Lherzolite Source Research Articles

Geology and Petrology of Omzha Block, Zhob Ophiolite, northern Balochistan, Pakistan

Abstract The Zhob Ophiolite is divided into three detached blocks including the Omzha block. The Omzha block is mapped and divided into lithological units such as ultramafic rock, mafic-felsic rock, and volcanic–volcaniclastic–pelagic rocks. These units are quite deformed and mixed up and are associated with one another by thrust faults. Petrography and geochemistry divide them into gabbro, diorite, plagiogranite, pheno-tephrite and trachy-andesite basalt, trachy basalt, chert, limestone, and mudstone. The ultramafic rocks are dominantly serpentinized harzburgite, dunite, and a minor lherzolite. Petrography of peridotite shows that it may be depleted in nature and may have residual after processes such as partial melting and the melt-rock reaction of a lherzolitic source. The gabbroic rocks are less well-developed and highly deformed. They are cross-cut by diorite, plagiogranite and anorthosite’ intrusions. The gabbro may be the plutonic section of Omzha block’ crust while the intermediate-felsic igneous rocks may have formed by the anataxis of crustal gabbro. The volcanic–volcaniclastic–pelagic rocks unit may be corrected with Bagh complex found underneath the Muslim Bagh Ophiolite. The metamorphic sole rocks of Omzha block are highly deformed and dismembered are comprising of metamorphic facies such as amphibolite, quartz-mica schist, and greenschist.

The Deccan Volcanic Province (DVP), India: A Review

The Deccan Volcanic Province (DVP), India: A Review

Petrogenesis and the evolution of Pliocene Timar basalts in the east of Lake Van, Eastern Anatolia, Turkey: A consequence of the partial melting of a metasomatized spinel–rich lithospheric mantle source

The Eastern Anatolian region provides a good example for collision–related volcanic activity. This activity started during the Middle Miocene and continued through the Pliocene and Quaternary. The new K − Ar ages reveal that the post–collisional Pliocene Timar basalts located in the East Anatolian Accretionary Complex (EAAC) erupted between 4.72 (Zanclean) and 3.29–2.93 Ma (Piacenzian). The Pliocene Timar basalts consist of plagioclase, clinopyroxene (augite), and olivine minerals and have glomeroporphyritic, intergranular, and ophitic textures. They display enrichment in light rare earth elements (LREEs) with (La/Yb)N values that range between 3.19 and 6.66 to 21.63–5.61. 87Sr/86Sr(i) and 143Nd/144Nd(i) isotopic ratios of the Timar basalts are 0.70599–0.70436 and 0.51277–0.51259, respectively. The εNd(t) values of these rocks range between 1.0116 and 1.0121. 87Sr/86Sr(i) and Nd(t) contents indicate crustal contamination during the emplacement of the Timar basalts. According to the Energy Constrained–Assimilation Fractional Crystallization (EC–AFC) model, this contamination rate is between ~1.4 and 4.4%. The melting model shows that the least evolved samples of the Timar basalts were formed by ~0.1–2% partial melting of the lherzolitic source. Primitive melts that constituent Timar basalts were occurred with partial melting of a magma source that has predominantly spinel bearing mineralogy. All data indicate that the origin of the parental melts of the Timar basalts is metasomatized lithospheric mantle source and then these melts reached the surface from the fissures that is related to extensional tectonics in Pliocene.

Evolution of Late Cretaceous to Palaeogene basalt–andesite–dacite–rhyolite volcanic suites along the northern margin of the Ladakh magmatic arc, NW Himalaya, India

This paper describes a comprehensive geochemical study of basalt–andesite–dacite–rhyolite volcanic association in the Khardung volcanic suite along the northern margin of the Ladakh magmatic arc. This volcanic association is outcropped mainly in the segment of the further north of the Khardung village to Khalsar delineating from the Ladakh magmatic arc by ~2 km thick porphyritic sill. The closed association of basalt–andesite–dacite–rhyolite volcanic within a volcanic suite suggests that these rocks may be genetically inter-related and might have derived from the same parental magma source. Felsic lavas (dacite–rhyolite) show SiO2 range from 64.75 to 79.11 wt.%, while intermediate lavas (basaltic andesite–andesite) ranges from 50.80 to 51.81 wt.% with mafic lavas (basalt) span from 53.39 to 62.05 wt.%. These volcanic suites show enrichment in LIL elements (Rb, Ba, Th, U, and Pb) and depletion in Nb, P, and Ti, which can be evident in spider diagrams with pronounced to mild Eu negative anomalies in REE patterns. Previous reports on zircon U–Pb ages of the Khardung volcanics range between 60 and 69.7 Ma confirm an upper bound eruption age of this volcanic suite as pre-collision continental arc magmas. Hence, the results of geochemical modelling suggest that the Khardung mafic–intermediate-felsic lavas were generated from the melting of 1–4% spinel and garnet-bearing lherzolite sources. The generated parental magmas were modified by crustal materials during the magma ascent along with fractional crystallization and were metasomatized by slab-derived fluids released from the subducting Neo-Tethyan oceanic crust during the Late Cretaceous to Palaeogene in the northern margin of the Ladakh magmatic arc.

