J.G. Mitchell

Genesis of collision volcanism in Eastern Anatolia, Turkey

Late Cenozoic volcanism in Eastern Anatolia extends in a broad SW - NE trending belt across the Arabia - Eurasia collision zone, from the Arabian foreland basin in the southwest and to the Kars Plateau and Lesser Caucasus in the northeast. Foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic - alkaline composition. Volcanism on the thickened crust north of the Bitlis Thrust Zone varies from the mildly alkaline volcano, Nemrut, and older Mus volcanics in the south, through the transitional calc-alkaline/alkaline volcanoes Bingol and Suphan and the alkaline volcano Tendurek to the calc-alkaline volcano Ararat and older Kars plateau volcanics in the north. Isotope (Sr, Nd) and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the Arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time; and that lavas from the alkaline volcanic area around Mus, and the volcanoes Nemrut and Tendurek north of the Bitlis Thrust Zone were derived from a lithospheric source of similar composition, either from the same, underthrust, Arabian continent or from the Bitlis Massif microcontinent. By contrast, the transitional lavas from Bingol and Suphan and the calc-alkaline lavas from Ararat and Kars were derived from lithosphere carrying a distinct subduction signature inherited from pre-collision subduction events. Positive correlations between 87Sr/86Sr and SiO2 and Rb/Nb and SiO2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone. Trace element covariation diagrams such as Y-Rb indicate the importance of hornblende crystallization at depth (and orthopyroxene at shallow levels) within the calc-alkaline provinces, in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas. Trace element diagrams, based on the covariation of compatible and incompatible elements, point to moderate - low degrees of partial melting with residual clinopyroxene throughout, and residual garnet in the foreland province. Consideration of mineral stabilities, mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths, probably beneath the amphibole and dolomite breakdown curves. Perturbation of the thickened lithosphere by delamination of the thermal boundary layer, perhaps coupled with local stretching associated with pull-apart basins on strike-slip fault systems, is then sufficient to generate melt, the composition of that melt being largely dependent on the enrichment history of the lithosphere in question. In Eastern Anatolia, volcanism appears to have started between about 8 and 6 Ma ago, some 5 Ma after the start of rapid uplift of the East Anatolian Plateau.

Petrogenetic evolution of late Cenozoic, post-collision volcanism in western Anatolia, Turkey

Following an Eocene continent-arc collision, the Western Anatolia region experienced a complete cycle of thickening and orogenic collapse. The early stage of collision-related volcanism, which was most evident during the Early Miocene (<21 Ma), produced a considerable volume of lavas and pyroclastic deposits of basaltic andesite to rhyolite composition. The volcanic activity continued into the Middle Miocene with a gradual change in eruptive style and magma composition. The Middle Miocene activity formed in relation to localised extensional basins and was dominated by lava flows and dykes of basalt to andesite composition. Both the Early and Middle Miocene rocks exhibit calc-alkaline and shoshonitic character. The Late Miocene volcanism (<11 Ma) was marked by alkali basalts and basanites erupted along the zones of localised extension. The Early–Middle Miocene volcanic rocks exhibit enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and have high 87Sr/86Sr (0.70757–0.70868) and low 143Nd/144Nd (0.51232–0.51246) ratios. Modelling of these characteristics indicates a mantle lithospheric source region carrying a subduction component inherited from a pre-collision subduction event. Perturbation of this subduction-metasomatised lithosphere by either delamination of the thermal boundary layer or slab detachment is the likely mechanism for the initiation of the post-collision magmatism. Petrographic characteristics and trace element systematics (e.g. phenocryst assemblages and relative depletion in MREE and heavy rare earth elements (HREE)) suggest that the Early–Middle Miocene magmas underwent hydrous crystallisation (dominated by plagioclase+pyroxene+pargasitic amphibole) in deep crustal magma chambers. Subsequent crystallisation in shallower magma chambers follows two different trends: (1) anhydrous (pyroxene+plagioclase-dominated); and (2) hydrous (edenitic amphibole+plagioclase+pyroxene dominated). AFC modelling shows that the Early–Middle Miocene magmas evolved through assimilation combined with fractional crystallisation, and that the effects of assimilation decreased gradually from the Early Miocene into the Middle Miocene. This may indicate a progressive crustal thinning related to the extensional tectonics that prevailed from the latest Early Miocene onwards. In contrast, the Late Miocene alkaline rocks are characterised by low 87Sr/86Sr (0.70311–0.70325) and high 143Nd/144Nd (0.51293–0.51298) ratios and have OIB-type like trace element patterns characterised by enrichment in LILE, HFSE, LREE and MREE, and a slight depletion in HREE, relative to average N-MORB. REE modelling indicates that these rocks formed by partial melting of a garnet-bearing lherzolite source. Trace element and isotope systematics are consistent with an origin by decompression melting of an enriched asthenospheric mantle source.

