[Show abstract][Hide abstract] ABSTRACT: LA-ICP-MS U-Pb zircon geochronological and geochemical data of meta-igneous and metasedimentary rock types of the Katuma Block of the Paleoproterozoic Ubendian Belt in Tanzania are used to unravel the crustal evolution of this metalliferous terrain. The protoliths of the metabasites and orthogneisses previously considered to be Paleoproterozoic are in fact mostly Neoarchean in age (2713 ± 11 Ma to 2638 ± 5 Ma), from which the oldest rocks experienced their first metamorphism during the same Neoarchean orogenic cycle at ca. 2650 Ma. A second event of mafic magmatism (2021 ± 11 Ma) was concomitant with the migmatization of the Neoarchean orthogneisses and was succeeded by granitic intrusions at 1990–1940 Ma. All rocks of the Katuma Block experienced their main metamorphic reworking during several Paleoproterozoic orogenic events, which were recognized by dating of various metamorphic zircon growth zones and the age of magmatic events dated at ca. 2050, 1960 and 1880 Ma. The detritus of the high-grade metasedimentary rocks derived from Neoarchean (Katuma Block or Tanzania Craton?) and Paleoproterozoic provenances and the minimum age for the deposition is constrained by its first metamorphism at ca. 1960 Ma. The Neoarchean and Paleoproterozoic metabasites, gabbronorites and orthogneisses are sub-alkaline in composition displaying a REE and trace element geochemistry akin to those of rocks formed in modern-arc settings. On the basis of the geochemical data, the presence of eclogites, deformation and metamorphic ages, we suggest that in Paleoproterozoic time the Katuma Block was again at an active continental margin, below which a Paleoproterozoic oceanic lithosphere was subducting.
Precambrian Research 01/2014; · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This article presents LA-ICPMS U-Pb zircon and U-Th-Pb monazite ages and geochemical data for felsic orthogneisses and granites from the Southern Granulite Terrane of India, a key area for reconstructing the evolution of the late-Neoproterozoic Gondwana supercontinent. The data identify two distinct crustal domains in the Madurai Province, the major part of the Southern Granulite Terrane, and we present a new geotectonic model for their crustal evolution. The Western Madurai Domain is widely composed of late-Neoarchaean (2.53-2.46 Ga) subduction-related, magnesian charno-enderbites, which were reworked in the early-Palaeoproterozoic during granulite facies metamorphism and partial melting (2.47-2.43 Ga). The Eastern Madurai Domain is dominated by a vast supracrustal sequence, which was deposited on a late-Palaeoproterozoic (1.74-1.62 Ga) basement of magnesian charnockites and Hbl-Bt gneisses that formed through reworking of underlying Archaean rocks. Both domains of the Madurai Province were intruded by voluminous mid-Neoproterozoic A-type charnockites and felsic orthogneisses (0.83-0.79 Ga), which formed through reworking of Archaean to Palaeoproterozoic rocks, together with minor leuco-gabbros and meta-rhyolites, during a major phase of extensional rifting. Regional mid-Neoproterozoic (0.82-0.78 Ga) metamorphism is temporally and spatially related to A-type granite magmatism. Convergence of the two domains in the late-Neoproterozoic (ca. 0.55 Ga) culminated in their collision along a SSW-NNE-trending belt of ultrahigh-temperature metamorphic rocks, the Kambam UHT Belt, and was associated by the localized emplacement of leucogranites (0.56 Ga).
Accretion of the Madurai Province at the Archaean Dharwar Craton occurred in the earliest Palaeoproterozoic along the Moyar-Bhavani-Cauvery Suture, a distinct crustal domain characterized by late-Neoarchaean (2.50 Ga) subduction-related, magnesian enderbites. They resemble the age and composition of the Western Madurai Domain charno-enderbites and intrude Mesoarchaean charnockites (2.85 Ga) and metabasites, representing remnants of oceanic crust. The suture zone was, coeval with the Western Madurai Domain, affected by early-Palaeoproterozoic (2.49-2.45 Ga) HP granulite facies metamorphism. Just as the Madurai Province the suture zone was intruded by ferroan mid-Neoproterozoic (0.84 Ga) metagranitoids but was only locally affected by an early-Cambrian overprint (0.54 Ga). The inferred early-Palaeoproterozoic accretion of the Madurai Province at the Dharwar Craton and the extension-related origin of the voluminous mid-Neoproterozoic A-type charnockites are difficult to match with the recently established concept of a Neoproterozoic subduction zone and suture between the craton and the Madurai Province.