Shallow marine basaltic volcanism of the Machadodorp Member (Silverton Formation, Pretoria Group), Transvaal Basin, South Africa — An example of Paleoproterozoic explosive intraplate volcanic activity in an epeiric embayment

The early Paleoproterozoic Machadodorp Member of the Silverton Formation (Pretoria Group, Transvaal Supergroup), South Africa, is an example of ancient submarine explosive volcanism in an intra-cratonic setting under incipient rifting conditions. The Silverton Formation consists of three different members, which include, in ascending order, the Boven Member, the Machadodorp Member and the Lydenburg Member. The formation is primarily characterized by the muddy sediments of the Boven and Lydenburg members, which formed in a relatively shallow submarine environment, predominantly as sub-storm wave-base pelagic deposits, within an epeiric embayment on the Kaapvaal Craton. The subaqueous Machadodorp Member volcanic activity is mostly distributed in the eastern part of the basin. So far, the volcanism of the Machadodorp Member has not been studied in detail. Therefore, a detailed facies analysis of its volcanism was initiated, allowing constraints on its volcanic evolution.The Machadodorp Member consists of a basal unit of basaltic sheet lava, followed by a fining-upward succession of volcaniclastic rocks, ranging from breccias, tuff-breccias and tuffs, rich in fluidal-shaped clasts, to laminated tuffs that are related to subaqueous mass flows, and a final sheet lava that characterized the end of volcanic activity in the section studied.The Machadodorp Member rocks are tholeiitic basalts in composition, with all samples plotting in the low-TiO2 suite. Fractional crystallization of clinopyroxene, olivine and plagioclase was the predominant evolutionary process as illustrated by mineral chemistry and major element geochemical modeling. AFC (Assimilation and Fractional Crystallization) processes could not account for all the trace element variations, nor for the arc-like signatures observed on the primitive mantle normalized plots. Variable ancient mantle enrichment beneath this area is a more plausible cause for the trace elements variations observed in the Machadodorp rocks. Trace element modeling indicates that the parental composition of these rocks was a variably enriched spinel lherzolite source. Such enrichment must have occurred before 3 Ga during episodes of arc collision and subduction.

Geochemistry and Mineralogy of Basalts from the South Mid-Atlantic Ridge (18.0°–20.6°S): Evidence of a Heterogeneous Mantle Source

The South Mid-Atlantic Ridge is a typical slow-spreading ridge that represents a modern example to understand mantle composition and the evolution of mid-ocean ridge magmatism. In this paper, we investigate basalt samples dredged from four locations along the South Mid-Atlantic Ridge ranging from 18.0° to 20.6°S. The basalts belong to the tholeiitic series and exhibit normal mid-ocean ridge basalt (N-MORB) geochemical features with variable enrichments of Rb, Th, U, and Pb and depletions of Ba and Sr relative to primitive mantle. Some samples have negative Nb–Ta anomalies whereas others have positive Na–Ta anomalies to average N-MORBs. Plagioclase phenocrysts, microphenocrysts, and microlites occur in the in the matrix; phenocrysts and microphenocrysts are bytownite and labradorite in composition. Olivine phenocrysts are forsterite (Fo87 to Fo96). Chemical zoning in phenocrysts are interpreted to record crystal fractionation and magma mixing. Cores of plagioclase phenocrysts have higher anorthite values (An72–83) and estimated crystallization temperatures (~1180–1240 °C) that may suggest a xenocrystic origin. The lower anorthite proportions of rims of plagioclase phenocrysts (An65–71) and microphenocrysts (An54–72) yield lower estimated crystallization temperatures of ~1090–1120 °C and ~980–1060 °C, respectively. Rims of plagioclase phenocrysts and microphenocrysts may be generated in different environments such as magma chambers or magma channels, respectively. The basalt samples probably originated from partial melting of a depleted mantle spinel lherzolite source with a minor contribution of enriched materials possibly derived from the Saint Helena plume and subcontinental lithospheric mantle in the asthenosphere. Variable compositions of the basalt samples suggest heterogeneous mantle that includes depleted and enriched components at the South Mid-Atlantic Ridge between 18.0°–20.6°S.

