Negative Ti Anomalies Research Articles

Archean and paleoproterozoic crust generation events, Amparo complex and Serra Negra orthogneiss in southern Brasília Orogen, SE Brazil

The Amparo Complex and Serra Negra orthogneisses consists largely of Archean and Paleoproterozoic domains, which represent the older tectonizied basement of Ribeira Belt, southeastern portion of the Brasília Neoproterozoic Orogenesis. This is the subject of UPb SHRIMP and Nd and Sr isotopic and geochemical investigations aiming to increase the knowledge of these juvenile crustal domains. In this region, older Meso and Neoarchean rocks occur in physical proximity to younger Paleoproterozoic rocks of Serra Negra Orthogneiss. Mesoarchean gneisses with UPb zircon ages of 2956 Ma have a composition of TTG suites with TDM of 3176 Ma and a ƐNd(t) of −0.14, suggesting derivation from the partial fusion of older mafic rocks. Mafic (amphibolite) and ultramafic rocks are associated with these domains. Neoarchean porphyritic granites are associated with the former rocks, occurring as intrusive high-K granites. They have a TDM value of 3404 Ma and a ƐNd (2.7Ga) of −4.29, which demonstrates their derivation from older Archean crust. Paleoproterozoic units occur throughout the belt represented by granodioritic or tonalitic orthogneiss with intercalations of amphibolites and ultramafic rocks. In the extension to the west the units crop out in the Campinas region, and to the east are denominated Pouso Alegre Complex. They constitute a TTG-type granite-tonalite suite, as verified by their geochemical signatures, REE and incompatible elements, high Na/Ca ratios, and negative anomalies of Ta, Nb, and Ti. SHRIMP UPb zircon ages are between 2.09 and 2.16 Ga (Rhyacian) with TDM between 2.11 and 2.36 Ga and ƐNd(2.1 Ga) between +0.88 and + 3.18, indicating juvenile nature. Low 87Sr/86Sri ratios disclose a possible contribution of mafic precursors in their origin. Neoproterozoic Ediacaran ages of Ribeira Belt event are seen in zircon crystals of all samples analyzed for this study. The effects of this Neoproterozoic orogenic event on basement rocks resulted in anatexis, banding and folding of the older rocks. As suggested for the Paleoproterozoic Pouso Alegre Complex, it is possibly an important period of juvenile continental growth that, in association with the Archean gneisses, may be related to the southern end of the São Francisco Craton.

Geodynamic setting, structure, and composition of continuous trachybasalt-trachyandesite-rhyolite series in the north of Altai-Sayan area: the role of crust-mantle interaction in continental magma formation

The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO2 contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO2 (<1.7 wt.%) and Fe2O3tot and higher contents of Al2O3 in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.

Condensate refractory inclusions from the CO3.00 chondrite Dominion Range 08006: Petrography, mineral chemistry, and isotopic compositions

We have found two refractory inclusions in the CO3.00 carbonaceous chondrite Dominion Range (DOM) 08006 that appear to be primary condensates from the early solar nebula. One, inclusion 56-1, contains the first four phases predicted to form by equilibrium gas-solid condensation: corundum; hibonite; grossite; and perovskite. The other, 31-2, contains nine predicted condensate phases: hibonite; grossite; perovskite; melilite; spinel; FeNi metal; diopside; forsterite; and enstatite. Except for melilite/spinel, the phases occur in the predicted sequence from core to rim of the inclusion, which has an irregular shape inconsistent with a molten stage. This inclusion preserves the most complete record of condensation in the early solar nebula that has yet been found. The physical evidence reported here supports equilibrium condensation calculations that predict the observed sequence as well as the assumptions upon which they are based, such as total pressure (∼10−3 atm), bulk system composition (solar), and COH proportions. All phases in both inclusions and the associated ferromagnesian silicates are 16O-rich, with Δ17O between −25 and −20‰, implying that this is the original composition of the vast majority of primary condensates and that 16O-poor compositions observed in many isotopically heterogeneous inclusions are largely due to subsequent isotopic exchange. While the nebula was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed early and was preserved during condensation. Inclusion 31-2 did not incorporate live 26Al and has nucleosynthetic anomalies in the heavy Ca and Ti isotopes (i.e., δ48Ca = 4.3 ± 1.9‰; δ50Ti = 8.8 ± 2.0‰). In contrast, inclusion 56-1 has radiogenic 26Mg excesses yielding a (26Al/27Al)0 ratio of (1.0 ± 0.1) × 10−5 and negative nucleosynthetic isotopic anomalies in Ca (δ48Ca = −10.3 ± 4.2‰) and Ti (δ50Ti = −4.3 ± 2.9‰). Thus, it represents a deviation from the mutual exclusivity relationship between 26Al incorporation and large nucleosynthetic anomalies. The reservoirs in which these inclusions formed had similar O-isotopic and different Al-, Ca– and Ti-isotopic compositions, suggesting that while the CAI-forming region was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed and was preserved during condensation.