In a Palaeogeographic model the widely Neoarchaean Western Madurai Domain is correlated with the Antananarivo Domain of central Madagascar, whereas the Proterozoic Eastern Madurai Domain is linked with the Ongole Domain, the southernmost segment of the Eastern Ghats Belt, and the Wanni Complex of Sri Lanka
Precambrian Research 01/2014; · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To quantify the Mesoproterozoic pressure-temperature-time evolution of the granulite-facies Ongole domain, a deep crustal Proterozoic magmatic arc in the Eastern Ghats Belt, LA-ICP-MS U-Pb zircon dating and in-situ electron microprobe U-Th-total Pb monazite dating are integrated with the petrologic observations. Zircon grains in the metapelites often preserve oscillatory-zoned, sometimes partly resorbed cores yielding a spread of 207Pb/206Pb ages between ca. 2700 and ca.1750 Ma, interpreted to reflect the age of detrital grains from the surrounding crustal blocks. Metamorphic zircon overgrowths and rims yield Palaeoproterozoic concordia ages from about 1625 to 1600 Ma, indicating the timing and duration of an ultrahigh-temperature (UHT) metamorphic event.
Texturally controlled in-situ monazite dating however reveals two metamorphic events separated by 60-80 Ma. In metapelites, the monazite inclusions in garnet and cores of some matrix monazite yield weighted mean ages of ca. 1610 Ma, similar to the concordia ages from zircon and indicate the timing of the UHT metamorphism. The monazite grains in the matrix, growing along with a second generation of garnet and in late-stage symplectites are highly recrystallized and yield weighted mean ages of ca. 1540 Ma, indicating the timing of a second metamorphism at higher pressures but lower temperatures. The weighted mean ages obtained from metapelites in the adjoining Vinjamuru domain are the same as those obtained from the Ongole domain. The chemical differences between the two generations of monazite (Th, Y and HREE contents) support the above interpretation of monazite growth under different conditions and at different times. Monazite in the charnoenderbites records the second metamorphic event but only rarely the earlier UHT event. In addition, they show evidence for later imprints of ductile to brittle deformation between ca. 1450 and 1360 Ma, which may correlate with Mesoproterozoic crustal extension and alkaline plutonism along the western boundary of the Eastern Ghats Belt. Though the Ongole domain was not involved in major crustal reworking during the Neoproterozoic, monazites do record minor disturbances at ca. 730 Ma and ca. 510 Ma. Hence the combination of zircon and monazite dating unveils the sequence of tectonothermal events in the Ongole domain in a hitherto unknown exactness: UHT metamorphism at ca. 1610 Ma due to magmatic heat advection, subsequently, 60 to 80 Ma later, a second metamorphic event at medium-pressure and lower-temperature due to the collision of the arc with the Indian continent, and finally minor disturbances due to rifting between ca. 1450 and 1360 Ma.
Precambrian Research 01/2014; · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Paleoproterozoic basement rocks are thought to form the northwestern end of the Ubendian Belt in Tanzania that disappears towards the north below a Mesoproterozoic sedimentary cover. The northwestern end of the Ubendian Belt is known to constitute three litho-tectonic terranes of Katuma, Wakole and Ubende. Through dating of zircon (SHRIMP U–Pb) and monazite (U–Th–total Pb electron microprobe ages) of high-grade metasedimentary rocks of the Wakole Terrane we have detected solely Mesoproterozoic ages, showing no sign of reworking of older Paleoproterozoic basement. These findings signify that the Wakole Terrane hosts younger sediments of Mesoproterozoic times metamorphosed to high-grade P–T conditions (peaked at 670–680 °C/8.5–8.9 kbar).
