Melting Of Amphibolite Research Articles

Petrology and Geochemistry of Adakitic Dacites and High-MgO Andesites, and Related Calc-alkaline Dacites from the Miocene Okoppe Volcanic Field, N Hokkaido, Japan

In the Okoppe area of North Hokkaido, Japan, the eruption of adakitic and calc-alkaline dacites was followed by high-Mg andesite (HMA) and calc-alkaline dacite during the Middle Miocene (12–10 Ma). Adakitic dacite is characterized by high Sr/Y and low Y, with Sr and Nd isotopic compositions identical to those of mid-ocean ridge basalt. It has higher MgO contents than adakites generated by experimental melting of metabasalt and amphibolite, and higher Ni and Cr contents than either Archean trondhjemite–tonalite–granodiorite or Early Cretaceous adakitic granites, which are considered to represent partial melts of subducted oceanic crust. This provides compelling evidence that adakitic dacite magma from Okoppe resulted from interaction of a melt derived from subducted oceanic basaltic crust and the overlying mantle wedge peridotite, with little modification to the adakitic melt signature and Sr and Nd isotopic values. The compositional variations in the Toyono adakitic dacite and associated calc-alkaline dacite probably resulted from mixing of the reacted magma and an evolved silicic dacite magma formed by fractional crystallization of the reacted magma. A disequilibrium phenocryst assemblage in the HMA may result from mixing of boninite and silicic andesite that resulted from crustal melting. Calc-alkaline dacites associated with the HMA were derived by fractional crystallization of silicic andesite and assimilation of crust with an enriched Sr isotopic signature. The most likely tectono-magmatic model for the production of adakitic dacite and HMA involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath North Hokkaido and the back-arc side of the NE Japan arc, coincident with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This resulted in a high geothermal gradient in the mantle wedge beneath North Hokkaido. The subsequent melting of a limited part of the cool oceanic crust subducting beneath Hokkaido produced adakitic magmas, which interacted with the overlying mantle wedge peridotite. These magmas subsequently reacted with an evolved calc-alkaline melt en route to the surface. Boninitic magma derived from the ascending hot asthenosphere in part reacted with crust-derived silicic andesitic magma, undergoing simultaneous fractional crystallization.

Timing of subduction and exhumation in a subduction channel: Evidence from slab melts from La Corea Mélange (eastern Cuba)

High pressure igneous rocks (tonalites), generated by partial melting of subducted basaltic rocks accreted to the mantle wedge, are present in the La Corea serpentinite-matrix mélange (eastern Cuba) as centimeter- to meter-sized blocks and as concordant to crosscutting veins within high-pressure parent amphibolite blocks. The slab melts have adakitic signatures, in agreement with formation after partial melting of metabasite. Thermobarometric calculations indicate 620–680°C and 13–15kbar during crystallization of tonalites and down to 250–300°C, 6kbar during retrogression, indicating counter-clockwise P–T paths (hot subduction-cool exhumation). Free water required for melting of amphibolite at moderate temperature (700–750°C) and moderate pressure (13–16kbar) close to the wet basaltic solidus is inferred to have been provided after dehydration of sediments, altered basaltic crust and serpentinite of the subducting Proto-Caribbean lithosphere. Single zircon (SHRIMP) and phengite 40Ar/39Ar age data constrain the P–T–t evolution of the mélange from the timing of crystallization of melts at ~110–105Ma to cooling at ~87–84Ma, ca. 350°C, ca. 9kbar. These figures are consistent with subduction of an oblique ridge, shortly before 115Ma. Furthermore, our data indicate very slow exhumation (ca. 1mm/yr) in the subduction channel during the oceanic convergence stage (120–70Ma) until final fast exhumation to the surface occurred at 70–65Ma during a regional arc-platform collision event.

Post-collisional ore-bearing adakitic porphyries from Gangdese porphyry copper belt, southern Tibet: Melting of thickened juvenile arc lower crust