The onset of post-collisional magmatism in the Macururé Domain, Sergipano Orogenic System: The Glória Norte Stock

In the northern-central portion of the Sergipano Orogenic System there is an expressive Neoproterozoic granitic magmatism with high-K calc-alkaline and shoshonitic affinities. The Glória Norte Stock (GNS, 45 km2) is the most important representative of the shoshonitic magmatism in one the domains of the Sergipano System, the Macururé. The contacts of the stock with the host metasedimentary rocks are discordant and steep, with generation of amphibolite facies hornfels. The GNS is made up of predominantly porphyritic quartz-monzonite and monzogranite. It shows a magmatic flow foliation defined by oriented mafic enclaves and feldspar phenocrysts, without evidence for solid state regional deformation. Mafic microgranular enclaves (MME) are abundant and present different sizes and shapes. Minette and biotite diopside cumulate enclaves are also present. Coexistence between two different magmas is indicated by crystal corrosion and dissolution textures, compositional zoning of feldspar and presence of clusters of mafic minerals. Grain size decrease towards the rims of the MME indicates fast cooling of small drops of mafic magma, due to temperature contrast with the felsic magma. The monzonites and granites of the GNS have shoshonitic affinity, and the enclaves are related to ultrapotassic suites (MgO > 3%, K2O > 3%). LREE are enriched as compared to HREE, and there are remarkable negative anomalies of Ta, Nb, Ti, P, Sr and Eu, mostly in the enclaves. The MME have been probably formed from a mantellic magma with shoshonitic affinity. The observed evolution from MME to quartz-monzonites and monzogranites is essentially linked to a process of fractional crystallization. The relations between Ta/Yb and Th/Yb ratios suggest enriched mantle as a possible source of this magmatism. The relative enrichment in Rb, Th, Ce and Sm indicates that magma was generated in post-collisional events. The U-PbSHRIMP age of 588 ± 5 Ma in zircon crystals indicates that the emplacement of the GNS represents a post-collisional magmatism, marking the end of collisional processes in the Macururé Domain.

Geochemistry and geochronology of dolerite dykes from the Daba and Dongbo peridotite massifs, SW Tibet: Insights into the style of mantle melting at the onset of Neo-Tethyan subduction

This study reports on compositional, whole-rock SrNd isotopic, and zircon UPb geochronological data for dolerite dykes in the Daba and Dongbo ultramafic massifs of the southwest Yarlung–Zangbo Suture Zone (YZSZ), SW Tibet. The 121 ± 2 Ma dolerite dykes from the Daba peridotite exhibit primitive mantle (PM)-normalised multi-element patterns with (La/Yb)N = 0.43–0.72 and negative Nb anomalies. They have high initial 87Sr/86Sr ratios (87Sr/86Sr(i) = 0.707197 to 0.707879) and high εNd(t) values (+7.4 to +7.9). The 125 ± 2 Ma dolerite intrusions within the Dongbo peridotite show PM-normalised trace element profiles [(La/Yb)N = 0.65–0.84] that are characterised by apparent negative Nb anomalies and moderate negative Ti (±Y) anomalies. They also have high 87Sr/86Sr(i) ratios (0.706108–0.706793) and high εNd(t) values (+7.8 to +8.2). Semi-quantitative Sm/Yb vs. La/Sm modelling demonstrates that the parental magmas of the investigated dykes were derived from 10%–20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing normal mid-ocean ridge basalt (N-MORB)-like lherzolitic composition. The geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor inputs of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs can be linked to the upwelling of an asthenospheric source that caused continental rifting and subsequent seafloor spreading that was followed by the subduction initiation adjacent to a passive margin during the Early Cretaceous (~130–120 Ma). The results provide a more detailed and perhaps more elegant hypothesis for the tectonomagmatic evolution of the southwestern YZSZ “ophiolitic” peridotites after their accretion beneath a Neo-Tethyan marginal basin.