Two distinct phases of Mesoproterozoic metamorphic events separated by 160 Ma have been dated at 1166 ± 14 Ma and 1007 ± 6 Ma (SHRIMP U–Pb zircon). Zircon ages are supported by in-situ dating of monazite with ages at 1170 ± 10 Ma, and 1022 ± 5–1016 ± 10 Ma. The first age is closely related to the period of S-type granitoid emplacement at about 1200 Ma in the Karagwe-Ankolean and Kibaran Belts. The second age cluster overlaps with a period of global Mesoproterozoic orogenic cycle, also recorded in the neighboring Irumide and Kibaran Belts. This age group is associated with the assembly of the hypothetical Mesoproterozoic Rodinia Supercontinent.
The recent model for the evolution of the Kibaran Belt suitably explains the spatial and temporal settings of Mesoproterozoic metasedimentary rocks of the Wakole Terrane overlying the Paleoproterozoic Ubendian Belt. Due to its proximity to the Kibaran Belt and by being bound by the Paleoproterozoic Terranes of Ubende and Katuma, it can be interpreted that the Wakole Terrane metasediments initially were deposited in an intra-continental basin that was later squeezed between these old terranes by the regional ca. 1000 Ma compressional event recorded in the Irumide, Kibaran and Karagwe-Ankolean Belts.
Precambrian Research 01/2014; 249:215–228. · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Ongole domain of the Proterozoic Eastern Ghats Belt, India, is dominated by charnockites and enderbites with enclaves of migmatitic metapelites. Reaction textures indicate two generations of mineral growth. Spinel-hercynite solid solution + quartz-bearing Fe-Al granulites provide critical evidence for an ultrahigh-temperature (UHT) metamorphism. The association of spinel and quartz, now separated by garnet and sillimanite coronas, suggests that the rocks underwent T > 950 °C peak metamorphism at P = 6.5-7 kbar, as constrained from P-T pseudosection. Coarse-grained orthopyroxene in garnet + cordierite-bearing metapelites shows the highest Al2O3 content up to 8.2 wt.%, suggesting T = 950-1000 °C. Temperatures estimated from mesoperthite and plagioclase pairs and other geothermometers (T = 900-1000 °C) further support ultrahigh-temperature metamorphism in the Ongole domain. The absence of cordierite in retrograde mineral assemblages points to isobaric cooling, giving rise to a near isobaric heating-cooling trajectory. The second generation of garnet, occurring as thick overgrowths over coarse-grained garnet porphyroblasts, in metapelites and especially in charnoenderbites have higher grossular contents than the porphyroblasts. Thermobarometry and P-T pseudosections indicate that they formed at a higher pressure but lower temperature (ca. 780 °C, 9.5 kbar) during a second metamorphic event. Orthopyroxene + sillimanite ± kyanite ± spinel symplectites replacing cordierite is most likely formed during this second metamorphic event. The presence of sillimanite, kyanite as well as andalusite along with coarse-grained retrograde biotite in the same rock marks the last stage of the second metamorphic event characterized by isobaric cooling just below the aluminosilicate triple point. This further suggests near-isothermal decompression from higher pressures. UHT metamorphism at low pressures during the first metamorphic event is most likely caused by magma emplacement. The higher pressure but lower temperature conditions achieved during the second metamorphic event is due to crustal thickening during collision of continental blocks. Near-isothermal decompression to ca. 4 kbar points to subsequent rapid exhumation of the over thickened crust. The two-stage evolutionary history of the Ongole domain fits well to the expected evolution of a magmatic arc, in which the first can be attributed to magmatic heat advection during arc growth and the second to crustal thickening during collision.