Geochemistry and geochronology of mafic rocks from the Jinghe ophiolitic mélange, northwest China: Implications for plume-related magmatism and accretionary processes within the North Tianshan Ocean

We present geological, whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb geochronological data for mafic rocks found as angular fragments in the Jinghe ophiolitic mélange of the Northern Tianshan orogenic belt, northwest China. The investigated exotic blocks comprise layered and massive gabbros and pillow basalts that are embedded in a serpentinite-rich matrix. The 380.5 ± 4.9 Ma layered gabbros have chondrite-normalized rare earth element (REE) patterns characterized by significant depletions in the light rare earth elements (LREE) and positive Eu anomalies, suggesting that the petrogenesis of these rocks was controlled by cumulus processes. The 381.5 ± 3.7 Ma massive gabbros have near-flat normal-type mid-ocean ridge basalt (MORB)-normalized multi-element patterns with moderate to strong negative Nb–Ti anomalies. These rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703825–0.704758 and high εNd(t) values (+6.7 to +9.0). Two compositional groups of basalts can be recognized: ocean island basalt (OIB)- and enriched MORB-type rocks. Both groups display variable enrichments in large-ion lithophile elements (LILE) in N-MORB-normalized trace element diagrams. Semi-quantitative modelling demonstrates that the parental magmas of the massive gabbros were derived by 5%–10% partial melting of a spinel-bearing lherzolite source influenced by minor inputs of subducted crustal components. The parental magmas of the subalkaline and alkaline basalts were derived by 10%–20% and 1%–5% polybaric melting, respectively, of a lherzolite source containing spinel and garnet in equal amounts. The petrogenesis of the Jinghe basalts can be linked to continental rifting followed by seafloor spreading caused by upwelling of a mantle plume. The petrogenesis of the gabbros is linked to plate re-organization that provided the geodynamic impetus for southwestward subduction in the North Tianshan Ocean, and the onset of incorporation of oceanic materials with different origins into a growing accretionary prism during the Late Devonian. Accretion resulted in chaotic mixing of dismembered pieces of variably deformed oceanic rocks, which are now preserved as an extensive block-in-matrix ophiolitic mélange.

STUDYING THE GENESIS OF IGNEOUS ROCKS IN ZARIN-KAMAR REGION (SHAHROOD, NORTHEASTERN IRAN) BY RARE EARTH ELEMENTS

Zarin-Kamar region is located north east of Shahrood (36o37’-36o42’N, 55007’-55012’E). Plutonic rocks in this area belongs to syenite group and their texture is intergranular hypidiomorphic. Volcanic rocks in the area have porphyritic, amygdale intersertal texture. which quartz is also seen among their cavities and porosities. Total concentration of REEs (ΣREE) in the study igneous rocks varies between 450 and 683 ppm. Diagram of Eu/Eu* versus Sr and Eu/Eu* versus Ba show negative anomalies of Eu. This phenomenon as well as Ba and Sr trends show that plagioclase removal has happened during the magma evolution. These rocks have rock has originated from an enriched mantle source. The rate of Dy/Yb in the igneous rocks of the region varies between 1.32 and 2.62. it shows that it stemmed from a garnet lherzolitic source. The rate of (Tb /Yb )N was between 0.97 to 2.25 showing a garnet source. Also other related figures showed that the samples belonged to OIB (Oceanic Island Basalt). The rate of La/Ta was between 6.6 to 14.01. It also showed that they had a source from asthenosphere. The rate of La/Nb was 0.5 to 0.91. It also shows a less crustal contamination among these samples.

H2O-rich mantle melting near the slab–wedge interface

To investigate the first melts of the mantle wedge in subduction zones and their relationship to primitive magmas erupted at arcs, the compositions of low degree melts of hydrous garnet lherzolite have been experimentally determined at 3.2 GPa over the temperature range of 925–1150 °C. Two starting compositions with variable H2O contents were studied; a subduction-enriched peridotite containing 0.61% Na2O, 0.16 K2O% (wt%) with 4.2 wt% H2O added (Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015) and an undepleted mantle peridotite (Hart and Zindler in Chem Geol 57:247–267, 1986) with 14.5% H2O added (Till et al. in Contrib Mineral Petrol 163:669–688, 2012). Saturating phases include olivine, orthopyroxene, clinopyroxene, garnet and rutile. Melting extent is tracked from near solidus (~ 5 wt%) to 25 wt%, which is close to or beyond the point where clinopyroxene and garnet are exhausted. The beginning of melting is a peritectic reaction where 0.54 orthopyroxene + 0.17 clinopyroxene + 0.13 garnet react to produce 1.0 liquid + 0.88 olivine. The melt production rate near the solidus is 0.1 wt% °C−1 and increases to 0.3 wt% °C−1 over the experimentally studied interval. These values are significantly lower than that observed for anhydrous lherzolite (~ 1 wt% °C−1). When melting through this reaction is calculated for a metasomatized lherzolite source, the rare earth element characteristics of the melt are similar to melts of an eclogite, as well as those observed in many subduction zone magmas. Moreover, since rutile is stable up to ~ 8 wt% melting, the first melts of a hydrous lherzolite source could also show strong high field strength element depletions as is observed in many subduction zone lavas. The silicate melts measured at the lowest temperatures and melting extents (< 10 wt%) are high silica andesites (56–60 wt% SiO2) and contain very low Ca/Al and high alkalis. These deep low degree andesitic melts, if added to experimentally produced hydrous liquids from melting (20–25 wt%) of harzburgite residues at shallow pressures (1.0–1.2 GPa, Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015), can match the compositional characteristics of primitive natural basaltic andesite and magnesian andesite lavas found globally in arcs. In addition to a silicate melt phase, there is a small amount of silicate dissolved in the H2O supercritical fluid that coexists with the silicate liquid and solids in our experiments. The composition of this fluid is in equilibrium with the Mg-rich minerals and it is granitic. The results presented here are used to develop a model for producing hydrous arc magmas. We hypothesize that mantle wedge melting produced by the flux of hydrous fluid from the slab occurs over a range of depths that begins at the base of the mantle wedge and ends at shallow mantle depths. These melts ascend and remain isolated until they mix in the shallow, hottest part of the mantle wedge. In this melting scenario, the metasomatic “slab melt” contributions to arc magmas is small (~ 5 wt%), but its effect on the alkali, REE and incompatible trace element budget of the derivative magmas is large and able to reproduce the trace elemental characteristics of the primitive andesites. Higher proportions of slab or sediment melt do not resemble primitive high magnesian arc andesites and basaltic andesites.