Geochemistry of Early Cretaceous volcanic rocks in the Northeastern Great Xing’an Range, northeast China and implication for geodynamic setting

ABSTRACTThe mechanism that triggered large-scale Late Mesozoic magmatism in the northeastern Great Xing’an Range (NE GXAR) is strongly controversial. In this paper, we present whole rock geochemistry and zircon trace element, U-Pb and Hf isotopic data on the volcanic rocks in the Longjiang and Guanghua formations in the northeastern Xing’an Block. Zircons with ages of 120–119 Ma indicate that these volcanic rocks were formed in the Early Cretaceous. Combined with previous data, it is clear that volcanic rocks in the NE GXAR erupted between 128 and 108 Ma. The andesite samples of the Longjiang Formation show high contents of Al2O3, CaO, and MgO, significant negative Nb, Ta, and Ti anomalies; εHf (t) values of zircons from the andesite sample vary from +4.13 to +7.67, indicating an enriched mantle source. The rhyolites of the Guanghua Formation show high SiO2 and K2O concentrations, low P2O5, MgO, Cr, and Ni contents and Mg# values. The positive εHf (t) values (+5.72 to +10.58) with two-stage Hf model ages ranging from 939 to 701 Ma indicate that the rhyolites are derived from the partial melting of basaltic lower crust. Combined with the regional geological evolution, we conclude that the generation of the Early Cretaceous volcanic rocks in the NE GXAR might be triggered by the dehydration, disintegration, and foundering of the Mongol-Okhotsk Oceanic flat-slab and the subsequent upwelling of the asthenosphere.

Petrogenesis of the Early-Triassic quartz-monzodiorite dykes from Central Jebilet (Moroccan Meseta): Trace element and Nd-Sr isotope constraints on magma sources, and inferences on their geodynamic context

A dyke swarm composed of hypabyssal rocks of intermediate composition cross-cuts Variscan granites and Palaeozoic metasedimentary country-rocks of the Jebilet range. Based on K-Ar isotope dating on kaersutite phenocrysts, these dykes are Early Triassic in age (ca. 240 ± 10 Ma).Due to more or less pervasive alteration, the primary mineralogical assemblages are often difficult to unravel. Plagioclase and amphibole are ubiquitous, but the extent of biotite, for instance, is subject to discussion. Two distinctive types of rocks are clearly present according to their mineralogical composition: most samples are characterized by a light-brown to greenish hornblende, while some samples are characterized by the presence of kaersutite, suggestive of an alkaline affinity.Incompatible trace element data, restricted to those elements which are considered to be relatively immobile during alteration processes, allow two groups of samples to be distinguished. The largest one displays distinctive negative Ti and Nb anomalies similar to those typical for magmas extracted from mantle sources, which interacted with hydrous fluids and silicate melts, and were thus enriched in Th and LREE relative to the HFSE. However, Nd and Sr data preclude late stage crustal contamination as a suitable explanation of these features in all but the most evolved samples. The LREE enrichment combined with relatively radiogenic Nd isotope signatures are interpreted to reflect the characteristics of source materials, which evolved in a rather primitive, ensimatic setting.The second group comprises only three samples but is of major interest as it does not show any clear Nb negative anomaly and its Th/Nb systematics suggests that crustal components were not involved in their genesis. This group is interpreted as having been produced through partial melting of mantle sources which did not suffer hydrous metasomatism nor recycling of old crustal components, but were enriched in a within-plate setting by low-degree mantle melts.Based on these data, the following tentative geological scenario is proposed. Lithospheric stretching and decompression would have triggered partial melting at relatively deep level of the lithosphere, and produced magmas with within-plate, alkaline-affinities. The ascent of these hot mantle melts would have triggered partial melting of fertile, hydrated domains of Late Proterozoic age present in the overlying mantle and/or lower crust, thereby generating the volumetrically predominant group of samples with « calc-alkaline » geochemical features, namely Nb and Ti negative anomalies. The two types of magmas likely interacted to produce hybrid liquids with variably deep Nb anomalies but broadly constant εNd values, as a result of the weak difference of their isotope signatures.These magmas are much younger (ca. 240 ± 10 Ma) than the last Variscan granites dated at ca. 300 Ma. On the other hand, they largely predate the tholeiitic continental flood basalts of the Central Atlantic Magmatic Province (CAMP) emplaced at ca. 200 Ma, and were thus emplaced amidst the 100 Ma time span between the end of the Variscan orogeny and the lithospheric break-up that led to the formation of the Atlantic Ocean. Therefore, the Jebilet dykes neither represent the ultimate manifestation of the Variscan orogeny nor a premise of the Atlantic opening. Instead, they document a particular event within a protracted intra-continental rifting history spanning the whole Permo-Triassic period. In general terms, this distensive event might be viewed as a local consequence of the dextral mega-shear which separated Europe from North Africa in Late Palaeozoic times. In this scope, the studied dykes could be described as a set of tension gashes, resulting from a local change in stress regime. Their formation was accompanied by a small pulse of intraplate magmatism, possibly because favorable fertile materials occurred locally at depth. At a larger scale, the whole dyke swarm cross-cutting the Variscan basement in Morocco is indicative of an aborted rifting event, largely predating the successful break-up of Pangea and the early opening of the Central Atlantic Ocean.

