Petrogenesis Research Papers - Academia.edu (original) (raw)

The felsic lavas of the Bamenda Mountains, in the main part of the Cameroon Volcanic Line, are mainly represented by trachytes, with subordinated benmoreites and alkaline to peralkaline rhyolites. New K–Ar geochronological data define two... more

The felsic lavas of the Bamenda Mountains, in the main part of the Cameroon Volcanic Line, are mainly represented by trachytes, with subordinated benmoreites and alkaline to peralkaline rhyolites. New K–Ar geochronological data define two main volcanic episodes, the first one between 18 and 22Ma, and the second one from 12.5 to 13.5Ma. Geochemical data indicate that these felsic rocks

The Neoproterozoic basement of Wadi Al-Baroud area located at the northern Eastern Desert (ED) of Egypt, at the northernmost segment of the Arabian–Nubian Shield (ANS), is comprised of two different granite suites. A large batholith... more

The Neoproterozoic basement of Wadi Al-Baroud area located at the northern Eastern Desert (ED) of
Egypt, at the northernmost segment of the Arabian–Nubian Shield (ANS), is comprised of two different
granite suites. A large batholith ascribed to the Older Granite suite, extends across the boundary between
the northern and central ED, and is intruded by two isolated plutons of the Younger Granite suite. The
Older Granite suite includes gray-colored, massive to gneissose, granodiorites to tonalites typically containing
microgranular mafic enclaves. These are calc-alkaline, magensian, metaluminous I-type granitoids,
with high Sr contents, and depleted in Rb, Nb, Y and REE. The Younger Granite suite plutons are
pink to red, biotite and two-mica monzogranites. These are peraluminous A-type granites exhibiting a
high-K calc-alkaline nature, and varying between ferroan and magnesian type granites. The A-type granites
of the Younger Granite suite are enriched in Ga, Y, HFSE and REE elements, and depleted in the LILE
elements Ba, Sr and Rb and transition metals Cr, Ni, Co, Sc and V. Magmatic saturation temperatures indicate
early crystallization of apatite at high temperature in the metaluminous I-type Older Granite suite,
while in the peraluminous A-type Younger Granites its crystallization occurs later after separation of zircon
and monazite. The plutons of the Younger Granite suite were generated during the post-collisional
stage of the northern ANS, following collision between the juvenile ANS crust and the pre-Neoproterozoic
continental blocks of west Gondwana. The emplacement of the Older Granite suite took place earlier,
within a normally mature continental arc prior to the collision. These pre-collision granitoids evolved
through assimilation-fractional crystallization processes from mantle-derived parental magmas, which
have interacted with crustal materials during ascent and storage. The post-collisional Younger Granite
suite seems to have been derived by high degree, partial melting of metasedimentary sources, particularly
psammitic and pelitic metasediments.

Many visitors to the cliffs of Budleigh Salterton see the dark nodules within the mudstones and have no simple, unified source for information on their nature, origins or development. This paper is an attempt to draw together both field... more

Many visitors to the cliffs of Budleigh Salterton see the dark nodules within the mudstones and have no simple, unified source for information on their nature, origins or development. This paper is an attempt to draw together both field observations and prior academic material, in order to provide a simple, coherent description of the geological setting, the physical appearance and a possible mode of development of the nodules. Aimed toward the interested amateur, the finer details of the implications of environmental changes have been assumed, concentrating on the general processes only.

Wadi Hammuda is dominated by a variety of low grade regionally metamorphosed volcaniclastic metasediments pertaining to two different geotectonic settings and intruded by arc and late collision granitic rocks. Thus, the volcaniclastic... more

