Post-collisional Period Research Articles

Spatial–temporal framework for the closure of the Junggar Ocean in central Asia: New SIMS zircon U–Pb ages of the ophiolitic mélange and collisional igneous rocks in the Zhifang area, East Junggar

The closure time of the Junggar Ocean is one of the hottest topics surrounding the tectonic evolution of the Central Asian Orogenic Belt (CAOB). This paper reports SIMS zircon U–Pb ages of the ophiolitic mélange and collisional igneous rocks in the Zhifang area, East Junggar. Our new results reveal the following evidence: (1) the West Hill ophiolitic mélange in the Zhifang area contains segments of the 371Ma MORB-type layered rocks and 363Ma oceanic islands, which were intruded by the 348Ma syn-collisional quartz diorites; (2) the ophiolitic mélange and 348Ma syn-collisional quartz diorites are overlain by the 342Ma andesitic tuffs; and (3) the 342Ma andesitic tuffs and successive 332Ma granodiorites-gabbro were formed at late-collisional setting, whereas the 314Ma granitic porphyry at post-collisional setting. We suggest that the Junggar Ocean in the Zhifang area was opened before 371Ma and 363Ma, and possibly closed before 348Ma.By combining the published U–Pb ages of the Mayile–Tangbale–Darbute–Kalamaili–Zhifang–Daheishan (MTDKZD) ophiolite belt, Early-Carboniferous volcanic rocks unconformably overlying the MTDKZD ophiolite belt, bimodal volcanic rocks and granitoids within and adjacent to the MTDKZD ophiolite belt, it is suggested that the Junggar Ocean was possibly opened scissors-like from the Neoproterozoic–Ordovician Mayile–Tangbale ocean eastwards through the Silurian–Devonian Darbute–Karamay ocean finally to the Devonian–Early-Carboniferous Kalamaili–Zhifang ocean, and closed scissors-like from the Zhifang–Kalamaili area in the eastern segment westwards to the Darbute–Karamay area in the western segment. Tectonic collage between the Yemaquan–Xiemisitai arc and Junggar block occurred at approximately 343–348Ma. The Junggar orogenic belt went through late-collisional stage at 343–330Ma, post-collisional period after 330Ma, and post-collisional extensional regime after 320Ma with an intensive stage at around 303Ma.

Alojamento do granito Lavras e a mineralização aurífera durante evolução de centro vulcano-plutônico pós-colisional, oeste do Escudo Sul-riograndense: dados geofísicos e estruturais

<p>A construção do granito Lavras é analisada mediante integração de dados geológicos, geofísicos e estruturais originais e compilados, em consonância com a evolução do centro vulcano-plutônico. Esse centro engloba o complexo intrusivo Lavras do Sul e a sequência traquiandesítica da Formação Hilário (604 - 590 Ma), ambos formados em posição de antepaís durante o período pós-colisional da Orogênese Dom Feliciano, no oeste do Escudo Sul-riograndense. A análise de estruturas rúpteis e lineamentos magnéticos indica que o vulcanismo teve início próximo ao colapso dessa orogenia, condicionado por sistemas transtensivos dextrais NW-SE a WNW-ESE que invertem para sinistrais com o relaxamento tectônico. A formação do complexo intrusivo, principiando com a intrusão subvulcânica do monzonito Tapera no norte, acompanhou a inversão no regime de <italic>stress</italic> regional ao longo de zona de falha N70-75°W que o seciona. Ao final, ocorreu o posicionamento do granito Lavras no sul, o qual possui dimensões modestas (325 km³) e forma tabular (comprimento - L: espessura - E ≈ 3:1) afinando para sul, como deduzido dos dados gravimétricos. Dois domínios composicional-estruturais, equivalentes aos granitos magnesianos centrais (granodiorito e monzogranito) e os ferrosos da borda (sienogranito e feldspato alcalino granito), são definidos pela trama ASM (anisotropia de suscetibilidade magnética). Tais dados, aliados aos de estruturas rúpteis, apontam um <italic>pluton</italic>multicíclico construído em dois eventos de ressurgência, envolvendo: (1) o lacólito central decorrente do alojamento do granodiorito sob a soleira de monzogranito; e (2) as intrusões anulares de granitos ferrosos, induzidas pela expansão do reservatório epizonal em razão da recarga com magmas máfico-ultramáficos lamprofíricos. O controle estrutural, a distribuição espacial e a associação com diques lamprofíricos corroboram o vínculo da mineralização aurífera com o último episódio de ressurgência em um centro vulcano-plutônico maduro.</p>