Volcano-stratigraphy and geochemistry of collision-related volcanism on the Erzurum–Kars Plateau, northeastern Turkey

The Eastern Anatolia Region exhibits one of the worlds best exposed and most complete transects across a volcanic province related to a continental collision zone. Within this region, the Erzurum–Kars Plateau is of special importance since it contains the full record of collision-related volcanism from Middle Miocene to Pliocene. This paper presents a detailed study of the volcanic stratigraphy of the plateau, together with new K–Ar ages and several hundred new major- and trace-element analyses in order to evaluate the magmatic evolution of the plateau and its links to collision-related tectonic processes. The data show that the volcanic units of the Erzurum–Kars Plateau cover a broad compositional range from basalts to rhyolites. Correlations between six logged, volcano-stratigraphic sections suggest that the volcanic activity may be divided into three consecutive Stages, and that activity begins slightly earlier in the west of the plateau than in the east. The Early Stage (mostly from 11 to 6 Ma) is characterised by bimodal volcanism, made up of mafic-intermediate lavas and acid pyroclastic rocks. Their petrography and high-Y fractionation trend suggest that they result from crystallization of anhydrous assemblages at relatively shallow crustal levels. Their stratigraphy and geochemistry suggest that the basic rocks erupted from small transient chambers while the acid rocks erupted from large, zoned magma chambers. The Middle Stage (mostly from 6–5 Ma) is characterised by unimodal volcanism made up predominantly of andesitic–dacitic lavas. Their petrography and low-Y fractionation trend indicate that they resulted from crystallization of hydrous (amphibole-bearing) assemblages in deeper magma chambers. The Late Stage (mostly 5–2.7 Ma) is again characterised by bimodal volcanism, made up mainly of plateau basalts and basaltic andesite lavas and felsic domes. Their petrography and high-Y fractionation trend indicate that they resulted from crystallization of anhydrous assemblages at relatively shallow crustal levels. AFC modelling shows that crustal assimilation was most important in the deeper magma chambers of the Middle Stage. The geochemical data indicate that the parental magma changed little throughout the evolution of the plateau. This parental magma exhibits a distinctive subduction signature represented by selective enrichment in LILE and LREE thought to have been inherited from a lithosphere modified by pre-collision subduction events. The relationships between magmatism and tectonics support models in which delamination of thickened subcontinental lithosphere cause uplift accompanied by melting of this enriched lithosphere. Magma ascent, and possibly magma generation, is then strongly controlled by strike-slip faulting and associated pull-apart extensional tectonics.

On dating the magmatism of Maio, Cape Verde Islands

Conventional K-Ar and40Ar/39Ar studies of Mesozoic ocean floor basalts and Tertiary plutonic and volcanic rocks from Maio, Cape Verde Islands, have been determined to elucidate the magmatic evolution of this ocean island. Pillow lavas of the Basement Complex yield a minimum age of 113±8 Ma though thermal overprinting of their formation age by the younger Central Intrusive Complex (CIC) and subsequent sheet intrusions is in some cases almost total. Activity in the CIC began before 20 Ma and plutons continued to develop until about 8 Ma, the youngest ages possibly indicating a cooling history of more than 2 Ma for these bodies relative to their volcanic counterparts. Sheet intrusion occurred throughout the period 20 to 9 Ma though the peak of this activity probably occurred 11 Ma ago. Field relations allow the time of thrusting(s) on the Monte Branco Thrust to be bracketed between 9 and 7 Ma. Volcanic activity began in the Tertiary, probably before 12 Ma, and culminated in the development of a stratovolcano at 7 Ma.