Porphyry Cu–Mo deposits, related to the Miocene adakitic porphyries from the Gangdese porphyry copper belt in the southern Tibet, formed in a post-collisional setting. Here, we present the new zircon U–Pb ages, whole-rock chemical, and Sr–Nd and zircon Hf isotopic data for the ore-bearing adakitic porphyries from Gangdese porphyry copper belt. LA-ICP-MS zircon dating for six samples yielded ages ranging from 19 Ma to 14 Ma, indicating they formed in the Miocene. The ore-bearing adakitic porphyries show SiO 2 of 61.47–71.67%, K 2O of 3.29–4.74%, and high Sr content (394–1106 ppm), high Sr/Y ratios (63–158), and low Y (6.12–10.3 ppm) and heavy rare earth element contents (e.g. Yb = 0.52–0.91 ppm). They show steep fractionated REE and flat HREE patterns, and strong enrichment in large ion lithophile elements (Cs, Rb, Ba, Th, and U) and depletion of high field strength elements (Nb), with positive Sr and negative Ti anomalies. There are no linear variations of Ba, La, Sr/Y, Dy/Yb, ( 87Sr/ 86Sr) i, and εNd(t) with increasing SiO 2 content. Combined with the zircon positive εHf(t) values, and wide range of ( 87Sr/ 86Sr) i (0.70559 to 0.70908) and of εNd(t) (− 6.8 to 0) values for all the adakitic samples, they were likely derived from the melting of garnet-bearing amphibolite in the juvenile arc mafic lower crust. Additionally, the adakitic porphyries with low Th/Nb ratios have lower initial 87Sr/ 86Sr, and higher εNd(t) and εHf(t) values, and those with high Th/Nb show higher initial 87Sr/ 86Sr, and lower εNd(t) and εHf(t) values. This indicates that the juvenile arc lower crust is derived from the melting of the mantle wedge that was metasomatized by slab fluid or sediment melt during the Neotethyan ocean subduction. Significantly, the juvenile arc lower crust possibly inherited the arc magma characteristics of abundant in F, Cl, and Cu and high oxidation state, which contributed to the porphyry Cu–Mo deposits in the Gangdese porphyry copper belt.

Petrogenesis and tectonic evolution of metaluminous sub-alkaline granitoids from the Takab Complex, NW Iran

AbstractThe Takab complex is composed of a variety of metamorphic rocks including amphibolites, metapelites, mafic granulites, migmatites and meta-ultramafics, which are intruded by the granitoid. The granitoid magmatic activity occurred in relation to the subduction of the Neo-Tethys oceanic crust beneath the Iranian crust during Tertiary times. The granitoids are mainly granodiorite, quartz monzodiorite, monzonite and quartz diorite. Chemically, the magmatic rocks are characterized by ASI<1.04, AI<0.87 and high contents of CaO (up to ~14.5 wt%), which are consistent with the I-type magmatic series. Low FeOt/(FeOt+MgO) values (<0.75) as well as low Nb, Y and K2O contents of the investigated rocks resemble the calc-alkaline series. Low SiO2, K2O/Na2O and Al2O3accompanied by high CaO and FeO contents indicate melting of metabasites as an appropriate source for the intrusions. Negative Ti and Nb anomalies verify a metaluminous crustal origin for the protoliths of the investigated igneous rocks. These are comparable with compositions of the associated mafic migmatites, in the Takab metamorphic complex, which originated from the partial melting of amphibolites. Therefore, crustal melting and a collision-related origin for the Takab calc-alkaline intrusions are proposed here on the basis of mineralogy and geochemical characteristics. TheP–Tevolution during magmatic crystallization and subsolidus cooling stages is determined by the study of mineral chemistry of the granodiorite and the quartz diorite. Magmatic crystallization pressure and temperature for the quartz-diorite and the granodiorite are estimated to beP~7.8±2.5 kbar,T~760±75°C andP~5±1 kbar,T~700°C, respectively. Subsolidus conditions are consistent with temperatures of ~620°C and ~600°C, and pressures of ~5 kbar and ~3.5 kbar for the quartz-diorite and the granodiorite, respectively.

Crustal evolution in a cratonic nucleus: Granitoids and felsic volcanic rocks of the North Caribou Terrane, Superior Province Canada

Attempts to decipher the petrogenesis of Archean Trondhjemite–Tonalite–Granodiorite (TTG) and establish trends in their compositions over time typically do not assess the possible role of assimilation–crystal fractionation processes. Studies of TTG suites worldwide, however, demonstrate that reprocessing of older crust is common. Felsic volcanic and intrusive rocks from the North Caribou Lake area, located in the nucleus of the Superior Province, exhibit a range of compositions that are not accounted for by variable degrees of amphibolite melting alone. Characteristic of Archean TTG, the rocks mainly fall in the lower right hand quadrant of a Nb/Ta vs Zr/Sm plot but they also show clear trends between εNd T and ratios such as Al 2O 3/TiO 2, Th/Nb, [Tb/Yb] N and Eu/Eu*. The lowest Nb/Ta values in the TTG likely result from crystal fractionation including titanite, which strongly partitions and fractionates these elements.