Precambrian Research 01/2014; · 4.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Chah-Bazargan gabbroic intrusions are located in the south of Sanandaj–Sirjan zone. Precise U–Pb zircon SHRIMP ages of the intrusions show magmatic ages of 170.5 ± 1.9 Ma. These intrusions consist primarily of gabbros, interspersed with lenticular bodies of anorthosite, troctolite, clinopyroxenite, and wehrlite. The lenticular bodies show gradational or sharp boundaries with the gabbros. In the gradational boundaries, gabbros are mineralogically transformed into anorthosites, wehrlites, and/or clinopyroxenites. On the other hand, where the boundaries are sharp, the mineral assemblages change abruptly. There is no obvious deformation in the intrusions. Hence, the changes in mineral compositions are interpreted as the result of crystallization processes, such as fractionation in the magma chamber. Rock types with sharp boundaries show abrupt chemical changes, but the changes exhibit the same patterns of increasing and decreasing elements, especially of rare earth elements, as the gradational boundaries. Therefore, it is possible that all parts of the intrusions were formed from the same parental magma. Parts showing signs of nonequilibrium crystallization, such as cumulate features and sub-solidification, underwent fracturing and were interspersed throughout the magma chamber by late injection pulses or mechanical movements under mush conditions. The geological and age data show that the intrusions were formed from an Al-, Sr-, Fe-enriched and K-, Nb-depleted tholeiitic magma. The magma resulted from the partial melting of a metasomatized spinel demonstrated by negative Nb, P, Hf, and Ti, and positive Ba, Sr, and U anomalies typical of subduction-related magmas.
International Journal of Earth Sciences 01/2013; 102(5). · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Eclogites from the Tian Shan high-pressure/low-temperature (HP/LT) metamorphic belt show evidence for successively increasing metasomatic alteration with increasing retrograde, blueschist-facies overprint. To constrain the source(s) of the metasomatizing fluid and to evaluate elemental and isotopic changes during this overprint, two sequences of eclogite–blueschist transitions were investigated: A layered transition from eclogite to blueschist (FTS 9-1 sequence) and blueschist-facies overprinted pillow metabasalts (FTS 4 samples). Geochemical trends based on the relationships of K, Ba, Rb and Th are consistent with HP metasomatism, but distinct from typical seafloor alteration trends. In contrast, oxygen isotope ratios in garnet (δ 18 O V-SMOW = 7.3–8.7‰) and omphacite (δ 18 O V-SMOW = 8.2–9.7‰) are similar to δ 18 O V-SMOW in bulk low-temperature altered oceanic crust (AOC), suggesting O isotopic preservation of a seafloor alteration signature. Carbonate crystallization related to the metasomatic overprint demonstrates CO 2 mobility during subduction and potential C storage in HP metamorphic rocks. Carbon isotope ratios in the two sequences differ markedly: Disseminated calcite in the layered FTS 9-1 sequence has δ 13 C V-PDB = − 9.14 ± 0.19‰, whereas vein-forming ankerite in the pillow metabasalts has δ 13 C V-PDB = − 2.08 ± 0.12‰. The ankerite reflects an inorganic marine/ hydrothermal signature, as observed in ophiolites, whereas the low δ 13 C V-PDB values from the calcite point to a contribution of organic carbon. The time when the metasomatic overprint occurred is estimated to be ~ 320 ± 11 Ma based on a Rb–Sr iso-chron age of six blueschist samples from the pillow metabasalts, which is in agreement with active subduc-tion in this region. Initial (T = 320 Ma) 87 Sr/ 86 Sr ratios for all HP/LT rocks range from 0.7059 to 0.7085, and εNd 320 Ma varies from − 0.4 to + 10.9. Both eclogite–blueschist sequences have initial Sr isotope composi-tions (87 Sr/ 86 Sr ~ 0.707) that are significantly higher than those of typical oceanic mantle-derived basalts. They are thought to derive from a fluid that preserved the Sr isotopic signature of seawater by fluid–rock in-teraction with seawater-altered oceanic lithosphere in a subduction channel. Mixing models between eclo-gite and various fluids suggest that the contribution of a sediment-derived fluid was likely less than 20%. A fluid predominantly derived from seawater-altered oceanic lithosphere is also supported by the calculated O isotope composition of the fluids (10.2–11.2‰). It is thus evident that subduction channel fluids carry com-plex, mixed elemental and isotopic signatures, which reflect the composition of their source rocks modified by interaction with various other lithologies.