Geochemistry of peridotites from the Yap Trench, Western Pacific: implications for subduction zone mantle evolution

ABSTRACTThis study examines the major and trace elements of peridotites from the Yap Trench in the western Pacific to investigate mantle evolution beneath a subduction zone. Major element results show that the peridotites are low in Al2O3 (0.31–0.65 wt.%) and CaO (0.04–0.07 wt.%) contents and high in Mg# (Mg/(Mg+Fe)) (0.91–0.92) and have spinels with Cr# (Cr/(Cr+Al)) higher than 0.6 (0.61–0.73). Trace element results show that the peridotites have extremely low heavy rare earth element (HREE) contents compared with abyssal peridotites but have U-shaped chondrite-normalized rare earth element (REE) patterns. The degree of mantle melting estimated based on the major elements, HREEs, and spinel Cr# range from 19% to 25%, indicating that the Yap Trench peridotites may be residues of melting associated with the presence of water in the mantle source. In addition to light rare earth element (LREE) enrichment, the peridotites are characterized by high contents of highly incompatible elements, positive U and Sr anomalies, negative Ti anomalies, and high Zr/Hf ratios. The correlations between these elements and both the degree of serpentinization and high field strength element (HFSE) contents suggest that fluid alteration alone cannot account for the enrichment of the peridotites and that at least the enrichment of LREEs was likely caused by melt–mantle interaction. Comparison between the peridotites and the depletion trend defined by the primitive mantle (PM) and the depleted mantle (DM) suggests that the Yap Trench mantle was modified by subduction-related melt characterized by high contents of incompatible elements, high Zr/Hf ratios, and low HFSE contents. Hydrous melting may have been enhanced by tectonic erosion of the subducting Caroline Plate with complex tectonic morphostructures at the earliest stages of subduction initiation.

Composition and evolution of the lithospheric mantle beneath the interior of the South China Block: insights from trace elements and water contents of peridotite xenoliths

Major and trace elements and water contents were analyzed in 16 peridotite xenoliths embedded by the Cenozoic basalts in Pingnan (southeastern Guangxi Province), to constrain the chemical composition and evolution of the lithospheric mantle located in the central part of the South China Block (SCB). The peridotites are mainly moderately refractory harzburgites and lherzolites (Mg#-Ol = 90.3–91.7) and minor fertile lherzolites (Mg#-Ol = 88.9–89.9). Clinopyroxenes in the peridotites show LREE-depleted pattern, and commonly exhibit negative anomalies in Nb and Ti, suggesting the peridotites probably represent residues after 1–10% of partial melting without significant mantle metasomatism. Water contents range from 146 to 237 ppm wt. H2O in clinopyroxene, and from 65 to 112 ppm wt. H2O, in orthopyroxene but are below detection limit (2 ppm wt. H2O) in olivine. Calculated bulk water contents, based on the mineral modes and partition coefficient, range from 14 to 83 ppm wt. H2O (average 59 ppm wt. H2O). There is a correlation between melting indices (such as Mg#-Ol, Ybn in clinopyroxene) and water contents in clinopyroxene and orthopyroxene, but no correlation is observed between the whole-rock water contents and the redox state (Fe3+/∑Fe ratios in spinel), suggesting that water contents in the peridotites are mainly controlled by the degree of partial melting rather than by oxygen fugacity. The lithospheric mantle beneath the interior of the SCB may not be compositionally stratified; fertile and moderately refractory mantle coexist at the similar depths. Geochemical data and water contents of the studied peridotites are similar to the proposed MORB source and indicate that the ancient refractory lithospheric mantle was irregularly eroded or reacted by the upwelling asthenosphere, and eventually replaced by juvenile fertile accreted mantle through the cooling of the asthenosphere.