Chronology of Neogene and Quaternary uplift and magmatism in the Caucasus: constraints from K–Ar dating of volcanism in Armenia

Abstract The Greater Caucasus is one of Earths highest actively-uplifting mountain ranges; the adjoining Caspian Sea basin contains a substantial proportion of its hydrocarbon reserves. Like other parts of the former Soviet Union, the Neogene and Quaternary chronology of these important regions has not previously been well-defined. It has thus been impossible to obtain reliable estimates for rates of processes such as uplift of the Caucasus and sedimentation in the Caspian Sea. Previous studies have established the relative timings of events in the region, using correlation schemes between volcanism, glaciations, and the stratigraphy of the Caspian basin. However, a range of absolute chronologies has previously been proposed for these sediments and igneous rocks, based mainly on different interpretations of their magnetostratigraphic records. By K–Ar dating, we determine the ages of volcanism at three localities in Armenia as 1.1, 0.8 and 0.8 Ma. Using these data and other evidence, we propose a revision to the chronology of this region, in which a distinctive brief interval of normal magnetic polarity in the local sedimentary and volcanic magnetostratigraphic records is matched to the Cobb Mountain event in the global record rather than the Olduvai event or an earlier subchron as had previously been thought. We thus interpret a ∼1.5 Ma timing for the start of volcanism in the Lesser Caucasus, and also suggest a ∼1.2 Ma timing for the Late Akchagyl transgression of the Caspian Sea, a key event in the regional stratigraphy when this water body reached its greatest extent. We tentatively correlate this transgression with the melting event following glaciation during stage 36 of the oxygen isotope timescale, which was thus the first time during the Pleistocene when eastern Europe was covered by a lowland ice sheet. Time-averaged since ∼1 Ma, the flanks of the eastern Greater Caucasus mountains are shown to have uplifted at ∼0.6 mm a−1 and the Lesser Caucasus at ∼0.3 mm a−1. We show that the rate and spatial scale of this uplift are too great to be the result of plate convergence, and suggest instead that this uplift is caused by crustal thickening due to inward lower-crustal flow to beneath these mountain ranges. At the start of magmatism in both the Greater and Lesser Caucasus, the estimated crustal thickness was ∼45 km. We thus suggest that this magmatism has been caused by heating of the mantle lithosphere due to earlier crustal thickening, the temperature rise required to initiate magmatism being the same in both cases.

40Ar/39Ar and K/Ar geochronology of the dykes from the south Indian granulite terrain

Abstract 40Ar/39Ar and conventional K/Ar data on mafic dykes and a felsic dyke, together with previous data have been used to constrain the ages of the dyke magmatism in the south Indian granulite terrain. The age of the mainly ENE–WSW-trending Agali–Anaikatti swarm is estimated to be 1980±25 Ma, with a very limited occurrence of upper Cretaceous (80–90 Ma) dykes in the area. The NW–SE Dharmapuri swarm is dated as at least 1800 Ma (possibly 1855±9 Ma). Both the NW–SE and the NE–SW dyke swarms of Tiruvannamalai are dated at 1650±10 Ma. Only a few dykes in north Kerala are of middle Proterozoic (ca. 1700–1650 Ma) age. Furthermore, the 40Ar/39Ar results of Dharmapuri dykes exhibit age spectrum characteristic of partial degassing; they provide strong evidence of latest Proterozoic overprinting by a thermal event probably related to the Pan-African orogeny. Most of the Phanerozoic dykes are confined to the coastal region in Kerala province. The dolerites of central Kerala and the majority of north Kerala dolerites intruded during the 70–65 Ma period; whereas a large gabbro and a felsic dyke in central and north Kerala, respectively, are of upper Cretaceous age (ca. 85 Ma) and the mean 81±1.5 Ma age from these dykes may represent cooling age. The oldest Phanerozoic (Upper Jurassic; 144±6 Ma) dyke intrusions have a restricted occurrence in south Kerala, very close to the southern tip of India. The Proterozoic dykes are interpreted as being lateral intrusions from under-plating igneous bodies beneath the eastern shield region. The geological record of the terrain does not show indications of significant pre-magmatic extension and rifting, but dyke magmatism may have led to extension, rifting and the consequent development of the India–Antarctica–Sri Lanka Proterozoic high-grade terrain. The data are also interpreted to suggest that the south Indian granulite terrain is an early Proterozoic tectono-metamorphic terrane with strong thermal imprints of Neoproterozoic/early Palaeozoic age. It is difficult to reconcile its extensions into Proterozoic Sri Lanka or Antarctica where tectono-metamorphism is dated to be of Neoproterozoic age. The late Phanerozoic dykes are predominantly related to the Deccan trap event, and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the Seychelles and the Indian continent. The few older (ca. 145 Ma) dykes in south Kerala may be related to the rift tectonics that preceded the separation of Australia–Antarctica and India.