Geochemical and Isotope Sr-Nd Character of Dschang Biotite Granite: Implications for the Pan-African Continental Crust Evolution in West-Cameroon (Central Africa)

The biotite granite of Dschang composing of quartz, feldspar and biotite, is a haplogranite I-type calc-alkalinegranitoid. It is weakly peraluminous with a ferroan affinity and exhibits fractionated feature. This granite wasgenerated at the minimum temperature and pressure of crystallization of 650°C/4 Kb. Its emplacement issyn-central Cameroon shear zone like most of post-collisional granitoids of the Pan-African fold belt in centralAfrica, and it displays N-S to NNE-SSW magmatic foliation dipping 30° to 60° towards W or WNW. Theevolution of the major elements shows a generalized decrease correlation trends look firstly like fractionalcrystallization. The depleted Sr content related to decreasing Al2O3 and CaO concentrations, indicate the roleplayed by plagioclase during differentiation. The trace elements normalized to the primitive mantle, shownegative anomalies out of Nb, Ta, Sr and Ti, and positive anomalies out of Ba, Rb, Th, comparable with those ofthe continental crust. The isotopic data Sr-Nd are rather homogeneous suggesting a single source for the biotitegranite of Dschang. ?Nd600Ma (- 16 to -18) and TDM (1.9 to 2.1 Ga) confirm the formation of the magma of thebiotite granite of Dschang starting from an old Paleoproterozoic crust by partial melting around 600 Ma. Thismelting which seems to have spread in the area and at several levels of the continental lithosphere, hencemagmas were mixing throughout their evolution, considering the diversity of the granites and their sources, iscertainly caused by great movements along CCSZ, at the post-collisional period of Pan-African orogenesis incentral Africa.

Late Paleozoic–Mesozoic tectonic evolution of the Trans-Altai and South Gobi Zones in southern Mongolia based on structural and geochronological data

The Trans-Altai Zone in southern Mongolia is characterized by thrusting of greenschist-facies Silurian oceanic rocks over Devonian and Lower Carboniferous volcano-sedimentary sequences, by E–W directed folding affecting the early Carboniferous volcanic rocks, and by the development of N–S trending magmatic fabrics in the Devonian–Carboniferous arc plutons. This structural pattern is interpreted as the result of early Carboniferous thick-skinned E–W directed nappe stacking of oceanic crust associated with syn-compressional emplacement of a magmatic arc. The southernmost South Gobi Zone represents a Proterozoic continental domain affected by shallow crustal greenschist-facies detachments of Ordovician and Devonian cover sequences from the Proterozoic substratum, whereas supracrustal Carboniferous volcanic rocks and Permian sediments were folded into N–S upright folds. This structural pattern implies E–W directed thin-skinned tectonics operating from the late Carboniferous to the Permian, as demonstrated by K–Ar ages ranging from ~320Ma to 257Ma for clay fractions separated from a variety of rock types. Moreover, the geographical distribution of granitoids combined with their geochemistry and SHRIMP U–Pb zircon ages form distinct groups of Carboniferous and Permian age that record typical processes of magma generation and increase in crustal thickness. The field observations combined with clay ages, the geochemical characteristics of the granitoids and their ages imply that the E–W trending zone affected by tectonism migrated southwards, leaving the Trans Altai Zone inactive during the late Carboniferous and Permian, suggesting that the two units were tectonically amalgamated along a major E–W trending strike slip fault zone. This event was related to late Carboniferous subduction that was responsible for the vast volume of granitoid magma emplaced at 300–305Ma in the South Gobi and at 307–308Ma in the Trans-Altai Zones. The formation and growth of the crust was initially due only to subduction and accretion processes. During the post-collisional period from 305 to 290Ma the addition of heat to the crust led to the generation of (per-) alkaline melts. Once amalgamated, these two different crustal domains were affected by N–S compression during the Triassic and early Jurassic (185–173Ma), resulting in E–W refolding of early thrusts and folds and major shortening of both tectonic zones.