Geological, petrologic, isotopic, and geochemical constraints of geodynamic models simulating formation of the archean tonalite-trondhjemite-granodiorite associations in ancient cratons

The geological setting, geochemistry, and Nd isotopic systematics of tonalite-trondhjemite-granodiortite (TTG) series in ancient cratons are considered. It is shown that the TTG series were formed from ∼4.2 to 2.6 Ga ago in the oldest continental cores; many TTG series do not reveal chronological links to greenstone belts. This follows from the evolution of the Slave Craton in the Canadian Shield, the Vodlozero Craton in the Baltic Shield, and the Pilbara and Yilgarn cratons in the Australian Shield, where greenstone associations postdated TTG series. As has been established at the Baltic Shield, the primary melts of the Mesoarchean TTG associations were formed at a shallower depth (P 150 Ma) that separates the formation of the TTG melts from the age of the source involved in melting. Taking into account the calculated rate of cooling of the lithospheric plates, these data indicate that most Archean TTG series likely were not formed in the convergent subduction-related and accretionary geodynamic settings. The isotopic and geochemical data constrain compositions of the sources that produced Archean TTG series. Petrologic modeling of the formation conditions and Nd isotopic composition of the metabasalts in greenstone belts show that these rocks could not have been the source of TTG series. The most plausible isotopic and geochemical analogue of this source are the Archean amphibolites (ENd mafic rocks), which differ from the metabasalts of greenstone belts by their lower Sm/Nd ratio and enrichment in some lithophile elements. The available data suggest that the primary TTG melts were generated as products of melting of amphibolites and granulites forming the lower crust.

Petrology, geochemistry, and geochronology of trondhjemites from the Qori Complex, Neyriz, Iran

Metamorphism, magmatism, and thrusting were the result of subduction of Neotethys beneath the continental-margin arc of the Sanandaj–Sirjan shear zone (SSSZ) during the Mesozoic. The Qori metamorphic complex is a part of the southern SSSZ. Leuco-granitic (trondhjemitic) rocks crop out in the Qori metamorphic complex and are rare rock types in the SSSZ. These rocks have intruded into the marbles and garnet amphibolites, the highest grade metamorphic rocks of the Qori metamorphic complex, and in some outcrops, a transitional boundary between the amphibolites and the granitoids can be distinguished. The granitoids are granular in texture and consist of plagioclase (albite-oligoclase), quartz ± K-feldspar ±muscovite and subordinate garnet, spinel, rutile, and apatite which primarily occur as inclusions in the main phases. The peraluminous trondhjemitic rocks are enriched in Na 2O and SiO 2 and depleted in FeO, MgO, and CaO. Similarities with some trondhjemitic liquids produced through partial melting of amphibolites or hydrous basalts (i.e., low-Al 2O 3 content, less than 15 wt.%; low Ba, Sr, TiO 2, and Eu content, all with negative anomalies; moderately enriched LREEs and Y, and flat HREE patterns) suggest that the evolution of the parental magma was controlled by residual plagioclases during partial melting of a garnet amphibolite source. Concentrations of ferromagnesian elements, Mg, Fe, and Mn, are low, suggesting that the granitic rocks were not produced by high degrees of partial melting. Furthermore, they display low amounts of ferromagnesian components from the protolith (garnet amphibolite). This is supported by consideration of compatible elements, especially Cr, Ni and Ti (and the less robust HREE), which respectively show very high and high bulk partition coefficients for relatively small degrees (< 20%) of partial melting of the source. The partial melting of the garnet amphibolites occurred at pressures and temperatures between 7.5 and 9.5 kbar (at a depth of 25 to 32 km) and 680 and 720 °C, respectively, based on the Grt–Hbl and Hbl–Pl thermometers and a Grt–Hbl–Pl–Qtz barometer. Precise U–Th zircon SHRIMP ages of the trondhjemite show magmatic ages of 147.4 ± 0.76 Ma, Volgian, Late Jurassic, and suggest that Neotethys began to subduct beneath the continental-margin arc of the SSSZ. As a result of this process, an arc-related metamorphism occurred, leading to the development of the garnet amphibolites in the Qori metamorphic complex.

MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust: New perspectives on crustal growth, crust–mantle differentiation and chemical structure of oceanic upper mantle

We have examined the high quality data of 306 mid-ocean ridge basalt (MORB) glass samples from the East Pacific Rise (EPR), near-EPR seamounts, Pacific Antarctic Ridge (PAR), near-PAR seamounts, Mid-Atlantic Ridge (MAR), and near-MAR seamounts. The data show a correlated variation between Eu/Eu⁎ and Sr/Sr⁎, and both decrease with decreasing MgO, pointing to the effect of plagioclase crystallization. The observation that samples with MgO>9.5 wt.% (before plagioclase on the liquidus) show Eu/Eu⁎>1 and Sr/Sr⁎>1 and that none of the major phases (i.e., olivine, orthopyroxene, clinopyroxene, spinel and garnet) in the sub-ridge mantle melting region can effectively fractionate Eu and Sr from otherwise similarly incompatible elements indicates that the depleted MORB mantle (DMM) possesses excess Sr and Eu, i.e., [Sr/Sr⁎]DMM>1 and [Eu/Eu⁎]DMM>1. Furthermore, the well-established observation that DNb≈DTh, DTa≈DU and DTi≈DSm during MORB mantle melting, yet primitive MORB melts all have [Nb/Th]PMMORB>1, [Ta/U]PMMORB>1 and [Ti/Sm]PMMORB>1 (where PM indicates primitive mantle normalized), also points to the presence of excess Nb, Ta and Ti in the DMM, i.e., [Nb/Th]PMDMM>1, [Ta/U]PMDMM>1 and [Ti/Sm]PMDMM>1. The excesses of Eu, Sr, Nb, Ta and Ti in the DMM complement the well-known deficiencies of these elements in the bulk continental crust (BCC). These new observations, which support the notion that the DMM and BCC are complementary in terms of the overall abundances of incompatible elements, offer new insights into the crust–mantle differentiation. These observations are best explained by partial melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional (major and trace element abundances as well as key elemental ratios) similarity to the BCC, as revealed by andesites in southern Tibet produced during the India–Asia continental collision. An average amphibolite of MORB protolith consists of ~66.4% amphibole, ~29.2% plagioclase and 4.4% ilmenite. In terms of simple modal melting models, the bulk distribution coefficient ratios D2Eu/(Sm+Gd)=1.21, D2Sr/(Pr+Nd)=1.04, DNb/Th=44, DTa/U=57, DTi/Sm=3.39 and DNb/Ta=1.30 readily explains the small but significant negative Eu and Sr anomalies, moderate negative Ti anomaly and huge negative Nb and Ta anomalies as well as the more sub-chondritic Nb/Ta ratio in the syncollisional andesitic melt that is characteristic of and contributes to the continental crust mass. These results support the hypothesis that continental collision zones are primary sites of net continental crust growth, whereas the standard “island arc” model has many more difficulties than certainties. That is, it is the continental collision (vs. "island arc magmatism" or "episodic super mantle avalanche events") that produces and preserves the juvenile crust, and hence maintains net continental growth. The data also allow us to establish the robust composition of depleted and most primitive (or “primary”) MORB melt with 13% MgO. This, together with the estimated positive Eu and Sr anomalies in the DMM, further permits estimation that the DMM may occupy the uppermost ~680 km of the convective mantle following the tradition that the DMM lies in the shallowest mantle. However, the tradition may be in error. The seismic low velocity zone (LVZ) may be compositionally stratified with small melt fractions concentrated towards the interface with the growing lithosphere because of buoyancy. Such small melt fractions, enriched in volatiles and incompatible elements, continue to metasomatize the growing lithosphere before it reaches the full thickness after ~70 Myrs. Hence, the oceanic mantle lithosphere is a huge enriched geochemical reservoir. On the other hand, deep portions of the LVZ, which are thus relatively depleted, become the primary source feeding the ridge because of ridge-suction-driven lateral material supply to form the crust and much of the lithosphere at and in the vicinity of the ridge.

A study of rare earth element (REE)–SiO2 variations in felsic liquids generated by basalt fractionation and amphibolite melting: a potential test for discriminating between the two different processes