[Show abstract][Hide abstract] ABSTRACT: Two distinct types of eclogites from the Raspas Complex (Ecuador), which can be distinguished based on petrography and trace element geochemistry, were analyzed for their stable (Li, O) and radiogenic (Sr, Nd) isotope signatures to constrain metasomatic changes due to uid-overprinting in metabasaltic rocks at high-pressure conditions and to identify uid sources. MORB-type eclogites are characterized by a relative LREE depletion similar to MORB. High-pressure (HP) minerals from this type of eclogite have highly variable oxygen isotope compositions (garnet: + 4.1 to + 9.8‰; omphacite: + 6.1 to + 11.0‰; phengite: 8.7 to 10.4‰; amphibole: 6.2 to 10.1‰) and generally show equilibrium oxygen isotope fractionation. Initial 87 Sr/ 86 Sr isotope ratios are also variable (0.7037–0.7063), whereas εNd 130 Ma values (+ 8.3 to + 11.0) are relatively similar. Sr and O isotopic compositional differences among rocks on outcrop scale, the preservation of O isotopic compositions of low-temperature altered oceanic crust, and Sr–Nd isotopic trends typical for seaoor alteration suggest inheritance from variably altered oceanic crust. However, decreasing δ 7 Li values (− 0.5 to − 12.9‰) with increasing Li concentrations (11–94 ppm) indicate Li isotope fractionation by diffusion related to uid–rock interaction. Li isotopes prove to be a very sensitive tracer of metasomatism, although the small effects on the Sr–Nd–O isotope systems suggest that the uid-induced metasomatic event in the MORB-type eclogites was small-scale at low-water/rock ratios. This metasomatic uid is thought to predominantly derive from in situ dehydration of MORB-type rocks. Zoisite eclogites, the second eclogite type from the Raspas Complex, are characterized by the presence of zoisite and enrichment in many incompatible trace elements compared to the MORB-type eclogites. The zoisite eclogites have a homogenous Sr–Nd isotopic signature (initial 87 Sr/ 86 Sr = 0.7075–0.7081, εNd 130 Ma = −6.7 to −8.7), interpreted to reect a metasomatic overprint. The isotopic signature can be attributed to the metasomatic formation of zoisite because associated zoisite veins are isotopically similar. Relatively homogenous O isotope values for garnet (10.9–12.3‰), omphacite (9.4 to 10.8‰), amphibole (10.0–10.1‰) and zoisite (10.5–11.9‰) and inter-mineral O isotopic disequilibria are consistent with a metasomatic overprint via open-system uid input. Li concentrations (46–76 ppm) and δ 7 Li values of the zoisite eclogites overlap the range of the MORB-type eclogites. The large amount of uid required for isotopic homogenization, combined with the results from uid inclusion studies, suggests that deserpentinization played a major role in generating the metasomatic uid that altered the zoisite eclogites. However, inuence of a (meta)sedimentary source is required based on Sr–Nd isotope data and trace element enrichments. The signicant geochemical variation in the various eclogites generated by interaction with metasomatic uids has to be considered in attempts to constrain recycling at convergent margins.
Chemical Geology 01/2011; 281:151-166. · 3.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The metamorphic Raspas Complex of southwest Ecuador comprises blueschists, eclogites, and high-pressure ultramafic and sedimentary rocks. The Lu-Hf garnet-whole rock ages of a blueschist, a metapelite, and an eclogite cluster at ca. 130 Ma. Peak metamorphic conditions of the high-pressure rocks occurred at 1.8 GPa and 600 °C, which corresponds to a maximum burial depth of about 60 km. According to their geochemical signatures, the eclogites formed from typical mid-ocean ridge basalts (MORB), whereas the blueschists had seamount-like protoliths and the eclogite-facies peridotites apparently stem from depleted, MORB-source mantle. These rocks were contemporaneously subjected to similar peak PT conditions, suggesting that they were subducted together as a coherent section within the downgoing slab. We hypothesize that this section became dismembered from the slab during burial at great depth as the seamounts were being scraped off. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments is suggestive of an exhumed high-pressure ophiolite sequence.
[Show abstract][Hide abstract] ABSTRACT: This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50–90km depths. The eclogites contain 2–20ppm N with δ15Nair values ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1ppm N and δ15Nair∼−4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3–15ppm N with δ15Nair values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ15Nair∼−5‰). Based on their relatively high N contents and their homogeneous and positive δ15N values, admixing of sedimentary N is also indicated for the serpentinized peridotites.One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab–mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc–subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.