Wadi Hammuda is dominated by a variety of low grade regionally metamorphosed volcaniclastic metasediments pertaining to two different geotectonic settings and intruded by arc and late collision granitic rocks. Thus, the volcaniclastic metasediments which form extensive outcrops are considered as a member of island arc assemblages. This paper deals with the petrography, geochemistry, and tectonic setting of the island arc volcaniclastic metasediment rock units. The volcaniclastic metasediments consist of interbedded metagreywackes, metasiltstones, metamudstones, and schists as well as metapyroclastics. They are well foliated, crenulated and tightly folded, metamorphosed, and intruded by granitic rocks. Geochemical data support the petro-graphic classification and reveal that these volcaniclastic metasediments are generally low-K, essentially tholeiitic in character, with the exception of some metasediments and metapyroclastics which exhibits calc-alkaline and tholeiitic affinities and represent the first stage of island arc volcanism. The overthrusted oceanic lithosphere blocks with fragments of the fore arc and/or back-arc marginal basins volcaniclastic metasediments were incorporated among the island arc volcanics which supported by tectonically relationship between the different rock units in the study area. Contemporaneous with this deformation event, Wadi Hammuda was subjected to low grade regional metamorphism and the rocks document an early phase of shearing and/or foliation. Occasionally minor folds were developed particularly in the metasediments and schists. The subsequent emplacement of the syn-tectonic granites (tonalites and granodiorites) resulted in minor local thrusts. During the regional thrusting event which preceded the emplacement of the late-tectonic granites (alkali feldspar granites) and affected the whole region, low grade successions cover the study area similar to the Meatiq volcaniclastic metasediments.

The Liuyuan mafic and ultramafic rocks are exposed in Southern Beishan, which is along the southern branch of the Central Asian Orogenic Belt (CAOB). Zircon SHRIMP U–Pb dating showed that Liuyuan gabbros intruded during the early Permian... more

The Liuyuan mafic and ultramafic rocks are exposed in Southern Beishan, which is along the southern branch of
the Central Asian Orogenic Belt (CAOB). Zircon SHRIMP U–Pb dating showed that Liuyuan gabbros intruded during the early Permian (~270–295 Ma) coeval with the basalts and the ultramafic rocks were emplaced at about 250 Ma. The basalts are within–plate tholeiites with slight enrichment in light rare earth elements (LREE) relative to heavy rare earths (HREE) and small negative anomalies of Nb and Ta. Gabbros including
olivine gabbros, olivine gabbronorites and troctolites are grouped into two: the cumulate gabbros are depleted
in LREE and showsmall negative Nb and Ta anomalies but distinct positive Sr and Eu anomalies; non–cumulate
gabbros resemble tholeiitic basalts. Lamprophyres and cumulate ultramafic rocks are characterized by large
enrichment of LREE relative to HREE with depletion in Nb and Ta. The enriched Sr–Nd isotopic trend from DM towards the EM II end member component implies that the lithospheric mantle was progressively enriched with depth by the involvement of subducted crustal material due to the delamination of thickened mantle lithosphere after collision. The digestion of subducted crustal material into the mantle resulting in the metasomatized and enriched mantle is inferred to be an important process during crust–mantle interaction.

Geochronological, geochemical, whole-rock Sr–Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of... more

Geochronological, geochemical, whole-rock Sr–Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis and tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon 206Pb/238U age of 127±2 Ma and an 40Ar/39Ar age on hornblende of

Kulshan caldera (4.5×8 km), at the northeast foot of Mount Baker, is filled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas and is only the third Quaternary caldera identified in the Cascade arc. A gravity traverse across the... more

Kulshan caldera (4.5×8 km), at the northeast foot of Mount Baker, is filled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas and is only the third Quaternary caldera identified in the Cascade arc. A gravity traverse across the caldera yields a steep-sided, symmetrical, complete Bouguer anomaly of −16 mGal centered over the caldera. Density considerations suggest that the caldera fill,

The Albanide–Hellenide ophiolites and related ophiolitic mélanges include eight different types of volcanic and subvolcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB); 2) Triassic high-Ti mid-ocean ridge basalts showing... more