Time constraints for the closing of the Paleo-Asian Ocean in the Northern Alxa Region: Evidence from Wuliji granites

The Wuliji pluton in the Northern Alxa Region, Inner Mongolia, is the principal part of Shalazhashan Mountain. It belongs to the Zongnaishan-Shalazhashan Arc Zone, northwestern North China Plate, whose north is Engger Us Ophiolite Belt and south is Qagan Qulu Ophiolite Belt. The pluton was emplaced into Upper Carboniferous-Lower Permian Amushan Formation. According to the research about the original Carboniferous Amushan Formation, the lower and middle sections of the Carboniferous Amushan Formation consist of volcanic, clastic, and carbonate rocks, interpreted to represent the sedimentary association of a volcanic arc and back-arc basin; the upper section of the Amushan Formation is a molasse composed of silty shale, sandstone, gravel-bearing sandstone, and conglomerate. The Wuliji pluton consists mainly of biotite monzonitic granite, amphibole-bearing biotite monzonitic granite, and monzonitic granite. Geochemical analyses show that the pluton has both metaluminous and peraluminous characteristics, and on average has SiO2 > 70 wt%, Al2O3 > 14 wt%, and high contents of Na2O+K2O (8.5 wt%), which define a calc-alkaline series. In addition, REE patterns show enrichment of LREE and weak negative Eu anomalies (δEu = 0.3–1). Altogether, the samples are depleted in Nb, Ta, Ti, P, Sr, and Ba, and enriched in Rb, Th, and K. These geochemical traits are interpreted to reflect an arc component. A secondary ion mass spectrometry (SIMS) U-Pb zircon age of the biotite monzonitic Wuliji pluton in the Northern Alxa Region, Inner Mongolia, is 250.8 ± 2.0 Ma (1σ). Samples have ɛNd(t) values between −0.1 and 1.3, which suggests that the granites were derived from mixing between the crust and mantle. Based on the SIMS age and geochemical characteristics, Wuliji granite is interpreted to be a post-collisional granite, the result of mantle-derived melt and assimilated juvenile arc crust. However, according to the newest international stratigraphic classification standard, the upper section of the Amushan Formation is Lower Permian in age, indicating that the back-arc basin had already closed in Early Permian. We conclude that the Paleo-Asian Ocean represented by the Engger Us Ophiolite Belt subducted southward in Late Carboniferous, at the same time that the trench-arc-basin system formed in the Northern Alxa Region. The Paleo-Asian Ocean was closed in Early Permian and the Northern Alxa Region entered a post-collisional period in the Late Permian, as indicated by the Wuliji granites. This suggests that the genesis of the Wuliji granites is consistent with the pluton emplacement at the upper crust, which occurred widely in the northern margin of the North China Plate in Late Carboniferous to Triassic.

Petrogenesis of syntectonic granites emplaced at the transition from thrusting to transcurrent tectonics in post-collisional setting: Whole-rock and Sr–Nd–Pb isotope geochemistry in the Neoproterozoic Quatro Ilhas and Mariscal Granites, Southern Brazil