Cambrian shallow-marine to emergent alkaline volcanism near Ouinguigui (Ougnat inlier, eastern Anti-Atlas, Morocco): Volcanic facies, geochemistry and geodynamic setting

Various forms of volcanism occurred during the Middle Cambrian southeast of the Ougnat inlier in the eastern Anti-Atlas of Morocco. This study is the first to examine the formation of these volcanic deposits and interrelationships to the sedimentary host and tectonics on local and regional scales. The emplacement history of the volcanosedimentary succession of Ouinguigui has been characterised by a multi-disciplinary approach of field work, facies analysis of the volcanic rocks and their host and by geochemical analysis of coherent volcanic rocks. Additionally, we introduce a generic facies code system for subaqueous to emergent basaltic successions to make related deposits more comparable. The sedimentary host rock is composed of pre-transgressive conglomerates and sandstones of the Tikirt and Akerouz Member, which are interpreted as deposits of shelf-type fan-delta and shoreface deposits. The transgression is reflected by an abrupt decrease in grain size and the presence of packstones similar to the Micmacca Breccia Member. The post-transgressive clastic deposits of the Feijas Internes Group and Tabanit Group are the main host for the volcanic activity. They are composed of coarsening-upward sequences, which reflect gradual transitions from offshore to shoreface and foreshore facies.The alkaline volcanism comprises four main phases, which are induced by the SW-vergent tilting of a basement block during extensional tectonics. Phase I starts with the emplacement of diatremes within the Jbel Wawrmast Formation and Jbel Afraou Formation reaching an estimated depth of ca. 200 m. They are the result of phreatomagmatic eruptions caused by the rise of basaltic magma into the coastal environment of the host. Phase II is characterised by the emplacement of large volumes of basaltic lava through fissure eruptions and the formation of a lava delta in the eastern part as well as subaqueous to emergent phreatomagmatic volcanism in the middle and western part of the Ouinguigui area. Wave activity led to the formation of volcanogenic conglomerates and sandstones. Phase III and IV are reflected by the emplacement of subvolcanic intrusions consisting of trachy andesites and trachytes.Whole rock geochemical data of 13 samples confirm the alkaline character of the Ouinguigui magmatism, which derived from a garnet lherzolite source of within-plate volcanism.

The evolution of the mantle source beneath Mt. Etna (Sicily, Italy): from the 600 ka tholeiites to the recent trachybasaltic magmas

ABSTRACTThe spatial/temporal proximity of Mt. Etna to the Hyblean Plateau and the Aeolian slab makes the discussion on the nature of its mantle source/s extremely controversial. In this study, a detailed geochemical overview of the entire Mt. Etna evolutionary sequence and a comparison with the magmatism of the Hyblean Plateau was proposed to: (i) simulate the composition of Mt. Etna tholeiitic to alkaline primitive magmas in equilibrium with a fertile mantle source; (ii) model the nature, composition and evolution of the mantle source from the tholeiitic stage (600 ka) to present magmatism. According to our simulations, two amphibole + phlogopite-bearing spinel lherzolite sources are able to explain the wide range of Etnean primary magmas. The enrichment in LILE, 87Sr/86Sr, Rb and H2O of the magmas emitted after 1971 (but also discontinuously generated in both historic and prehistoric times) are caused by different melting proportions of amphibole and phlogopite in a modally and compositionally homogeneous mantle domain, with melting degrees analogous to those required to produce magmas erupted prior to 1971. The behaviour of the hydrous phases during melting could be ascribed to a variable H2O/CO2 activity in the mantle source, in turn related to the heat/fluxes supply from the asthenospheric upwelling beneath Mt. Etna. All these considerations, strengthened by numerical models, are then merged to review the complex Pliocene/Lower Pleistocene to present day’s geodynamic evolution of eastern Sicily.