The origin of felsic magmas (>63% SiO2) in intra-oceanic arc settings is still a matter of debate. Two very different processes are currently invoked to explain their origin. These include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite. Because both fractionation and melting can lead to similar major element, trace element and isotopic characteristics in felsic magmas, such lines of evidence have been generally unsuccessful in discriminating between the two processes. A commonly under-appreciated aspect of rare earth element (REE) solid–liquid partitioning behavior is that D REE for most common igneous minerals (especially hornblende) increase significantly with increasing liquid SiO2 contents. For some minerals (e.g., hornblende and augite), REE partitioning can change from incomptatible (D 1) at high liquid SiO2. When this behavior is incorporated into carefully constrained mass-balance models for mafic (basaltic) amphibolite melting, intermediate (andesitic) amphibolite melting, lower or mid to upper crustal hornblende-present basalt fractionation, and mid to upper crustal hornblende-absent basalt fractionation the following general predictions emerge for felsic magmas (e.g., ∼63 to 76% SiO2). Partial melting of either mafic or intermediate amphibolite should, regardless of the type of melting (equilibrium, fractional, accumulated fractional) yield REE abundances that remain essentially constant and then decrease, or steadily decrease with increasing liquid SiO2 content. At high liquid SiO2 contents LREE abundances should be slightly enriched to slightly depleted (i.e., C l/C o ∼ 2 to 0.2) while HREE abundances should be slightly depleted (C l/C o ∼ 1 to 0.2). Lower crustal hornblende-bearing basalt fractionation should yield roughly constant REE abundances with increasing liquid SiO2 and exhibit only slight enrichment (C l/C o ∼ 1.2). Mid to upper crustal hornblende-bearing basalt fractionation should yield steadily increasing LREE abundances but constant and then decreasing HREE abundances. At high liquid SiO2 contents LREE abundances may range from non-enriched to highly enriched (C l/C o ∼ 1 to 5) while HREE abundances are generally non-enriched to only slightly enriched (C l/C o ∼ 1 to 2). Hornblende-absent basalt fractionation should yield steadily increasing REE abundances with increasing liquid SiO2 contents. At high SiO2 contents both LREE and HREE are highly enriched (C l/C o ∼ 3 to 4). It is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra-oceanic arc environments where continental crust is absent.

Contribution of syncollisional felsic magmatism to continental crust growth: A case study of the Paleogene Linzizong volcanic Succession in southern Tibet

The Linzizong volcanic succession (~ 65–45 Ma) and the coeval batholiths (~ 60−40 Ma) of andesitic to rhyolitic composition represent a magmatic response to the India–Asia continental collision that began at ~ 70–65 Ma and ended at ~ 45–40 Ma with convergence continuing to present. These syncollisional felsic magmatic rocks are widely distributed along much of the > 1500 km long Gangdese Belt immediately north of the India–Asia suture (Yarlung–Zangbo) in southern Tibet. Our study of the Linzizong volcanic rocks from the Linzhou Basin (near Lhasa) suggests that syncollisional felsic magmatism may in fact account for much of the net contribution to continental crust growth. These volcanic rocks show a first-order temporal change from the andesitic lower Dianzhong Formation (64.4–60.6 Ma), to the dacitic middle Nianbo Formation (~ 54 Ma), and to the rhyolitic upper Pana Formation (48.7–43.9 Ma). The three formations show no systematic but overlapping Nd–Sr isotope variations. The isotopically depleted samples with ε Nd( t) > 0 indicate that their primary sources are of mantle origin. The best source candidate in the broad context of Tethyan ocean closing and India–Asia collision is the remaining part of the Tethyan ocean crust. This ocean crust melts when reaching its hydrous solidus during and soon after the collision in the amphibolite facies, producing andesitic melts parental to the Linzizong volcanic succession (and the coeval batholiths) with inherited mantle isotopic signatures. Ilmenite as a residual phase (plus the effect of residual amphibole) of amphibolite melting accounts for the depletion of Nb, Ta and Ti in the melt. The effect of ocean crust alteration plus involvement of mature crustal materials (e.g., recycled terrigeneous sediments) enhances the abundances of Ba, Rb, Th, U, K and Pb in the melt, thus giving the rocks an “arc-like” geochemical signature. Residual amphibole that possesses super-chondritic Nb/Ta ratio explains the sub-chondritic Nb/Ta ratio in the melt; residual plagioclase explains the slightly depleted, not enriched, Sr (and Eu) in the melt, typical of continental crust. These observations and reasoning plus the remarkable compositional similarity between the andesitic lower Dianzhong Formation and the model bulk continental crust corroborates our proposal that continental collision zones may be sites of net crustal growth (juvenile crust) through process of syncollisional felsic magmatism. While these interpretations are reasonable in terms of straightforward petrology, geochemistry and tectonics, they require further testing.

Source composition and melting temperatures of orogenic granitoids: constraints from CaO/Na2O, Al2O3/TiO2 and accessory mineral saturation thermometry