Accreted seamounts in the South Tianshan Orogenic Belt, NW China

The South Tianshan Orogenic Belt (STOB) is situated in the southwest margin of the Central Asian Orogenic Belt and is characterized by thedistributionof ophiolitic mélanges and metamorphic rocks. In this study, we focus on 5 ophiolitic mélanges with ocean island basalt (OIB) and aim to discuss the tectonic evolution of the South Tianshan Ocean. The Kynda, Kainar, Uchkel, Heiyingshan, and Guluogou ophiolitic mélanges in STOB mainly consist of metamorphic peridotite, gabbros, basaltic lavas (pillows), abyssal radiolarian cherts, and limestones. The basaltic rocks mainly occur as tectonic blocks within the mélanges. These basaltic rocks are characterized by LREE enrichment and HREE depletion, very weak or no Eu anomalies, and no obvious Nb, Ta, and Ti negative anomalies, positive εNd(t) values, relatively high initial87Sr/86Sr ratios, and wide206Pb/204Pb ratios, showing the signature of OIB. The OIB‐type rocks from Kynda, Kainar, and Guluogou are likely to be derived from garnet lherzolite by relatively high degrees of melting (10% to 20%). But the basaltic rocks of Uchkel and Heiyingshan formed at significantly lower degrees of partial melting (1% to 5%) of spinel and/or garnet lherzolite. The OIB‐type rocks in these ophiolitic mélanges are considered as accreted seamount fragments and may represent mantle plume‐related magmatism within the South Tianshan Ocean. In combination with previous work, we suggest an updated model, which is bidirectional oceanic subduction with seamounts accretion and subduction system, is suitable for Palaeozoic tectonic evolution of the South Tianshan Ocean.

Petrogenesis of the Early Cretaceous Kada igneous rocks from Tethyan Himalaya: Implications for initial break‐up of eastern Gondwana

Early Cretaceous magmatic rocks associated with the Kerguelen mantle plume are widespread in the Comei area with a chronology concentrated from 134 to 130 Ma. However, less data for the earliest Early Cretaceous magmatic rocks cropping out in the Comei area have been reported. The earliest Early Cretaceous magmatic rocks from the Kada area comprise mafic rock (diabase) and felsic rocks (granite, dacite, and quartz monzonite). This paper presents a detailed LA‐ICP‐MS zircon U–Pb chronology with major and minor (trace) elements and Lu–Hf isotope data from Kada igneous rocks to identify the extent of the early activity of the Kerguelen mantle plume and the initial break‐up of eastern Gondwanaland. The LA‐ICP‐MS zircon U–Pb dates reveal that the age of Kada magmatic rocks range from 136.9 to 143.5 Ma. Kada mafic rocks exhibit both OIB (Samples KD3 and KD06) and N‐MORB (Sample KD7‐1) characteristics. The N‐MORB‐type mafic rocks were derived from partial melting of depleted spinel lherzolite. However, the OIB‐type mafic rocks were derived from partial melting (~60–80 km) of spinel + garnet lherzolite. Kada felsic rocks show obvious enrichment of light rare earth elements and negative Eu anomaly features. Among these, dacite and quartz monzonite rocks show flat heavy rare earth elements and clear Nb, Ta, P, and Ti negative anomalies with depleted εHf(t) characteristics (−11.0 to −21.0). This is indicative of partial melting of specific crustal rocks in an intraplate environment caused by underplating of coeval basaltic magmas. However, granite is characterized by high heavy rare earth element fractionation and high and dispersed εHf composition (−1.5 to +8.9), thus indicating that granite is derived from extensive fractional crystallization of mantle‐derived melts from an AFC process. The Kada felsic rocks show A‐type granitic characteristics in an extensional environment. Based on the evidence from palaeomagnetism reconstructions, Kada rocks and Bunbury basalts are both distributed in and influence the scope of the Kerguelen mantle plume. They are also coeval with the initial break‐up of eastern Gondwana. We suggest that Kada magmatic rocks in southern Tibet represent the earliest activity of the Kerguelen mantle plume and that they played a role in the initial break‐up of eastern Gondwana.

Geochronology and Geochemistry of the Early Permian A‐type Granite in the Hongol Area, Central Inner Mongolia: Petrogenesis and Tectonic Implications

We performed geochronological and geochemical analyses of the A‐type granite in the Hongol area, central Inner Mongolia, to determine its age, petrogenesis and tectonic setting, which are significant for clarifying the Late Paleozoic tectonic evolution of the Xing'an Mongolian Orogenic Belt (XMOB). The rock type of the A‐type granite in the Hongol area is alkali‐feldspar granite, and it constitutes a western part of the Baiyinwula–Dongujimqin A‐type granite belt. Zircon U–Pb geochronology yields 206Pb/238U ages ranging from 293 to 286 Ma for the alkali‐feldspar granite, indicating this granitic pluton formed in the Early Permian. The alkali‐feldspar granite is high in silica (SiO2=75.13wt%–80.17wt%), aluminum (Al2O3=10.59wt%–13.17wt%) and alkali (Na2O+K2O=7.33wt%–9.11wt%), and low in MgO (0.08wt%–0.39wt%) and CaO (0.19wt%–0.70wt%). It is obviously enriched in LILEs such as Rb, Th and K, depleted in HFSEs such as Nb, Ti, La and Ce, with pronounced negative anomalies of Nb, Ti, P, Eu, Sr and Ba. Its Sr–Nd–Pb isotopic compositions show positive εNd(t) (+0.72–+3.08), low TDM2 (805–997 Ma), and high radioactive Pb with (206Pb/204Pb)i of 18.710–19.304, (207Pb/204Pb)i of 15.557–15.604 and (208Pb/204Pb)i of 37.887–38.330. Petrological characteristics and geochemical data suggest that the alkali‐feldspar granite in the Hongol area belongs to aluminous A‐type granite. This A‐type granite formed in a post‐collisional extensional setting and was generated by the partial melting of felsic rocks in the middle‐lower crust resulting from post‐collisional slab breakoff. It is suggested that the Paleo‐Asian Ocean was closed before the Permian in central Inner Mongolia.