Geochemical and petrological diversity of mafic magmas from Mount St. Helens

Quaternary eruptive products in the Cascade arc include a variety of different basalt types. At Mount St. Helens (MSH), the most active volcano in the Cascades throughout the last 35 ka, three different mafic endmembers erupted at the end of the Castle Creek period (1900–1700 years B.P.): (1) high-field strength element (HFSE)-rich basalt enriched in K, Ti, P, and incompatible trace elements; (2) low-K olivine tholeiite (LKOT) with lower amounts of incompatible trace elements; and (3) calc-alkaline (arc-type) basaltic andesite with a typical subduction signature, i.e., enrichment in fluid-mobile large ion lithophile elements (LILE) relative to immobile high-field strength elements (HFSE). Each type has compositions projecting backwards to more primitive endmembers in the Cascades. Single units encompassing basaltic-to-basaltic andesitic compositions with intermediate trace-element abundances form two almost continuous trends towards basaltic andesite. These trends are interpreted to result from assimilation of pre-existing, more evolved, calc-alkaline material (and in one case mixing of different mafic magma types) during migration of the magmas through the crust. Most of the erupted basalts are porphyritic (10–30%) with an assemblage dominated by olivine and plagioclase and show disequilibrium textures preventing detailed reconstruction of mantle melting processes. Although typical hydrous arc basalt produced by flux melting in the mantle is absent in the eruptive products of MSH, arc-type basaltic andesite suggests its presence at depth. LKOT magmas are interpreted as decompression melts from the upper mantle, whereas HFSE-rich basalts are likely derived from the water-poor periphery of the main flux melting regime, potentially tapping a trace-element-enriched source. Primitive spinel compositions and whole-rock trace-element variations indicate at least two distinct, relatively fertile lherzolite sources for these two basalt types. Weak crustal zones associated with an old fracture system beneath MSH likely provide conduits for fast and isolated ascent of distinct batches of magma, bypassing the lower crustal mush zone. The eruption of the basalts through the upper crustal magma system and main edifice is consistent with an offset plumbing system suggested by geophysical data.

The oxidation state of iron in Mid-Ocean Ridge Basaltic (MORB) glasses: Implications for their petrogenesis and oxygen fugacities

The oxidation state of a basaltic liquid may be obtained from the average valence state of Fe in its quenched glass. Suitable glasses are widely available from the basalts erupted at Mid-Ocean Ridges (MORB). Measurements of Fe3+/∑Fe, where ∑Fe=Fe2++Fe3+, by XANES spectroscopy of a globally representative sample of these MORB glasses are in good agreement with the most recent literature values determined by wet chemistry, but have improved precision, with an expected standard deviation of ±0.01 in Fe3+/∑Fe for each independent measurement. These precise data allow the geochemical controls on MORB redox systematics at both global and local scales to be evaluated. At the global scale, the relationship between log[Fe2O3] and [MgO] shows that Fe3+ behaves like a lightly incompatible element (LICE) during the crustal evolution of MORB, with its incompatibility between those of the redox-insensitive elements Li and In. The variability of Fe3+ about this global trend is also consistent with that of redox-insensitive elements of similar incompatibility, implying no external buffering of oxygen fugacity (fO2). The variability of Fe3+ is anti-correlated with the variabilities of Sr and Na, which are compatible in plagioclase, but is positively correlated with the variabilities of elements that partition preferentially into clinopyroxene and/or olivine rather than plagioclase, showing that its variability is controlled at least partly by varying ratios of plagioclase to clinopyroxene and olivine during the crustal evolution of MORB. Deviations of Fe3+ concentrations from the global trend are, like those of other LICE, anticorrelated with the deviations from their global trends of the very incompatible elements (VICE). Extrapolation of the global trend to an assumed parental melt at 10.4 wt% [MgO] gives [Fe2O3]o, the concentration of Fe2O3 in the global average MORB parental melt, of 0.6 wt%, which is consistent with 15–20% partial melting of a spinel lherzolite source with 0.2 to 0.3 wt% Fe2O3. This estimate of source Fe2O3 agrees with the estimate for fertile upper mantle lherzolite deduced from measurements of Fe3+/∑Fe and total Fe in the minerals of spinel peridotite xenoliths from the lithosphere.The fO2 of the MORB glasses at 1 bar may be calculated from their measured Fe3+/∑Fe using a new parameterization of 478 experimental data from the literature with <60 wt% SiO2. The need for the new parameterization arises from recent experimental studies, which, among other aspects, are consistent with the ideal stoichiometry governing the thermodynamic relationship between Fe3+/Fe2+ and fO2, namely Fe3+/Fe2+ ∝ (fO2)0.25. The parameterization gives:log10⁡(Fe3+/Fe2+)=0.25ΔQFM−1.35+0.034[Na2O]+0.044[K2O]+0.023[CaO]−0.18[P2O5] where ΔQFM is the difference between the fO2 of the silicate melt and the quartz–fayalite–magnetite buffer in log10 units, logfO2(QFM) =8.58−25050/T, relative to the conventional standard state of pure O2 at 105 Pa, T is in K, and [Na2O] etc., are the concentrations of the oxide components in weight percent. The global average ΔQFM recorded by MORB glasses in the compositional range of 5 to 10 wt% MgO is +0.2 ±0.3, with a small but resolvable increase with decreasing MgO. There is no evidence that Fe3+/∑Fe systematics in MORB are influenced by interactions with other polyvalent elements like S or Cr.