Granitoid melts, generated experimentally from various materials by fluid-absent melting, show characteristic major-element compositions that can be used to infer source characteristics and melting temperatures. CaO/Na2O ratios distinguish between pelite-derived melts (CaO/Na2O < 0.5) and melts derived from greywackes or igneous sources (CaO/Na2O: 0.3–1.5). Distinctly more mafic melts (granodiorites and quartz diorites) generated by fluid-absent melting of amphibolite can show even higher CaO/Na2O ratios, up to 10, although the majority of the melts have CaO/Na2O ratios between 0.1 and 3. Al2O3/TiO2 ratios reflect the melting temperature, and mathematical formulations are presented that allow using this ratio as a geothermometer for given source compositions. A comparison of temperatures from melting experiments with corresponding Al2O3/TiO2 values indicate a reasonably good correlation ( r 2: 0.70–0.91), demonstrating the usefulness of temperature estimates in granitoid rocks based on Al2O3/TiO2 systematics. Application to well investigated S-type and A-type granites and quartz diorites from the Damara Belt (Namibia) shows different CaO/Na2O and Al2O3/TiO2 ratios for all rock types, supporting their origin from different sources at different temperatures. For the quartz diorites, temperature estimates derived from Al2O3/TiO2 ratios, and those derived from apatite solubility in mafic rocks, agree within ± 20 °C. On the other hand, temperature estimates for A-type and S-type granites derived from Al2O3/TiO2 ratios are systematically higher by 50–150 °C compared with those from accessory mineral saturation, suggesting disequilibrium during partial melting of the lower crust.

Early Paleozoic tectonic evolution of the Chinese South Tianshan Orogen: constraints from SHRIMP zircon U–Pb geochronology and geochemistry of basaltic and dioritic rocks from Xiate, NW China

Traditionally the Chinese South Tianshan has been regarded as a late Paleozoic orogenic belt. However, little is known about the early Paleozoic tectonic architecture of the region. This paper presents the first evidence of Cambrian–Ordovician MORB-type basalts and adakitic diorites on the southern margin of the Yili plate in China. Basalts from Xiate in southwestern Tianshan show a typical transitional (T-) MORB and ferrobasalt composition, which indicate a formation at a propagating spreading ridge. The basalts give a weighted mean 206Pb/238U crystallization age of 516.3 ± 7.4 Ma by SHRIMP U–Pb zircon dating and have experienced contact metamorphism due to the intrusion of a dioritic pluton. The dioritic pluton has a weighted mean 206Pb/238U crystallization age of 470 ± 12 Ma and geochemical characteristics resembling that of adakitic rocks. The pluton is considered to have been formed by partial melting of garnet amphibolites from thickened lower crust in arc or continental collision settings. The basalts and diorites are considered to outline the eastern extension of the early Paleozoic suture zone, the Nikolaev Line, which stretches east–west for hundreds of kilometers between the Northern Tianshan and Central Tianshan terranes of Kyrgyzstan. Our findings substantiate that the Yili and Central Tianshan plates were separated by the early Paleozoic Terskey ocean. The Terskey ocean probably closed during the early stage of the late Ordovician (Lomize et al. in Geotectonics 31(6):463–482, 1997), resulting in the final amalgamation of the Yili and Central Tianshan plates. Consequently, an early Paleozoic suture zone is documented in the Chinese Tianshan region, which is most likely represented by the North Nalati fault.

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex, Central Eastern Desert, Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.

Geodynamic setting of the Birimian volcanism in central Ivory Coast (western Africa) and its place in the Palaeoproterozoic evolution of the Man Shield

AbstractPalaeoproterozoic volcanic formations having different geochemical features are described in the Katiola-Marabadiassa area (Central Ivory Coast). They consist of (i) metamorphic mafic rocks belonging to the greenstone belts, (ii) rhyodacites intruded into the greenstone belts and interpreted as sub-volcanic apophyses of TTG plutons, and (iii) calc-alkaline andesitic lava interbedded in the lower sedimentary pile of the Bandama Basin. The greenstone belt rocks have a magmatic signature of ocean floor tholeiites. They were generated from moderate partial melting of a fairly depleted spinel lherzolite source. The rhyodacites show the same compositions as the TTG granitoids of the belt. These TTGs generated from melting of garnet-bearing amphibolite, derived from subducted oceanic crust, but also from peridotite melting of the fertilized mantle wedge. The andesites show a common composition of active continental margin potassic calc-alkaline lavas. However, they do not constitute a volcanic belt, but belong to an intracontinental basin settled in a wrench tectonic system. They probably generated from remelting of sub-crustal metasomatised mantle.The tectono-magmatism story of the Katiola-Marabadiassa area comprises four stages. A similar evolution is found in other belt-and-basin areas in Ivory Coast. A review of lithostratigraphic and chronological data of the Birimian terrains leads us to compare the four-stage model of the Central Ivory Coast to the West-African Palaeoproterozoic evolution. The first stage corresponds to ocean crust formation, by mid-ocean ridge accretion and volcanic arc building. The second stage, partly overlapping the previous one, is convergent and lasted from oceanic crust stacking to TTG and associated greenstone belt formation. It led to a protocontinental crust accretion by docking of granitized blocks, from ca 2200 Ma to 2110 Ma. The third stage corresponds to the formation of intracontinental basins by reorganization of the granitoid-greenstone shields and of the remnant oceanic basins, between 2118 Ma and 2093 Ma. It was associated with calc-alkaline magma production. The fourth stage is characterized by transtensional stress, with the closure of the basins by left-lateral wrenching along N-S faults, and by emplacement of new potassic granitoids until 2072 Ma.

Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece

Research Article| January 01, 2009 Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece Georgia Pe-Piper; Georgia Pe-Piper * 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada †E-mail: gpiper@smu.ca. Search for other works by this author on: GSW Google Scholar David J.W. Piper; David J.W. Piper 2Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Search for other works by this author on: GSW Google Scholar Ioannis Koukouvelas; Ioannis Koukouvelas 3Department of Geology, University of Patras, Patras 26110, Greece Search for other works by this author on: GSW Google Scholar Lila M. Dolansky; Lila M. Dolansky 4Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Search for other works by this author on: GSW Google Scholar Sotiris Kokkalas Sotiris Kokkalas 5Department of Geology, University of Patras, Patras 26110, Greece Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (1-2): 39–54. https://doi.org/10.1130/B26317.1 Article history received: 23 Aug 2007 rev-recd: 06 Apr 2008 accepted: 22 Apr 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Georgia Pe-Piper, David J.W. Piper, Ioannis Koukouvelas, Lila M. Dolansky, Sotiris Kokkalas; Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece. GSA Bulletin 2009;; 121 (1-2): 39–54. doi: https://doi.org/10.1130/B26317.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Potassium-rich volcanic rocks of the shoshonite suite are common features of postorogenic extensional settings inboard from subduction zones. Various petrogenetic processes and tectonic settings have been proposed for their origin. Early Miocene volcanic rocks of Limnos, part of the northeast Aegean shoshonite belt, show distinctive geochemical features that allow their petrogenesis to be well constrained. The rocks are principally trachyandesites and dacites. Very strong fractionation of light and middle rare earth elements (REEs), similar to that found in adakites, is inconsistent with a mantle source, but it can be modeled by melting of meta-basalt enriched in incompatible elements. A comparison with experimental melting of metabasaltic amphibolite requires small degrees of dehydration melting of amphibole, plagioclase, clinopyroxene, and minor garnet at a temperature >950 °C. Melting was triggered by mantle-derived magma, evidenced by repetitive zoning in clinopyroxene with Cr-rich cores. Nd and Sm isotopes suggest that some of this magma was similar to lamproite found elsewhere in this shoshonite belt and some was of asthenospheric origin. The amphibolite source is inferred to be subduction-enriched metabasalt that underplated the crust during pre-Mesozoic subduction. The regional trigger for dehydration melting was upwelling of asthenosphere as a result of slab detachment. The geochemistry and radiogenic isotopes of other shoshonitic rocks in the northeastern Aegean suggest a similar origin, but with higher degrees of partial melting of base-of-crust metabasaltic amphibolite. Similar processes appear likely for shoshonitic magmatism in some postcollisional settings elsewhere. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dehydration melting of solid amphibolite at 2.0 GPa: Effects of time and temperature

Two sets of dehydration-melting with a natural solid amphibolite, collected from North Himalayan structure zone, Tibet, have been carried out in multi-anvil apparatus at 2.0 GPa and 800―1000℃, for 12―200 h. One is keeping the pressure at 2.0 GPa and the annealing time of 12 h, changing the temperature (800―1000℃). The other is keeping the pressure at 2.0 GPa and temperature at 850℃, varying the annealing time (12―200 h). The products are inspected with microscope and electron probe. The results indicate that at 2.0 GPa, annealing time of 12 h, garnets, melts and clinopyroxenes occur in amphibolite gradually with increasing temperature and the chemical compositions of melt vary from tonalite to granodiorite, and then to tonalite. However, at 2.0 GPa and 850℃, with the annealing time increasing, the garnets, melts and cli-nopyroxenes also occur in amphibolite gradually and the chemical compositions of melt vary from tonalite to granodiorite. In both cases, melts interconnect with each other when the contents of melt are over the 5 vol.%. the viscosities of the melt produced in amphibolite at temperature higher than 850℃ are on a level with 104 Pa·s. The interconnected melt with such a viscosity may segregate from the source rock and form the magma over reasonable geological time. Therefore, it is believed that at the lower part of the overthickened crust, the tonlitic and grano-dioritic magma may be generated through the dehydration melting of amphibolite.