Geological, petrological and geochemical framework of Miravalles-Guayabo caldera and related lavas, NW Costa Rica

The Miravalles-Guayabo caldera is located in the Guanacaste Cordillera and its genesis and evolution is related with the geodynamic setting of northwestern Costa Rica. The development of a N-S graben, and related NE-trending fracture zones, was essentially coeval with caldera forming eruptions and the multiple collapses which dismembered earlier volcanoes.The lavas of the Miravalles-Guayabo Complex (MGC) are two-pyroxene calc-alkaline andesites and basaltic andesites with subordinate high alumina basalts and rare dacites. Interstitial matrix glasses span from andesite (in basalts) to dacite (in andesite s.l.) and rhyolite (in dacite) that overlap the bulk chemical compositions of the lavas. REE patterns and spider diagrams show typical features of a subduction-related volcanism at convergent plate boundaries with spiked incompatible-element patterns, negative anomalies for Nb and Ti in spider diagrams, enrichment in LILE (with the exclusion of Sr) and LREE, and relative depletion in HFSE. The geochemical signatures of the MGC volcanic rocks are essentially consistent with recent petrogenetic models.Thermobarometric estimates suggest that basaltic and magmas equilibrated with their phenocrystic phases at 1080–1120 °C, and pressures ranges of 380–430 MPa, whereas andesitic melts are confined at 200–230 MPa and temperatures of 1050–1070 °C. In turn, amphibole-biotite dacites equilibrated at 240–290 MPa and temperatures of 920–950 °C approaching water saturated conditions (with H2O contents up to 5.7–7.2 wt%). Thermobarometry indicate that contiguous andesitic-dacitic magma lenses are stored within the subvolcanic region at depths below 10 km, whereas basaltic magmas are stored and degas at slightly higher depths (up to ~15 km). Single eruptions associated with the emplacement of lava flows seem to be related to the tapping of portions of single magma lenses during minor unrest episodes. Conversely, caldera forming eruptions involved the ejection of differentiated rhyolitic pumices (of La Ese member) and likely occurred during recharge and pressurization of this heterogeneous reservoir under critical stress conditions.

2.85 Ga and 2.73 Ga A-type granites and 2.75 Ga trondhjemite from the Zhongxiang Terrain: Implications for early crustal evolution of the Yangtze Craton, South China

The discovery of the Neoarchean Yangpo Group and Jinshan A-type granites in the Zhongxiang Terrain opened a new research window on the early crustal evolution of Yangtze Craton. We present here new zircon dating and detailed geochemical evidences to further reveal the magmatic events and their implication to related tectonic regimes. The combined study of zircon U-Pb ages, Hf isotope compositions, and whole-rock geochemistry were carried out for gneissic granites from the Huachong and Huashanguan complexes. Two groups of granite samples from the Huachong Complex have high SiO2, alkali (K2O + Na2O) and TFeO contents and relatively low Al2O3, CaO, MgO, TiO2 and P2O5 contents; all samples are noticeably peraluminous (A/CNK = 1.21–1.48) with negative anomalies in Nb, Sr, P and Ti, and positive anomalies of HFSE (Zr, Hf, Y) and LILE (Rb, U, Th, La) contents. These samples also show very high zircon saturation temperatures (Tzr (°C) = 910–942 °C), high 104 × Ga/Al (2.62–3.51), and high Zr + Nb + Y + Ce contents (571–767 ppm). Collectively, these observations illustrate typical characteristics of A-type granites. The samples from the Qinchong gneissic granites have high sodic, Sr/Y, Nb/Ta and (La/Yb)N with steep REE patterns, identical to those of TTG (tonalite–trondhjemite–granodiorite). These observations suggest that these rocks were generated by partial melting of juvenile crust materials that left a rutile-bearing eclogite residue. Zircon U–Pb dating for two A-type gneissic granites from the Huachong Complex yields weighted mean 207Pb/206Pb ages of 2856 ± 12 Ma and 2732 ± 18 Ma, and the Qinchong trondhjemite yields a upper intercept age of 2749 ± 46 Ma. Thus, we recognize three types of Archean magmatic events in the Zhongxiang Terrain as follows: ① ca. 2.85 Ga A-type granite, which is the oldest A-type granite currently known in South China; ② ca. 2.75 Ga trondhjemite, which may be resulted from arc-continental collision; ③ ca. 2.73 Ga A-type granite, which may be associated with back-arc extension. Therefore, combined with recent studies conducted in the Zhongxiang Terrain, the ca. 2.65 Ga Jinshan A-type granite in Huji region, these magmatic events suggest that the Yangtze Craton (including major Zhongxiang and Kongling terrains, and other microblocks) might have experienced two tectonic cycles of continental assembly and breakup from the Mesoarchean to Neoarchean era (ca. 2.90–2.85 Ga and ca. 2.75–2.65 Ga).

