown an increase of Ti and P contents eastward of the Chile-Peru trench.     

The lesser antilles — A discussion of the Island arc magmatism

The active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. With the exception of the alkali basalts of Grenada, the volcanics of the arc can be regarded as belonging to the low-K, island arc, calc-alkaline suite. Although compositions ranging from basalt to rhyolite have been described, porphyritic andesite appears to be the dominant rock type on most volcanoes (intermediate centers). Variable amounts of basalt and basaltic andesite occur and rarely predominate over andesite (latter are basic centers), whereas the more silicic members are only occasionally found. The calc-alkaline suite is characterized by relatively high Al₂O₃ and CaO and low K₂O, Rb and Ni. Variations, especially in the alkali elements, occur both with space and time. A characteristic feature of many of the volcanoes is the occurrence in the basalt and basaltic andesite volcanics of plutonic blocks, often showing cumulate textures. The blocks which ware composed of plagioclase — amphibole — olivine — clinopyroxene — magnetite are thought to be the products of fractionation. The differences between basic and intermediate centers is probably due to the frequency that the magma ascended to the surface or remained in high level chambers where fractionation occurred.     

Geochemical and Sr–Nd isotopic constraints on the petrogenesis of the Goesan monzodiorite pluton in the central Okcheon belt,Korea

The Permian–Triassic high pressure metamorphism and potassic magmatism in central Korea attest to the extension of the Dabie‐Sulu collision belt in central‐eastern China towards the Korean Peninsula and possibly the Japanese Islands. We present major and trace element and Sr–Nd isotope data for a ca. 230 Ma monzodiorite pluton emplaced in the Goesan area, central Okcheon belt, Korea. This pluton shows geochemical features comparable with those of the coeval monzonite–syenite–gabbro–mangerite suite documented recently in the Gyeonggi massif. The metaluminous and alkali–calcic signatures of the Goesan intrusives correspond to the Caledonian‐type post‐orogenic granitoids. The K₂O/Na₂O ratios of all analyzed samples are greater than 1, and are not correlative with their SiO₂ contents. The enrichment of both large‐ion‐lithophile elements and highly compatible elements in the Goesan pluton is probably indicative of metasomatized mantle origin. The elemental fractionation in the source region must have occurred in the distant past, possibly the Paleoproterozoic, to generate significantly negative ε_Nd(t) values (< –16). Chondrite‐normalized rare earth element patterns as well as Rb/Sr and Ba/Rb ranges suggest that the source consists of amphibole‐bearing rocks. Progressive decreases in negative Eu anomaly and Ba, Sr, Ni, Cr and V contents with increasing SiO₂ contents reflect an important role of plagioclase, biotite and hornblende for the fractionation process. Zr is undersaturated in the potassic, metaluminous melt. The initial Sr–Nd isotopic compositions of the samples are correlated with their SiO₂ contents, substantiating a role of crustal assimilation during the magmatic differentiation. The Sr–Nd elemental and isotopic modeling suggests that the Goesan pluton was initially slightly heterogeneous in its isotopic composition, and underwent concurrent assimilation and fractional crystallization. The occurrence of the Goesan pluton provides further evidence corroborating the amalgamation of allochthonous terranes within the Okcheon belt during the Permian–Triassic collisional orogeny.     

Geochemistry of upper cretaceous volcanic rocks from the pontic chain,northern turkey

Preliminary data on major elements, Cs, Ba, Rb, Pb, Sr, REE, Y, Th, U, Zr, Ht, Sn, Nb, W, Mo, Cr, V, Sc, Ni, Co and Cu contents for eight samples coming from the Upper Cretaceous volcanic belt of the Pontic Chain (Northern Turkey) are reported. SiO, versus K₂O relationship shows that the analyzed samples belong to the calc-alkaline and shoshonite series. The calc-alkaline rocks appear to represent two distinct magma types one close in composition to typical island are calc-alkaline magmas and one with high incompatible elements concentration and tractionated heavy REE patterns which suggest a genesis by partial melting at high pressure with a garnet bearing residue. Shoshonitic rocks show Na₂O/K₂O close to one, high incompatible elements concentration, and TiO₂%. Al₂O₃%, Ni and Co contents, Ni/Co and V/Ni ratios and REE patterns similar to typical island are andesites which suggest for these rocks similar genetical processes as the island are calc-alkaline magmas.     

Systematic petrochemical differences in andesite suites

When the chemical compositions of andesites are computed by a least mean squares technique at 60 % silica for different volcanoes and provinces, systematic differences appear. While island arcs in general have lower K, Rb, Ba, Th, Sr and higher Ca than the continental andesites, individual volcanoes within a province often show up to orders of magnitude differences in these elements. Of the seven main Quaternary volcanic provinces of the western American Cordillera (Alcutian, Cascade, Central Mexican, Central American, Columbian, North Chilean and South Central Andes) the Cascade region has the lowest K₂O and highest Sr (1.25%, 1400 ppm) and the North Chilean region has the highest K₂O (2.8) and low Sr. Ba appears to follow K while K, Rb, Sr appear to vary independently in different regions though K and Sr often vary inversely within a region.