The Neoproterozoic post-collisional period in southern Brazil (650–580Ma) is characterized by substantial volumes of magma emplaced along the active shear zones that compose the Southern Brazilian Shear Belt. The early-phase syntectonic magmatism (630–610Ma) is represented by the porphyritic, high-K, metaluminous to peraluminous Quatro Ilhas Granitoids and the younger heterogranular, slightly peraluminous Mariscal Granite. Quatro Ilhas Granitoids include three main petrographic varieties (muscovite–biotite granodiorite — mbg; biotite monzogranite — bmz; and leucogranite — lcg) that, although sharing some significant geochemical characteristics, are not strictly comagmatic, as shown by chemical and Sr–Nd–Pb isotope data. The most primitive muscovite–biotite granodiorite was produced by contamination of more mafic melts (possibly with some mantle component) with peraluminous crustal melts; the biotite monzogranite, although more felsic, has higher Ca, MgO, TiO2 and Ba, and lower K2O, FeOt, Sr and Rb contents, possibly reflecting some mixing with coeval mafic magmas of tholeiitic affinity; the leucogranite may be derived from pure crustal melts. The Mariscal Granite is formed by two main granite types which occur intimately associated in the same pluton, one with higher K (5–6.5wt.% K2O) high Rb and lower CaO, Na2O, Ba and Zr as compared to the other (3–5wt.% of K2O). The two Mariscal Granite varieties have compositional correspondence with fine-grained granites (fgg) that occur as tabular bodies which intruded the Quatro Ilhas Granoitoids before they were fully crystallized, and are inferred to correspond to the Mariscal Granite feeders, an interpretation that is reinforced by similar U–Pb zircon crystallization ages. The initial evolution of the post-collisional magmatism, marked by the emplacement of the Quatro Ilhas Granitoids varieties, activated sources that produced mantle and crustal magmas whose emplacement was controlled both by flat-lying and transcurrent structures. The transition from thrust to transcurrent-related tectonics coincides with the increase in the proportion of crustal-derived melts. The transcurrent tectonics seems to have played an essential role in the generation of mantle-derived magmas and may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of orthogneiss protoliths.

Early post-collisional Brasiliano magmatism in Botuverá region, Santa Catarina, southern Brazil: Evidence from petrology, geochemistry, isotope geology and geochronology of the diabase and lamprophyre dikes

Abstract The post-collisional magmatism related to Brasiliano orogeny represented the final stage of the Dom Feliciano Belt in Rio Grande do Sul and Santa Catarina states, southern Brazil, presenting high-K calc-alkaline to shoshonitic and alkaline chemical signatures. Magmatic episodes related to this early period were found in Botuvera region, Santa Catarina state, represented by diabase and lamprophyre (spessartite-type) dikes intrusive in metavolcano–sedimentary rocks of the Brusque Metamorphic Complex (CMB). These dikes have massive structure and igneous textures ranging from very fine equigranular to porphyritic, and the latter is characterized by the presence of phenocrysts of plagioclase and hornblende. The dikes have northeast direction and sharp contacts with the metamorphic rocks, indicating that its position was after the main orogenic regional metamorphism that affected the CMB, interpreted as of collisional nature. The diabase has a basic composition, whereas spessartite lamprophyres are intermediate, with geochemical affinities to the tholeiitic series, with a significant enrichment in light rare-earth elements (LREE) and large ion lithophile elements (LILE) such as Cs, Rb, Ba, K and Sr, and negative anomalies for high-field-strength elements (HFS) such as Nb, Ta, U and T. The concentration of standard trace elements and the Th/Yb and Ta/Yb ratios indicate that these magmas were derived from an enriched mantle source and were strongly contaminated by crust. Except for higher values of K, these features are similar to those found in basaltic volcanic rocks associated with the post-collisional period in south Brazil. The widely dispersed values of ɛND (618), ranging between −13.74 and +5.52, highlights the heterogeneity of the source and reinforces the importance of a crustal component in the generation of these rocks. The extremely low value of eNd (618), of −21.67 obtained for spessartite lamprophyres supports the importance of the involvement of crust in the genesis of this rock. Using the U–Pb and LA–ICP–MS method, a concordant age of 618 ± 8.7 Ma was obtained in zircon crystals of a diabase dike of the region of Barra do Areia, in Botuvera, SC. The existence of inherited zircon grains older than 1800 Ma in this sample supports the involvement of Paleoproterozoic continental crust. The data analysis characterizes the first magmatic age related to the post-collisional period of the Brasiliano orogeny in this region of the Santa Catarina Shield.