Petrogenesis of the two phases of intrusive rocks at Chodarchay, NW Iran: using trace and rare earth elements

The Chodarchay area is located 50 km east of the city of Zanjan, within the Tarom-Hashtjin subzone, in the Western Alborz structural zone of NW Iran. Igneous rocks in the area include intrusive granitic (phases I and II), and volcanic rocks. Geochemical characteristics of the granitic rocks were determined using their trace and rare earth element (REE) compositions. The phase I intrusive rocks at Chodarchay include quartz monzonite and quartz syenite, while the phase II mainly consists of granite and alkali granite. These intrusive rocks have SiO2 contents ranging between 60.11 and 69.14 wt%, high K2O + Na2O (8.20 to 10.21 wt%) and K2O/Na2O ratios of 1.24 to 2.56. Petrological and geochemical studies indicate that the granitoids have high-K to shoshonitic affinity, they are metaluminous to mildly peraluminous and characterize as I-type granitoids. The investigated granitoids (phases I and II) have fractionated REE patterns characterized by enrichment in light REEs and varying degrees of depletion in heavy REEs. Large-ion lithophile element (LILE) enrichment, high-field strength element (HFSE) depletion, and distinct positive Pb anomaly in the primitive mantle-normalized trace element patterns of the rocks imply subduction-related arc magmatic signatures. It is suggested that the granites formed in a volcanic arc to post-collision setting from a garnet spinel lherzolite source.

Petrology and geochemistry of sheeted dykes and pillow lavas from the Sabzevar Ophiolitic Mélange (Northeast Iran): new Constraints for the Late Cretaceous evolution of the Neo-Tethys Oceanic Basin between the Central Iranian Microcontinent and Eurasia

TheSabzevarophiolite-bearingmelangemainlyincludestectonic slicesofophiolitesregardedas remnants of the Late Cretaceous oceanic basin located between the northern margin of the Central Iran Microcontinent and the southern margin of the Turan (Eurasia) plate, as well as remnants of its northern continental margin. We present here new mineral chemistry and whole rock chemical data on the sheeted dyke complex and pillow lavas series from this ophiolite. The sheeted dykes consist of basalts and basaltic andesites showing island arc tholeiitic (IAT) affinity (Group 1). They display low TiO 2 (0.37-0.81 wt%) contents and N-MORB normalized incompatible element patterns featuring Th enrichment and Ta and Nb depletion. The pillow lavas show different chemical features compared to the sheeted dykes and can be subdivided in two distinct geochemical groups. The first Group of pillow lavas (Group 2) mainly consists of andesites showing a clear calc-alkaline (CA) affinity with low TiO 2 (0.71-0.81 wt%) contents. Their N-MORB normalized incompatible element patterns display marked enrichments in Th, U, La, and Ce, and depletion in Ta, Nb, and Ti. The second Group of pillow lavas (Group 3) is mainly represented by alkaline basalts with high TiO 2 (1.54-3.45 wt%) contents. N-MORB normalized incompatible element patterns show significant Th, Ta, and Nb enrichments. Trace element modeling shows that CA primary melts were generated by 20% partial melting of a depleted lherzolite mantle source that had been enriched by continental crust chemical components. IAT basalts were generated from various degrees of partial melting of either depleted lherzolitic or harzburgitic mantle sources bearing no or negligible influence from continental crust components. Alkaline basalts were generated by polybaric partial melting of an enriched within-plate oceanic mantle source. A new tectono-magmatic model that can explain the different rock association cropping out in the Sabzevar ophiolite is proposed. This model implies the existence, on the southern margin of the Turan plate during the Late Cretaceous, of a continental arc-forearc system where CA and IAT rocks were erupted, respectively. Alkaline basalts were erupted in a subduction-unrelated oceanic basin and then tectonically accreted to the forearc as a consequence of the collision between the seamount and the arc-forearc system.