Early Cretaceous granitic rocks from the southern Jiaodong Peninsula, eastern China: implications for lithospheric extension

ABSTRACTZircon U–Pb ages, major element and trace element compositions, and Sr, Nd, and Pb isotopic compositions for late Mesozoic granites from the southern Jiaodong Peninsula (eastern China) were determined. Ages for the Wulianshan, Xiaozhushan, and Dazhushan plutons are 119.1–122.3, 114.2, and 108.9 Ma, respectively. Major and trace element characteristics show that these granitic rocks belong to alkaline, A-type granites formed in an extensional setting. Trace element compositions show strong, variable negative anomalies in Ba, K, P and Ti, and positive anomalies in Rb, Th, U, Pb, Ce, Zr, and Hf, which are typical characteristics of A-type granites. Variable Sr and Nd isotopic compositions, 87Sr/86Sr(i) = 0.70540–0.7071 and εNd(t) = −14.5 to −20.9. Whole-rock Pb isotopic compositions have the following ranges, (206Pb/204Pb)t = 15.707–16.561, (207Pb/204Pb)t = 15.376–16.462, and (208Pb/204Pb)t = 36.324 to 37.064. Isotopic modelling indicates an origin that lies between mantle tapped by Cenozoic basalts around the Tan-Lu megafault and lower continental crust (LCC), and which can be explained by mixing of 11–18% mantle and 82–89% LCC. Based on new and compiled data, we suggest that the southern Jiaodong Peninsula, as well as the Laoshan area, was in a regional extensional setting of an orogenic belt during 106–126 Ma. The granitic rocks may be the result of late Mesozoic lithospheric thinning and decratonization (i.e. late Mesozoic craton destruction event occurring throughout eastern China).

Petrogenesis of Early Cretaceous dioritic dikes in the Shanyang-Zhashui area, South Qinling, central China: Evidence for partial melting of thickened lower continental crust

The dioritic dikes distributed in the Shanyang-Zhashui area of the South Qinling region play an important role in understanding the deep magmatic processes and tectonic evolution during the orogenic process. The zircon UPb ages of the dioritic dikes indicate that they were emplaced at ∼144 Ma and therefore postdate the dikes that formed in the intracontinental orogenic background after the continental collision between the North China Block (NCB) and the South China Block (SCB). The dioritic dikes have SiO2 contents of 56.86–64.93 wt%; K2O contents of 1.65–3.21 wt%; low MgO (1.50–2.66 wt%), Y (14.4–25.5 ppm) and heavy rare earth element (HREE) contents; low Mg# values (39.9–49); high Sr contents (528–4833 ppm); and high Sr/Y ratios (32.8–189). They exhibit highly fractionated REE and flat HREE patterns, strong enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, and U) and depletion in high field strength elements (HFSEs) (e.g., Nb), as well as positive Sr and negative Ti anomalies. Furthermore, these dioritic dikes exhibit (87Sr/86Sr)i ratios ranging from 0.7048 to 0.7083, εNd(t) values ranging from −3.3 to −1.4, and εHf(t) values ranging from −4.1 to 1.6. The geochemical patterns of the dioritic dikes indicate that they possess adakitic characteristics. Moreover, the low MgO contents, Mg# values, Ni contents, Th/Ce ratios, and SrNdHf isotopic features all indicate that these dioritic dikes were generated by the partial melting of thickened mafic lower crust. The high La/Yb and Sr/Y ratios, low Y and Yb contents, absence of significant Eu anomalies, flat HREE patterns, and low Nb/Ta ratios of these rocks suggest that the adakitic melts were derived from the melting of garnet-bearing amphibolite. The geochronologic, elemental and isotopic evidence suggests that the dioritic dikes may have formed in a locally extensional environment within an overall N-S compressional setting or during the transition from compressional to extensional environments in the Early Cretaceous. This process resulted in the upwelling of the asthenospheric or lithospheric mantle, causing partial melting of the mafic lower crust and forming the adakitic dioritic melts.