Geochemical and geochronological studies of granitoid rocks from the Western Tianshan Orogen: Implications for continental growth in the southwestern Central Asian Orogenic Belt

The Western Tianshan Orogen in northwestern China is part of the southwestern margin of the Central Asian Orogenic Belt (CAOB), which represents the largest tectonic assembly of accretionary complexes and the most significant area of Phanerozoic continental growth in the world. Granitoid rocks have a widespread occurrence and occupy about 30% of the whole exposure of the orogen. Ages obtained for the granitoids vary from 896 Ma to 247 Ma, while initial 87Sr/ 86Sr ratios vary from 0.70329 to 0.72070 and ε Nd values from − 14.1 to + 7.3. The occurrence of Neoproterozoic granitic gneisses implies the presence of Precambrian basement in this area. During oceanic subduction, continental growth occurred as a result of several combined processes, i.e. by addition of oceanic crustal melts (adakites), the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement. However, during the post-collisional period, vertical accretion of underplated juvenile mantle material may have been accomplished during ‘slab breakoff’ delamination. This study suggests a two stage model of continental growth of ‘syn-subduction lateral accretion of arc complexes’ and ‘post-collisional vertical accretion of underplated mantle material’, which may also be relevant for the western segment of the CAOB in the Phanerozoic. The reworking and involvement of old crust are more obvious in the Western Tianshan Orogen compared to other parts of the CAOB comprising the Western Junggar, Eastern Junggar, Alataw and Altay terranes.

Petrologia e geocronologia (U/Pb-Sm/Nd) do Granito Passagem, Complexo Granitóide Pensamiento, SW do Cráton Amazônico (MT)

The Passagem granite includes stocks, plugs and dikes located in the Ricardo Franco hill - Vila Bela da Santissima Trindade region -state of Mato Grosso, central Brazil. The Passagem Granite is included in the Paraguá terrane - SW Amazonian Craton. It consists of isotropic monzogranite, sienogranite and more rarely granodiorites with leucocratic dark gray to white color. These rocks range from hypidomorphic inequigranular to xenomorphic texture, fine to medium grained. Biotite is the only primary mafic present as essential phase and characterize an expanded slightly acid sequence formed by a sub-alkaline magmatism of high-potassium calc-alkaline, slightly peraluminous composition from arc magmatic tectonic environment during a post-collisional period. Mechanism of fractional crystallization of plagioclase, biotite, titanite, apatite and zircon associated with simultaneous crustal assimilation are suggested for the evolution of these rocks. The results support the hypothesis of a post-collisional magmatism in the Paraguá terrane at 1284 ± 20 Ma corresponding to the crystallization age of the Passagem granite. This paper propose that Passagem Granite represents as an extension in Brazilian terrane of the Pensamiento Granitoid Complex

Pre- and post-collisional depositional history in the upper and middle Bengal fan and evaluation of deepwater reservoir potential along the northeast Continental Margin of India

The Bengal fan is the largest submarine fan in the world that has formed as a result of high sediment transport from the Himalaya by the Ganga–Brahmaputra river system. The Himalaya was formed as a result of the collision between the Eurasian and Indian plates. The initiation of this collision known as “soft” collision occurred around 59 Ma, whereas, the major collision, known as “hard” collision took place around 15 Ma ago. Prior to the collision, sediments into the Bay of Bengal were derived from the northwest by relatively smaller river system like Mahanadi–Godavari. The switching of river systems with time was not distinct but gradational. In the post- collision period, the sediment input from the NW was masked in most instances because of rapid sediment supply from the Himalaya to the north. Pre-collisional sediment dispersal pattern from the NW was largely affected by pre-existing basement high known as 85°E Ridge; this ridge was submerged during the post-collisional period. Post-collisional sediments are commonly referred to as the Bengal fan sediments and show huge accumulation along the shelf and beyond. High resolution 2D seismic data acquired along a corridor covering the upper, middle and distal parts of the present day active Bengal fan system indicates that the fan has prograded southward with time because of continuously increasing sediment supply and has, therefore, masked the effect of eustacy. The present day geometry of the fan shows a single active canyon and an associated single active fan. The active channel shows typical meandering pattern that shifts laterally with time. The seismic facies analysis indicates that both the pre- and post-collision basin has significant hydrocarbon potential. The thermogenic model is best suited for modeling source rock maturity in the pre-collision basin whereas both biogenic and thermogenic models best explain source rock maturity in the post-collision, younger Bengal fan. The wedge out against the 85°E Ridge is considered to be one of the important play types for hydrocarbon exploration in the deeper part of the basin. On the other hand, the channel levee complexes and frontal splay/basin floor fan are the possible target areas for petroleum exploration in relatively younger Bengal fan deposits.