The Albanide–Hellenide ophiolites and related ophiolitic mélanges include eight different types of volcanic and subvolcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB); 2) Triassic high-Ti mid-ocean ridge basalts showing enriched compositions (E-MORB); 3) Triassic and Jurassic high-Ti mid-ocean ridge basalts showing normal compositions (N-MORB); 4) Jurassic basalts with geochemical features between MORB and island arc tholeiites; hereafter defined as medium-Ti basalts (MTB); 5) Jurassic low-Ti, island arc tholeiitic (IAT) rocks; 6) Jurassic very low-Ti (boninitic) rocks; 7) Jurassic backarc basin basalts and basaltic andesites (BABB); 8) Triassic and Jurassic calc-alkaline rocks (CAB). The geochemical and petrogenetic features of these rock-types, as well as the results from REE modelling of mantle sources, primary melt generation, and mantle residua indicate that they have formed in distinct tectonic settings within an oceanic environment.Both Triassic and Jurassic N-MORBs primary magmas derived from ~ 10 to 20% partial melting of a primitive asthenosphere, whereas Triassic alkaline WPB basalts originated from low degrees of partial melting of an OIB-type mantle source and were most likely erupted in seamounts. Triassic E-MORBs originated from ~ 12% partial melting of a primitive asthenosphere influenced by the OIB-type component. The residual MORB mantle is represented by depleted lherzolites, which are commonly found in the Albanide–Hellenide ophiolites.Mid Jurassic MTB and IAT primary magmas derived from ~ 10% and 10–20% partial melting of the MORB residual mantle, respectively with the variable addition of subduction components and were erupted in an intra-oceanic, supra-subduction zone setting. The residual mantle associated with these magmatic events is represented by harzburgites. Mid Jurassic boninitic primary magmas may have originated either from 10 to 20% partial melting of the MTB and IAT residual mantle or from ~ 30% partial melting of the MORB residual mantle. In both cases, the depleted mantle sources were enriched in light rare earth elements (LREE) by subduction-derived fluids. The extremely depleted harzburgites, which are widespread in the Albanide–Hellenide ophiolites, are interpreted as the residual mantle associated with boninite formation.Mid-Late Jurassic CABs originated from ~ 15 to 20% partial melting of a depleted peridotite mantle significantly enriched in Th and LREE by subduction-derived fluids, whereas BABBs originated from 10 to 20% partial melting of a primitive asthenosphere somewhat enriched in Th and LREE by a nearby subduction. Both these rock-types were erupted in a continental arc-backarc setting.The different rock-types of the Albanide–Hellenide ophiolites record the fundamental stages of the Triassic–Jurassic evolution of the Neo-Tethys in the Dinaride sector: from sea-floor spreading, after continental break-up, to intra-oceanic subduction initiation and supra-subduction zone (SSZ) lithospheric accretion.► The Triassic Dinaride Tethys was characterized by oceanic spreading. ► During the Triassic, alkaline within-plate and mid-ocean ridge basalts were erupted. ► The Jurassic Dinaride Tethys was characterized by an intra-oceanic arc setting. ► Supra-subduction zone volcanics were erupted in the intra-oceanic arc. ► Mid-ocean ridge basalts were also erupted in the intra-oceanic arc.

Combined Hf-O isotopic analyses of zircons from tuffs and lavas within the Sierra Madre Occidental (SMO) silicic large igneous province are probes of petrogenetic processes in the lower and upper crust. Existing petrogenetic and... more

Combined Hf-O isotopic analyses of zircons from tuffs and lavas within the Sierra Madre Occidental (SMO) silicic large igneous province are probes of petrogenetic processes in the lower and upper crust. Existing petrogenetic and tectonomagmatic models diverge, having either emphasized significant crustal reworking of hydrated continental lithosphere in an arc above the retreating Farallon slab or significant input of juvenile mantle melts through a slab window into an actively stretching continental lithosphere. New isotopic data are remarkably uniform within and between erupted units across the spatial and temporal extent of the SMO, consistent with homogeneous melt production and evolution. Isotopic values are consistent with enriched mantle magmas (80%) that assimilated Proterozoic paragneisses (~20%) from the lower crust. 18Ozircon values are consistent with fractionation of mafic magma and not with assimilation of hydrothermally altered upper crust, suggesting that the silicic ...