Episodic mesozoic volcanism in Namibia and Brazil: A K—Ar Isochron study bearing on the opening of the south atlantic

Abstract An attempt is made to find a more objective and precise basis for the correlation of volcanics from southwestern Africa and South America than is possible by frequency diagrams of individual K—Ar ages. This leads to a critical appraisal of conventionally calculated K—Ar ages with the conclusion that a priori assumption regarding the isotopic composition of non-radiogenic argon and, hence, the standard atmospheric correction, are no longer tenable. K—Ar isotoopic data on Mesozoic basalts and dolerites from Namibia and Brazil are presented in terms of an isochron model. Plots for cogenetic rocks are unacceptably scattered on a “radiogenic” 40 Ar vs. K diagram, but show a high degree of collinearity on 40 Ar/ 36 Ar diagrams0K/ 36 Ar diagrams. Using the latter plots, a number of isochrons are generated which indicate that Mesozoic volcanism in these regions occured as several discrete episodes of fairly short duration. Effusion of the extensive Serra Geral basalts of Brazil and the Kaoko basalts of Namibia is shown to have occured simultaneeously at 121 m.y.B.P. Basalts from a series of boreholes along the central Parana Basin, as well as a group of dykes from Sao Paulo, yield isochrons of 128 m.y., which coincides with the postulated onset of separation of Africa and South America based on marine magnetic anomalies. Linear dyke swarms along the Namibian seaboard, interpreted as an expression of the earliest rift phase, have an isochron age of 134 m.y. Sills and dykes, mainly from southern Namibia, with isochron ages of 183 m.y. are considered to be the westernmost manifestation of Stormberg volcanism, not necessarily related to rifting. Most of the igneous suites examined have initial 40 Ar/ 36 Ar ratios significantly different from the modern atmospheric value.

KAr ages of clay-size concentrates from the mineralisation of the Pedroches Batholith, Spain, and evidence for Mesozoic hydrothermal activity associated with the break up of Pangaea

Abstract The K Ar ages of 32 clay concentrates extracted from samples of ore, gangue and wallrock associated with mineralisation in the Pedroches Batholith lie in the range 119–285 Ma. Although some of the mineralisation is of Permian age more than half of the ages lie between 210 and 230 Ma and indicate a hydrothermal event at this (Triassic) time. A comparison with age data for mineralisation and certain anorogenic magmatism in other areas of the North Atlantic reveals a consistent pattern of a major event at ca. 210–230 Ma and a minor event at ca 160 Ma with little Cretaceous or Tertiary activity. It is proposed that the ca. 210–230 Ma event was related to an initial rapid fracturing of the crust associated with the break up of Pangaea, which was related with an increase in the geothermal gradient and penetration of the deep crust by surface waters which returned via both new and rejuvenated fissure systems.

Palaeomagnetism and radiometric age of the Jurassic Chon Aike formation from Santa Cruz Province, Argentina: implications for the opening of the South Atlantic

At Camarones Bay, the Chon Aike formation of rhyolites, andesites and porphyries exhibits normal remanent magnetization at nine sites and reversed remanence at eight sites. A K-Ar age of 166±5 my was obtained from a pair of determinations on each of ten samples. This puts the emplacement of these lavas in the Bathonian Stage of the Middle Jurassic. The reversed magnetization may therefore correspond to the Pospelovskaja Reversed Zone in the predominantly normal Jurassic geomagnetic field [1]. The mean direction of the normal group of sites is the same as that already reported for normally magnetized samples from Puerta Deseado [2], but the reversed group yields a different mean direction. A single population formed of the two sets of VGPs falling near the present south poles has a mean at 129°W 82°S ( α95 = 12°), and when compared with the African mean Jurassic pole could be interpreted in terms of a post Jurassic opening of the South Atlantic.

Neogene and Quaternary Volcanism of Southeastern Turkey

Abstract Potassium-argon dating indicates two episodes of basaltic magmatism in south eastern Turkey at c. 19–15 and c. 2.3–0.6 Ma. Each produced olivine-titanaugite basalts, whose chemical compositions are difficult to classify using any conventional model in both the Anatolian continental fragment and the Arabian Platform. It is proposed here that both episodes of volcanism, and the associated crustal thickening and surface uplift, result from heating of the mantle lithosphere by crustal thickening caused by inflow of plastic lower crust from adjoining regions. Thus, although this study region has remained in a plate boundary zone for tens of millions of years, its volcanism has no direct relationship to local plate motions. It is suggested instead that both episodes of volcanism are the result of loading effects caused by glacial to interglacial sea-level variations, which will cause net flow of lower crust from beneath the offshore shelf to beneath the land: the moderate glaciations of Antarctica which began in the Early-Middle Miocene, and the more intense lowland glaciations of the northern hemisphere which began around c. 2.5 Ma.