Post-collisional Volcanism of Kalamaili Suture Zone

Kalamaili suture zone is an important plate boundary in north Xinjiang, with plenty of Neopaleozoic volcanic rocks around. These volcanic rocks are enriched in LILEs and relatively depleted in HFSEs, and they are characterized by high Nb, Zr, TiO 2 contents and Zr/Y, Nb/Y ratios, and display depleted Sr-Nd isotopic characteristics. The characteristics of these volcanic rocks suggest that they were produced in post-collisional period. The post-collisional volcanic rocks around Kalamaili suture zone became younger from west to east, which indicated that this suture zone entered post-collisional period gradually from west to east, and this reflected that the Kalamaili Ocean had been closed gradually from west to east. As to the Kalamaili suture zone in post-collisional period, the lithosphere extended and thinned, and the asthenospheric materials upwelled and partially melted. In the case of the intensive mantle convection, the suture zone was underplated by plenty of mantle-derived magmas. The lithospheric mantle metasomatized by fluid in subduction period partially melted under decompression and high temperature, the resulting melts contaminated the underplating asthenospheric materials, and the erupted lavas came into being the Neopaleozoic post-collisional volcanic rocks.

O uso de Imagens do Sensor Aster no Mapeamento de Unidades Vulcano- Sedimentares do Platô da Ramada, Vila Nova do Sul, RS

The remote sensing products which are generated by orbitals sensors of middle spatial and spectral resolution and the development of new technique of digital image processing has been a auxiliary tool in the basic geological surveys. In this work was utilized images obtained from the ASTER sensor (Advanced Spaceborne Thermal Emission and Reflection Radiometer) to identify the lithologies in the Ramada Plateau area, situated closed to Vila Nova do Sul town, in the southernmost Brazil, giving emphasis to the volcanic sequence of the plateau and the encase sedimentary rocks unit. The volcanic unit represent a important part of the Neoproterozoic alkaline magmatism in the Sul-rio-grandense Shield and is interpreted as one of the volcanic cycles of the post-collisional period of Brasiliano Cycle in the southern Brazil. The selective principal component analysis technique was used in the digital image processing, because it provided the best results enhancing the existent spectral differences of the studied lithologies. In this processing were utilized only two spectral bands to reduce the data dimensionality, permitting a better interpretation of them. The first generated image (PC1) concentrate the common information of the two bands (albedo and topographic shading), while PC2 image present the differential information of the scene, representing the spectral contrast of the two original bands. From this technique was generated a color composite image from the PC’s2 images of the pair of bands 4-5 (R), 3-4 (G) and 8-9 (B). This image has shown the spectral differences between volcanic rocks of the Ramada Plateau and the sedimentary rocks of the Maricá Group and its individualization in two sub-units, mainly considering the concentration of rhyolitic dykes and sills in the north of the area. In this study was identify too in the southwestern portion of the Ramada Plateau, a dioritic intrusive body.

Methane-rich fluid inclusions from ophiolitic dunite and post-collisional mafic–ultramafic intrusion: The mantle dynamics underneath the Palaeo-Asian Ocean through to the post-collisional period