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local... more

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local complexity of the upper mantle beneath the French Massif Central. Ultramafic nodules have a metamorphic, magmatic or pyrometamorphic origin; they display different stages of

The belt consisting of deep-water meta-sediments of the Aravalli Supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels. These are the basal Bagdunda volcanics and the Gopir magmatic rocks... more

The belt consisting of deep-water meta-sediments of the Aravalli Supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels. These are the basal Bagdunda volcanics and the Gopir magmatic rocks represented by Gopir dikes and flows associated with the ultramafic rocks along the Kaliguman lineament. The Bagdunda and Gopir mafic volcanic rocks are sub-alkaline, varying in composition from high Mg-tholeiite to basaltic komatiite. They are predominantly LREE depleted, but some flat REE patterns are also observed. Bulk geochemical data, especially the incompatible trace elements discount the possibility of crustal contamination. Gopir dikes are highly enriched in terms of their REE [(Ce/Sm) N and (Ce/Yb) N] and other incompatible trace elements and closely resemble the basal Aravalli volcanics. Their ratio (rock)/ratio (PM) and normalized multi-element abundance patterns depict their overall enriched nature compared to N-MORB and primitive mantle. The Gopir and Bagdunda volcanics reflect trace element characteristics transitional between E-MORB and OIB. Gopir dikes show remarkable similarity with continental tholeiite and Continental Flood Basalts (CFBs) with negative Nb, P and Ti anomalies and low Nb/Ce ratio unlike the volcanics of both the suites. Petrogenetic modeling based on the compositionally corrected [Mg] and [Fe] abundances for Gopir dikes, Gopir and Bagdunda volcanics indicates (a) their derivation from non-pyrolitic sources (b) their derivation from sources that were variably enriched in [Fe/Mg] ratios with large variation in their [Fe] contents and (c) olivine was a major phase to fractionate, followed by a lesser amount of clinopyroxene in the case of Gopir volcanics, whereas in Bagdunda volcanics a combination of olivine and clinopyroxene fractionation is suggested. Based on the geochemical data, supported by field evidence, we propose a geodynamic model for the development of the Jharol Belt in which we suggest that the basement rock, i.e. the Banded Gneissic Complex (BGC) started rifting probably under influence of a mantle plume during the late Archaean-early Proterozoic period. During the opening of the basin, magma derived from asthenospheric mantle reached the surface contemporaneously with sedimentation. The first phase of volcanism is represented by Bagdunda volcanics. With continuous rifting, the crust became highly attenuated and facilitated asthenospheric upwelling, causing high degrees of melting (indicated by a large volume of mafic and ultramafic rocks) during the second phase of magmatism (Gopir volcanics) that occurred at later stages of Jharol sedimentation. Coeval melting of the sub-continental lithosphere under the adjoining BGC craton probably caused the emplacement of dikes along with the Gopir volcanics. At this stage of progressive rifting, an oceanic crust very similar to that in marginal basins developed in the Jharol Belt.

The most recent volcanism in the Valles caldera is represented by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), and Banco Bonito Flow (BBF) as well as the VC-1 rhyolite, which are collectively known as the East... more

The most recent volcanism in the Valles caldera is represented by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), and Banco Bonito Flow (BBF) as well as the VC-1 rhyolite, which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted at approximately 55 ka and 40 ka after an approximate 460 ka lull in volcanism. Previous studies suggested a mafic intrusion at depth triggered the eruptions. This thesis represents the first detailed study of the EFM. Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides. Electron probe microanalysis and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. Large (1 mm), typically resorbed or subhedral crystals represent one population, and small (0.5 mm), generally euhedral crystals represent the other. The large resorbed plagioclase crystals typically have rim over...

Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from ∼1900 14C yr BP... more

Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from ∼1900 14C yr BP to the present. Radiocarbon dating of pyroclastic deposits indicates an almost continuous activity with periods of high eruption rates alternating with shorter time spans of distinctly reduced eruptive frequency since the first appearance of high-K volcanic rocks. Geochemical data of 28 well-dated, prehistoric pyroclastic flows of the Merapi high-K series indicate systematic cyclic variations. These medium-term compositional variations result from a complex interplay of several magmatic processes, which ultimately control the periodicity and frequency of eruptions at Merapi. Low eruption rates and the absence of new influxes of primitive magma from depth allow the generation of basaltic andesite magma (56–57 wt% SiO2) in a small-volume magma reservoir through fractional crystallisation from parental mafic magma (52–53 wt% SiO2) in periods of low eruptive frequency. Magmas of intermediate composition erupted during these stages provide evidence for periodic withdrawal of magma from a steadily fractionating magma chamber. Subsequent periods are characterised by high eruption rates that coincide with shifts of whole-rock compositions from basaltic andesite to basalt. This compositional variation is interpreted to originate from influxes of primitive magma into a continuously active magma chamber, triggering the eruption of evolved magma after periods of low eruptive frequency. Batches of primitive magma eventually mix with residual magma in the magmatic reservoir to decrease whole-rock SiO2 contents. Supply of primitive magma at Merapi appears to be sufficiently frequent that andesites or more differentiated rock types were not generated during the past ∼2000 years of activity. Cyclic variations also occurred during the recent eruptive period since AD 1883. The most recent eruptive episode of Merapi is characterised by essentially uniform magma compositions that may imply the existence of a continuously active magma reservoir, maintained in a quasi-steady state by magma recharge. The whole-rock compositions at the upper limit of the total SiO2 range of the Merapi suite could also indicate the beginning of another period of high eruption rates and shifts towards more mafic compositions.