Using In Situ Trace-Element Determinations to Monitor Partial-Melting Processes in Metabasites

Peak metamorphism (∼800–850°C, 8–10 kbar) in the Harts Range Meta-Igneous Complex (Harts Range, central Australia) was associated with localized partial melting by the reaction hornblende + plagioclase + quartz + H2O = garnet + clinopyroxene + titanite + melt. In situ trace-element determinations of prograde, peak and retrograde minerals in migmatitic metabasites and associated tonalitic melts using laser-ablation ICP–MS has allowed monitoring of a range of partial-melting processes (melting, melt segregation and back-reaction between crystallizing melt and restitic minerals). Mass balance calculations indicate that titanite is a major carrier of trace elements such as Ti, Nb, Ta, Sm, U and Th, and therefore may be an important accessory phase to control the redistribution of these elements during the partial melting of amphibolites. Titanite preferentially incorporates Ta over Nb and, hence, residual titanite might assist in the formation of melts with high Nb/Ta. The fact that single minerals record different rare earth element (REE) patterns, from prograde to peak to retrograde conditions, demonstrates that REE diffusion is not significant up to ∼800°C. Therefore, trace-element analysis in minerals can be a powerful tool to investigate high-grade metamorphic processes beyond the limits given by major elements.

226Ra/230Th excess generated in the lower crust: Implications for magma transport and storage time scales

Excesses of 226 Ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs, followed by fast ascent rates of magma from slab to surface (up to 1000 m/yr). However, we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle-derived 226 Ra excesses, and so reduce inferred vertical transport rates. We developed an incongruent, continuous melting model, and both the incongruent melting reaction and ingrowth effects contribute to the 226 Ra excess. In particular, we found that dehydration melting of amphibolite can produce modeled 226 Ra excesses greater than 300%. Mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 U excess (subducted slab) signature. This amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements, similar to those observed in several arc settings, which may distinguish these magmas from those with 226 Ra excesses produced by slab dewatering alone.

U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations

As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean 206Pb/ 238U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO 2 gap between 55% and 65%. The mafic rocks display negative ɛ Nd (755 Ma) values (−9.9 to −10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ɛ Nd (755 Ma) values (−7.9 to −9.2), negative Nb, Ta, P and Ti anomalies, very high La N/Yb N (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks.

The dependence of Nb and Ta rutile–melt partitioning on melt composition and Nb/Ta fractionation during subduction processes

Partition coefficients between rutile and silicate melts were determined experimentally for Nb and Ta with melt compositions varying from rhyolite to basalt. Experimental conditions were 1.7–2.5 GPa, 950–1300 °C, at oxygen fugacities between QFM−2 and QFM+3.5. Both rt/melt D Nb and rt/melt D Ta increase by almost one order of magnitude with SiO 2 content and polymerization, but decrease with TiO 2 content in the melt. The ratio rt/melt D Nb/ D Ta is 0.45–0.55 for basaltic melt compositions, around 0.6 for andesitic melts and 0.8–1.0 for more silicic melts, remaining ≤1 for all examined silicate melts. The fact that rt/melt D Nb/ D Ta is smaller than unity can be explained by a slightly smaller ionic radius of Ta 5+ than Nb 5+ and thus a preferred incorporation of Ta into rutile. The variation of rt/melt D Nb, rt/melt D Ta, and rt/melt D Nb/ D Ta strongly depends on melt composition without any significant correlation with rutile composition. The strong positive correlation of rt/melt D Nb and rt/melt D Ta with rt/melt D Ti and SiO 2 contents is explained with the decreasing solubility of high charge cations in an increasingly polymerized melt where the concentration of non-bridging oxygens decreases. The positive correlation of rt/melt D Nb/ D Ta with rt/melt D Ti and SiO 2 contents is more difficult to understand and might be related to the higher polarizibility of Nb 5+ compared to Ta 5+. Magmas resulting from slab melting with residual rutile are slightly Nb-enriched relative to Ta and do not explain the subchondritic Nb/Ta ratio of continental crust. Rutile in the residue during partial melting or dehydration of subducting crust is not capable of significantly enriching Nb over Ta in the residue. Excluding residual rutile as a reason for high Nb/Ta reservoirs outsources this problem to either partial melting of low-Mg amphibolite or metasomatic Nb-enrichment of rutile-bearing eclogite-lenses in the source region of kimberlites. However, both of these processes cannot produce Nb-enriched reservoirs sufficiently large to balance the silicate Earth's Nb/Ta ratio to chondritic, thus, this study supports previous suggestions that the “missing” Nb is stored in the core.