Geochemistry of Mesoarchean felsic to ultramafic volcanic rocks of the Lac Guyer area, La Grande Subprovince (Canada): Evidence for plume-related magmatism in a rift setting

The Lac Guyer area of the La Grande Subprovince (Superior Province) hosts Mesoarchean east-trending volcano-sedimentary sequences subdivided into 3 stratigraphic units (Mintisch and Rouget Formations and Guyer Group), which were emplaced between 2847 and 2806 Ma on a gneissic-tonalitic basement. These approximately 35 to 175 km-long sequences comprise felsic to ultramafic volcanic rocks that have been metamorphosed to the amphibolite and granulite facies and record mainly two Archean episodes of ductile deformation.The Mesoarchean tholeiitic basalt, komatiitic basalt, and komatiite appear to be genetically related based on petrologic modeling, their close spatial association, and their major and trace element signatures. They are characterized by the following: depletion to enrichment in LREE relative to HREE; Zr/Ypm, Ti/Vpm, and La/Smpm ratios higher than or close to 1; Ti/Zrpm and Nb/Thpm ratios lower than or close to 1; common negative anomalies in Nb-Ta ± Ti; and local positive to negative anomalies in Hf-Zr. Komatiitic basalt and komatiite are mostly Al-undepleted and rarely Al-depleted. According to current komatiite melting models, this suggests that these rocks formed by a high degree of partial melting of a garnet-bearing lherzolite source that consumed any residual garnet and progressively depleted the primitive mantle source in incompatible elements. Basalt and komatiitic basalt compositions were likely produced by assimilation and fractional crystallization (AFC) processes from komatiitic magmas (∼21–23 wt% MgO), as suggested by AFC modeling and their geochemical characteristics, whereas komatiite compositions were controlled by partial melting processes and crustal contamination.The Mesoarchean calc-alkaline rhyolite, dacite, and andesite are enriched in LREE relative to HREE, enriched in Th relative to LREE, and have negative anomalies in Nb-Ta-Ti, negative to positive anomalies in Hf-Zr, as well as high Zr/Y and Ti/V ratios and low Ti/Zr ratios. Such geochemical signatures could be explained either by concurrent assimilation and fractional crystallization of the basaltic magmas in the lower crust or by magma mixing between basalt and lower crustal anatectic magmas.The spatial and temporal distribution of Mesoarchean volcanic rocks within the Lac Guyer area and their major and trace element characteristics suggest that they were emplaced into a continental rift associated with a mantle plume. The geochemical similarities between these rocks and the rift-related volcanic rocks of the younger Yasinski Group (2751–2732 Ma), and the presence of other Mesoarchean volcanic sequences with rift affinity indicate that the La Grande Subprovince was affected by two main plume-related rifting events during the Archean (ca. 2.88–2.81 and 2.75–2.71 Ga), each produced by a single or multiple magmatic plumes.

Devonian volcanic rocks of the southern Chinese Altai, NW China: Petrogenesis and implication for a propagating slab-window magmatism induced by ridge subduction during accretionary orogenesis

Ridge–trench interaction is a common tectonic process of the present-day Pacific Rim accretionary orogenic belts, and this process may facilitate “slab-window” magmatism that can produce significant thermal anomalies and geochemically unusual magmatic events. However, ridge-trench interaction has rarely been well-documented in the ancient geologic record, leading to grossly underestimation of this process in tectonic syntheses of plate margins. The Chinese Altai was inferred to have undergone ridge subduction in the Devonian and a slab-window model is proposed to interpret its high-temperature metamorphism and geochemically unique magmatic rocks, which can serve as an excellent and unique place to refine the tectonic evolution associated with ridge subduction in an ancient accretionary orogeny. For this purpose, we carried out geochemical and geochronological studies on Devonian basaltic rocks in this region. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating results yield an age of 376.2 ± 2.4 Ma, suggesting an eruption at the time of Late Devonian. Geochemically, the samples in this study have variable SiO2 (43.3–58.3 wt%), low K2O (0.02–0.07 wt%) and total alkaline contents (2.16–5.41 wt%), as well as Fe2O3T/MgO ratios, showing typical tholeiitic affinity. On the other hand, the basaltic rocks display MORB-like REE patterns ((La/Yb)N = 0.90–2.57) and (Ga/Yb)N = 0.97–1.28), and have moderate positive εNd(t) values (+4.4 to +5.4), which collectively suggest a derivation from a mixing source comprising MORB-like mantle of a mature back-arc basin and subordinate arc mantle wedge. These basaltic rocks are characterized by Low La/Yb (1.26–3.69), Dy/Yb (1.51–1.77) and Sm/Yb (0.83–1.32) ratios, consistent with magmas derived from low degree (∼10%) partial melting of the spinel lherzolite source at a quite shallow mantle depth. Considering the distinctive petrogenesis of the basaltic rocks in this region, the Late Devonian basalts in the southern Chinese Altai is suggested to have witnessed the propagating process of slab-window magmatism that was induced by ridge subduction in a nascent rifting stage of a back-arc basin.