An ash layer newly discovered in Core MD01-2393 from the southwestern South China Sea has been studied in order to characterize its major element features. The layer, 4.0-cm thick, light grayish, and silt size, occurs right at the Marine... more

An ash layer newly discovered in Core MD01-2393 from the southwestern South China Sea has been studied in order to characterize its major element features. The layer, 4.0-cm thick, light grayish, and silt size, occurs right at the Marine Isotope Stage 4–5 transition, ca. 74 kyr ago. The morphology and geochemistry of glass shards, combined with oxygen isotope and carbonate stratigraphy, confirm the youngest Toba eruption in northern Sumatra as the origin of the ash layer. Major element data on mineral crystals (i.e. biotite, plagioclase, and hornblende) from the ash layer suggest that biotite is phenocrystic while hornblende and some plagioclase are xenocrystic, implying that these xenocrysts were incorporated into the youngest Toba magma before the eruption.

The Torud-Ahmad Abad magmatic belt is located in the south-southeast of Shahrood (East of Semnan Province, NE Iran) and lies in the northern part of the Central Iran Structural Zone (CISZ), where a thick sequence of Paleo-cene to middle... more

The Torud-Ahmad Abad magmatic belt is located in the south-southeast of Shahrood (East of Semnan Province, NE Iran) and lies in the northern part of the Central Iran Structural Zone (CISZ), where a thick sequence of Paleo-cene to middle Eocene volcanic and volcanosedimentary rocks cropped out. This sequence was intruded by numerous dikes, hypabyssal igneous domes and one small gabbrodioritic intrusion, with compositions ranging from trachybasaltic andesite, trachyandesite, dacite, trachyte, gabbro, diorite and syenite. Various enclaves (cogentic and noncogenetic) with different composition, size and shape have been found in these domes and dikes. These enclaves are evidence of magma mixing and crustal contamination. Geochemically, the studied rocks exhibit a calc-alkaline to high potassium calc-alkaline affinity, and are enriched in LREE and LILE and depleted in HREE and HSFE. Other geochemical characteristics, such as a silica content varying between 59-63 wt% and 51-59 wt%, a Na 2 O content N 3 wt%, Al 2 O 3 content N 16 wt%, Yb b 1.8 ppm, and Y b 18 ppm, make it possible to classify these rocks as high silica adakite in the Ahmad Abad region and low silica adakite in the Sahl-Razzeh region or at least, adakitic like rocks. Also, depletion of Nb and Ti, and high enrichment in Rb, Ba, K and Th, imply crustal contamination of the mentioned adakitic domes. The petrographical and geochemical evidence show that the magma forming of the high silica adakites has been originated from partial melting of the subducted oce-anic slab of Neo-Tethys (Sabzevar-Darouneh branch) in amphibolite to eclogite facies and the low silica adakites formed by partial melting of the metasomatized or modified mantle wedge, above the subduction zone. Gabbroic to syenitic rocks are the products of fractional crystallization of basic magma which originated from a nearly non-modified mantle wedge above the subducted oceanic slab. U-Pb dating of the dacitic and andesitic rocks belong to hypabyssal rocks yielded age of 41.4 ± 0.3 Ma, and 35.5 ± 0.2 Ma respectively and consistent to Middle to Late Eocene.