Geochimica et Cosmochimica Acta 01/2010; 74(5):1636-1652. · 3.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Geodynamic processes, like amalgamation and continental dispersal, control the thermobarometric history of the Earth's crust. Due to its central position, Madagascar is the ideal location for studying important aspects of the amalgamation and breakup of the Gondwana supercontinent. Using geochronological methods (e. g. U–Pb zircon and monazite as well as titanite and apatite fission-track dating), a detailed cooling path for the Bemarivo Belt of northern Madagascar has been deciphered. New SHRIMP zircon ages of ca. 540 Ma and published monazite ages of ca. 515 Ma allow to estimate the duration of the high/ultrahigh-temperature metamorphism related to the early Palaeozoic continental collision during Gondwana formation. The breakup of this supercontinent was associated with crustal thinning, brittle deformation and denudation along the evolving sedimentary basins recorded by the titanite and apatite fission-track ages ranging between ca. 320 Ma and 160 Ma.
Gondwana Research - GONDWANA RES. 01/2009; 16(1):72-81.
[Show abstract][Hide abstract] ABSTRACT: 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 Na2O and SiO2 and depleted in FeO, MgO, and CaO. Similarities with some trondhjemitic liquids produced through partial melting of amphibolites or hydrous basalts (i.e., low-Al2O3 content, less than 15 wt.%; low Ba, Sr, TiO2, 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.
[Show abstract][Hide abstract] ABSTRACT: During subduction, a significant amount of nitrogen (N) is fixed in the subducting altered oceanic crust (AOC) and, for some margins, the subduction input flux of N in AOC is thought to rival that in sediment. However, the ultimate fate of N during subduction-zone metamorphism remains unclear. N may be released from the AOC and added to arcs, it may be retained in the AOC and incorporated into the mantle, or it may enter fluids along the slab-mantle interface. Moreover, it is not known whether the increase in delta15N accompanying prograde metamorphism of sedimentary rocks also occurs during subduction of mafic igneous rocks and thus what the isotopic contribution of the metabasaltic rocks is when they enter the mantle. In this study, we have analyzed HP/UHP metabasaltic rocks from world-wide localities (Zambia, Italy, Ecuador, China and Spain; range of peak-metamorphic P-T conditions of 14-30 kbar and 500-800°C) for N concentrations and delta15N in an attempt to characterize subduction input flux of N in AOC. Eclogites have variable N concentrations (2 to 20 ppm) and delta15N ranging from -1 to +8. Blueschists contain up to 50 ppm N and overlap in delta15N with the eclogites. In both concentration and delta15N, the HP/UHP metamorphosed mafic rocks are distinct from fresh MORB (N = 1.1 ppm and delta15N = -4 ± 2), but overlap with AOC, consistent with retention of a significant proportion of N during prograde metamorphism. However, trends on diagrams that discriminate between seafloor alteration and metamorphic additions and the concurrent enrichment of N with Ba, Pb, Rb and Cs, together with delta15N values, suggest that some sample sequences (China, Ecuador) were enriched in metasedimentary N by HP/UHP fluid-rock interactions. Others (Italy, Zambia) lack those correlations and appear to more closely reflect the characteristics of the precursor AOC. A sample profile through a prograde blueschist-to-eclogite transformation from the Tianshan (China) reveals that N as well as Rb, Ba and Cs concentrations gradually decrease and N isotopic compositions gradually increase from the blueschist towards the eclogite, suggesting that these elements were leached from the rock and N isotopic fractionation occurs during fluid-induced eclogitization. Our results imply that, if the eclogites represent material that is eventually subducted into the deep mantle, an isotopically heavy N component may be added, which could be of significance for elevated delta15N values in plume-derived magmas (e.g., Society Islands, Hawaii).