Review of the Cambrian volcanic activity in Morocco: geochemical fingerprints and geotectonic implications for the rifting of West Gondwana

Volcanic activities related to the opening of a Cambrian rift in Morocco were widespread from the Fortunian to the Cambrian Epoch 3. Numerous data are available from northwestern volcanic sites, particularly in the western High Atlas, but they are scarce from the southeastern sites. New data are documented here from the volcanic formations exposed in the Jbel Tazoult n’Ouzina of the Tafilalt Province, eastern Anti-Atlas and dated to Cambrian Epoch 2–3. The Cambrian volcanic activities recorded in the High Atlas, Anti-Atlas, and Coastal Meseta are synthesized to refine their stratigraphic setting and to characterize their magmatic affinities and fingerprints. Six volcanic pulses are determined as tholeiitic, transitional, and alkaline suites. The tholeiitic and transitional magmas originated from primitive mantle and E-MORB-type sources with a spinel- and garnet-bearing lherzolite composition. Some of them were modified by assimilation–fractional crystallisation processes during crust–mantle interactions. The alkaline magmas fit with an OIB-type and a garnet-bearing lherzolite source. The palaeogeographic distribution of the magmatic suites was controlled by the lithospheric thinning of the Cambrian Atlas Rift and lithospheric constraints of the Pan-African metacraton and West African craton.

Phase equilibria and geochemical constraints on the petrogenesis of high-Ti picrite from the Paleogene East Greenland flood basalt province

Phase equilibrium experiments have been performed on an extremely high-Ti (5.4wt.% TiO2) picrite from the base of the Paleogene (~55Ma) East Greenland Flood Basalt Province. This sample has a high CaO/Al2O3 ratio (1.14), a steep rare-earth elements (REE) profile, is enriched in incompatible trace elements, and is in chemical equilibrium with highly primitive olivine. This all suggests that the picrite is a near-primary melt that did not suffer major chemical evolution during ascent from the mantle source and through the crust. Near-liquidus phase relations were determined over the pressure range of 1atm, 1 to 1.5GPa and at temperatures from 1094 to 1400°C. They provide an important constraint on the petrogenesis of these lavas. The high-Ti picritic melt is multi-saturated with olivine (Ol)+orthopyroxene (Opx) at 1GPa but has only Ol or Opx on the liquidus at lower and higher pressures, respectively. This indicates the primitive melt was last equilibrated with its mantle source at relatively shallow pressure (~1GPa). Melting probably started at 2-3 GPa and the picritic melt was produced by 15-30% melting of the mantle source. Such a degree of partial melting requires a mantle with a high potential temperature (1480-1530˚C). The relatively low CaO content and high FeO/MnO ratios of the most primitive East Greenland picrites, the high Ni content of olivine phenocrysts and the presence of low-Ca pyroxene (i.e., pigeonite) at high pressure in our experiments all suggest that the mantle source contained a major component of garnet pyroxenite. Residual garnet in the source could adequately explain the low Al2O3 content (7.92 wt.%) and steep REE patterns of the picrite sample. However, simple melting of a lherzolitic source, even with a major pyroxenite component, cannot explain the formation of magmas with the very high Ti contents observed in some East Greenland basalts. We therefore propose that magmas highly-enriched in Ti were produced by melting of a metasomatized mantle source containing Ti-enriched amphibole and/or phlogopite.

The Triassic-Liassic volcanic sequence and rift evolution in the Saharan Atlas basins (Algeria). Eastward vanishing of the Central Atlantic magmatic province

We investigate the Triassic-Liassic sequence in ten diapirs from the Saharan Atlas (Algeria). Based on detailed mapping, two episodes are identified. The first one consists of a volcano-sedimentary sequence in which three volcanic units were identified (lower, intermediate and upper units). They are interlayered and sometimes imbricated with siliciclastic to evaporitic levels which record syn-sedimentary tectonics. This sequence was deposited in a lagoonal-continental environment and is assigned to the Triassic magmatic rifting stage. The second episode, lacking lava flows (post magmatic rifting stage), consists of carbonate levels deposited in a lagoonal to marine environment during the Rhaetian-Hettangian. The volcanic units consist of several thin basaltic flows, each 0.5 to 1m thick, with a total thickness of 10–15m. The basalts are low-Ti continental tholeiites, displaying enrichment in large ion lithophile elements and light rare earth elements [(La/Yb)n= 2.5-6] with a negative Nb anomaly. Upwards decrease of light-rare-earth-elements enrichment (e.g. La/Yb) is modelled through increasing melting rate of a spinel-bearing lherzolite source from the lower (6–10wt.%) to the upper (15–20wt.%) unit. The lava flows from the Saharan Atlas share the same geochemical characteristics and evolution as those from the Moroccan Atlas assigned to the Central Atlantic magmatic province. They represent the easternmost witness of this large igneous province so far known.