The protracted development history of the Palaeo-Asian Ocean starting from ∼1000 Ma to ∼320 Ma, and continuation of mantle-derived magmatism right through the post-collisional period as manifested by the widespread mafic–ultramafic intrusion and alkaline–peralkaline granite suggest a vigorous, long-running mantle dynamo. Using the micro-laser Raman spectrometer, we analyzed olivine- and plagioclase-hosted fluid inclusions from ophiolitic dunite, Kudi of the western Kunlun Range, and post-collisional mafic–ultramafic intrusions of the Xiangshan and the Huangshandong, eastern Chinese Tienshan Mountains. Our results show that fluids brought from the mantle by these rocks are rich in H 2 O and CH 4 with variable amounts of N 2 . Estimation of the redox state, pressure and depth of formation for the melt–CH 4 –H 2 O + N 2 fluid systems from the 3 rock suites of the Xinjiang, and comparison of these parameters with those of the peridotite–fluid parageneses for the sub-cratonic upper mantle give meaningful results. These fluids were formed in the asthenosphere, were originally CH 4 –N 2 -rich, primordial mantle fluid, but have been progressively diluted with H 2 O and become oxidized by repeated subduction of oceanic slab. We propose that repeated redox melting in the mantle due to dilution of CH 4 with H 2 O and oxidation caused formation of the long-lived Palaeo-Asian Ocean, brought about extensive intrusion of mafic–ultramafic complexes and alkaline–peralkaline granites right through the post-collisional period, and, consequently, resulted in significant Phanerozoic continental growth in the Central Asian Orogenic Belt. Rise of atmospheric O 2 and the Earth's major oxidation occurred at about 2.4 to 2.2 Ga. Oxidation of the Earth's mantle promoted the mantle melting and magma input to the crust, resulting in major continental growth after 2.5 Ga. However, much-postponed oxidation of the primordial mantle underneath the Central Asian Orogenic Belt led to significant continental growth in the Phanerozoic times.

CARACTERÍSTICAS GEOQUÍMICAS E DE POSICIONAMENTO TECTÔNICO DO MAGMATISMO NEOPROTEROZÓICO AFLORANTE NO PLATÔ DO TAQUAREMBÓ - RS

The Neoproterozoic-Cambrian Taquarembo Plutonovolcanic Association, exposed in the Taquarembo Plateau, southern Brazil, is composed of volcanoclastic deposits, lavas and shallow intrusive rocks of silica-saturated alkaline affinity. Metaluminous basalts, basaltic andesites, dacites to trachytes, monzodiorites and rhyolites are associated with peralkaline syenites and rhyolites of comenditic affinity. Basic rocks were discriminated as low and high Ti-P, the former probably related with the metaluminous more evolved liquids and the second one with the peralkaline intermediate and acid rocks. REE, HFS and particularly Zr-P, discriminate the metaluminous and peralkaline rocks, increasing in the second group. Isotope and trace element based geochemical interpretation, suggests that both basic magmas are probably derived from the same EM1 mantle sources, and represent different melt fractions. The comenditic character, Nb- Ta negative anomalies in MORB normalised spidergrams, low Zr/Nb ratios, and LREE enriched patterns are suggestive of mantle sources modified by crustal subduction. Crustal influence was mostly inherited from a previous subduction and the consequent mantle metasomatism. The differentiation of parental magmas was dominantly through mineral crysytallization, involving mainly pyroxene and feldspar, with minor amounts of apatite, zircon and ilmenite. This magmatism was generated during the postcollisional period of Brasiliano Cycle, overlying the foreland of a continental margin arc.

A Review of the Nature of Magmatism in Central Anatolia during the Mesozoic Post-Collisional Period

Magmatism in central Anatolia is characterized by petrographically and chemically distinct granitic and syenitic rocks. The granitic magmatism comprises C-type (crustal-derived) and H-type (hybrid) monzogranites and monzonites. Garnet-bearing C-type leucogranites represent the oldest magmatic phase, but younger hornblende ± biotite ± K-feldspar H-type plutons dominate the geology of the Central Anatolian Crystalline Complex (CACC). These typically include mafic microgranular enclaves. The granitic magmatism predates syenitic intrusions, among which quartz-bearing syenites were emplaced prior to feldspathoid-bearing ones. The nature of magmatism in central Anatolia varies through time from peraluminous to metaluminous to alkaline. These different magma types reflect distinct stages of postcollisional magmatism, in which interaction between crust and mantle varied considerably. The C-type granites of the early stages of postcollisional magmatism were likely derived by partial melting of the lower continental crust induced by mafic magma underplating as a result of lithospheric delamination. The H-type granites and syenites of the mature and advanced stages of postcollisional magmatism indicate a significant contribution from mande-derived magma within a continuous or episodic extensional tectonic regime.

Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Abundant high-K calc-alkaline (HKCA) magmatism appears to be post-collisional and often shifts to shoshonitic or alkaline–peralkaline compositions in the final stages of orogeny. The nature and the causes of this transition are studied on the basis of 308 major element and of 86 unpublished trace element (including REE) analyses of the Pan-African granitoids from the Tuareg shield (Adrar des Iforas, Mali and Aı̈r, Niger). This database covers a wide variety of magmas from subduction-related to intraplate-type including abundant HKCA batholiths. Literature data from geodynamically well-constrained cases are also included. In addition to a conventional geochemical approach of the studied magmatism, the sliding normalization method is proposed. This tool aims at comparing magmatic series: each studied rock is normalized to the interpolated composition of the reference series that has the same SiO 2 content as the sample. This method amplifies differences in sources and in fractionation processes and allows comparison of rocks from basic to acid composition. Two distinct juvenile sources are proposed: a previously enriched phlogopite-K richterite bearing lithospheric mantle or a lower juvenile crustal equivalent for HKCA-shoshonitic magmas, and a lowest lithospheric-upper asthenospheric OIB-type mantle for alkaline-peralkaline magmatism. The first source is melted only shortly after its generation when the lithosphere was still hot, which restricts HKCA magmatism mainly to post-collisional settings. The second asthenospheric/lowest lithosphere source is by definition close to its melting temperature and can generate magma ubiquitously both in space and time. The main melting triggers are lithospheric major structures which are not only operative in a post-collisional setting but also in other environments such as intraplate setting. Geochemistry thus gives indications about the nature of the source and on geotectonic settings. However, the latter is a second rank information, which is partly model-dependant. The post-collisional period differs from other settings by a propensity to generate large amounts of magma of various kinds, among which HKCA magmatism is volumetrically the most prominent.

A OROGÊNESE BRASILIANA NO SEGMENTO CENTRAL DA FAIXA RIBEIRA, BRASIL

The structure of the central segment of the Ribeira belt is defined by three large thrust sheets (Lower, Intermediate and Upper) which override the autochthonous domain, towards the southern portion of the Sao Francisco Craton. Four lithotectonic associations are individualized within the domains: basement rocks (pre-1,8 Ga rocks); orthogneisses of unknown age; Post-1,8 Ga metasedimentary cover and Brasiliano granitoid rocks. Continental tholeiitic deformed dykes and disrupted lenses within the cover indicate extensional periods. Investigation of the tectono-metamorphic and magmatic aspects, associated with new U/Pb geocronological data support the subdivision of Brasiliano Orogeny within three main tectonic periods: a) the syn-collisional period (590-563 Ma) is represented by the main deformation (D 1+2 ), coeval with the M 1 metamorphic stage and γl -γ3 granitoid rocks. Crustal shortening took place initially through ductile thrusts and tight recumbent folds. The late stage of this collisional event is characterized by oblique convergence, with pervasive deformation throughout the belt. M 1 had its thermal peak during D 2 , and is characterised by intermediate to high pressures. The spatial distribution of the metamorphic zones defines an inverted gradient. Deformed I and S type granitoid rocks occur during this period, b) the post-collisional period (535-520 Ma) is characterized by D 3 deformation, coeval with M 2 metamorphic stage. During D 3 phase, the oblique convergence continued, but was accomodated by late to post-metamorphic peak strike slip ductile shear zones. Between these shear zones, shortening was achieved by steep folding of S 2 foliation. The Ma stage with lower pressure regime is associated with intense melting of the basement and the cover, resulting in the generation of I-type magmatism and subordinated S-type; and c) the post-tectonic period is represented by the Post-D4 γ 5 magmatism registered between 503 to 492 Ma. This magmatism may be related to melting of crustal rocks associated with mantelic contribution related with high temperature conditions and uplift just after the Brasiliano Orogeny