[Show abstract][Hide abstract] ABSTRACT: Chemical changes associated with the rehydration of dry eclogites to form blueschists were studied to obtain information about the chemistry of the fluids infiltrating during this retrograde metamorphic overprint. The studied eclogites of the Tian Shan were formed during Carboniferous subduction of pillow basalts that show typical ocean island basalt geochemical signatures. The retrograde P–T path is characterised by decompression associated with cooling, typical for fragments of a subducting slab that are ascending in the cool and hydrated subduction channel. The fluids infiltrating the eclogites are interpreted to represent dehydration fluids of the down going slab that ascended to the overlying subduction channel. The rehydration of the eclogites proceeded from the pillow margins producing two concentric shells consisting of glaucophane-dominated blueschists (inner shell) and phengite–ankerite blueschists (outer shell) replacing omphacite and garnet of the eclogite. Mass-balance calculations based on major and trace element compositions of eclogites and blueschists point to high element mobility during fluid infiltration and associated metamorphic reactions. Blueschists show lower contents of REE, Y, Sr, Pb, U, P, Ca, Na, Al and Si compared to eclogites, indicating a loss of these elements, but they show higher contents of volatiles, Mg, transition metals and LILE, indicating a gain of these elements. The chemical changes point to a composition of the infiltrating fluid that has an affinity to fluids derived from serpentinites, i.e. with low concentrations in Si, Al, Ca and Na and high concentrations in Mg, Co and Ni. The gain of Cu, Mn, Zn and LILE in the blueschists points to an addition of a sediment-derived component in the serpentinite-derived fluid. The pronounced enrichment of K2O and of Ba at constant Ba/Th ratios in the blueschists resembles those of island arc basalts derived from a fluid-enriched source. The low element load of the fluid equilibrated with Mg-rich mélange zones induces the mobility of most of the elements during the eclogite–blueschist transformation to compensate for disequilibrium between the mafic rock and the infiltrating fluid. The associated volume loss enhances the fluid infiltration into the eclogite interior. Increasing precipitation rates and/or diminishing diffusion rates for element removal brought the eclogite–blueschist transformation to an end, which resulted in the preservation of eclogite in the pillow cores.
[Show abstract][Hide abstract] ABSTRACT: A multi-thermochronometer study of basement rocks, using combined Pb-Pb zircon, U-Th-total Pb monazite, U-Pb SHRIMP titanite, 40Ar/39 Ar biotite, and apatite fission track data, was performed to derive a detailed cooling history for the Vohibory terrane, southwest Madagascar. The main metamorphism at ca. 640–610 Ma is interpreted to represent the closure of the Mozambique Ocean and the subsequent accretion of the Vohibory arc terrane to the active con-tinental margin of Azania (proto-Madagascar). Contemporaneous with the assumed main Gondwana-forming collision between Azania/India and the Congo craton at ca. 535 Ma, slow cooling (8.1–3.6C/m.yr.) indicates extrusion of the Vohibory block. This central part of the East African orogen reached thermal equilibrium between 500 and 350 Ma. During late Carboniferous/Triassic rifting between East Africa and Madagascar, the Vohibory terrane was the favored starting point for crustal extension, causing basement rock cooling (up to 5.3C/m.yr.) and heating (up to 1.6C/m.yr.). The Jurassic passive margin evolution was accompanied by rift locus migration to the west of the Vohibory block. The resulting rift geometry and associated sedimentation caused flexural isostatic response and inversion of the Vohibory part of the late Carboniferous/Triassic rift.
The Journal of Geology 01/2008; 116:21-38. · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Vohibory Block of south-western Madagascar is part of the East African Orogen, the formation of which is related to the assembly of the Gondwana supercontinent. It is dominated by metabasic rocks, which have chemical compositions similar to those of recent basalts from a mid-ocean ridge, back-arc setting and island-arc setting. The age of formation of protolith basalts has been dated at 850–700 Ma by U–Pb SHRIMP analysis of magmatic cores in zircon, pointing to an origin related to the Neoproterozoic Mozambique Ocean. The metabasic rocks are interpreted as representing components of an island arc with an associated back-arc basin. In the early stage of the Pan-African orogeny, these rocks experienced high-pressure amphibolite to granulite facies metamorphism (9–12 kbar, 750–880 °C), dated at 612 ± 5 Ma from metamorphic rims in zircon. The metamorphism was most likely related to accretion of the arc terrane to the margin of the Azania microcontinent (Proto-Madagascar) and closure of the back-arc basin. The main metamorphism is significantly older than high-temperature metamorphism in other tectonic units of southern Madagascar, indicating a distinct tectono-metamorphic history.