Geochemical types of subvolcanic dacite – rhyolite complexes from the сentral part of the Vedlozero-Segozero greenstone belt (Karelian Craton)

The paper presents the results of detailed geological, petrographic and geochemical studies of Archean acidic subvolcanic rocks from the Koikary domain located in the central part of the Vedlozero-Segozero greenstone belt. Three petrologically contrasting groups are clearly distinguished in the investigated dacite-rhyolite association: 1) dacite with high content of Cr, Ni, Ti, Nb, and heavy REE, which are characterized by εNd (2935 Ma) from +0.72 to +2.93 and contain porphyritic phenocrysts of several generations; 2) rhyolite with a strongly fractionated REE distribution, negative anomalies in Nb, Sr and Ti, and with εNd (2935 Ma) from +1.15 to +3.37; 3) dacite and rhyolite with a moderately fractionated REE distribution, negative anomalies in Nb, Sr and Ti, and with εNd (2935 Ma) = –6.47. The above mentioned features of the rocks reflect the contrasting conditions of generation of acidic subvolcanic rocks in the Koikary domain and mark different time stages of the greenstone belt development.

Litogeoquímica e geocronologia dos ortognaisses migmatíticos do Domo de Itabaiana, Sergipe: uma suíte do tipo tonalito, trondhjemito e granodiorito?

O Domo de Itabaiana encontra-se inserido no Domínio Vaza Barris da Faixa de Dobramentos Sergipana, no Estado de Sergipe. Está representado por um complexo gnáissico-migmatítico, constituído por ortognaisses migmatizados, com intercalações de níveis anfibolíticos. O conjunto foi retrabalhado e soerguido durante o processo de deformação Neoproterozoica dessa faixa. Estudos petrográficos revelam que os ortognaisses são de composição tonalítica a granodiorítica, por vezes com feições desde protomiloníticas a miloníticas. A litogeoquímica evidenciou um caráter sódico nessas rochas, as quais foram analisadas em dois grupos distintos, em relação aos seus padrões em ETR e teores de K2O. Os litotipos com teores de K2O &lt; 2,5% compreendem termos da série cálcio-alcalina de baixo K a cálcio-alcalina normal, enquanto aqueles com teores de K2O ≥ 2,5% compreendem termos posicionados na interface da série cálcio-alcalina normal com a série cálcio-alcalina de alto K. Os dois grupos possuem anomalias negativas de Th-U, Ta-Nb e Ti, além de baixos teores de Y, apresentando um padrão fortemente fracionado, com o enriquecimento em terras raras leves e empobrecimento em terras raras pesadas. Apenas um grupo apresenta anomalia positiva de Sr e concavidade nos espectros dos terras raras pesados, feições também típicas dos TTGs arqueanos. Sugere-se que ambos possuem afinidade com TTGs e que o grupo mais enriquecido em K2O sofreu algum tipo de participação crustal em sua gênese. As análises geocronológicas determinaram uma idade de 2729 ± 12 Ma (LA-ICP-MS), interpretada como idade de cristalização dos ortognaisses.

The Bashgumbaz Complex (Tajikistan): Arc obduction in the Cimmerian orogeny of the Pamir

We present a new interpretation of the Bashgumbaz Complex (BSC), a fragment of a large mafic-ultramafic nappe exposed along the Alichur Valley in the Pamir, whose significance is of paramount importance for the understanding of the Cimmerian orogeny in South Pamir. The BSC is exposed along the contact between the SE and SW Pamir and consists of a low-grade metamorphic association dominated by gabbros and serpentinized harzburgites with minor bodies of quartzdiorite and plagiogranite tectonically coupled to basaltic and rhyolitic volcanic products. Tectonic slices of metasedimentary rocks are interleaved within the complex. The BSC is in contact with a sedimentary unit rich in olistostostromes with exotic blocks containing a Lower Permian fauna showing affinity with Central Pamir. The BSC is intruded by a Lower Cretaceous granitic body with a U-Pb zircon age of 117Ma, crosscutting the whole complex. Petrographic, mineralogical and geochemical data suggest a supra-subduction-zone affinity for the gabbros and diorites. Significant enrichment in LILE and LREE compared to HREE, coupled with negative anomalies of Nb, Ti, Zr and other HFSE support this interpretation. U-Th-Pb dating of zircon from a diorite provides a lower Norian (ca. 222Ma) youngest-crystallization-age constraint, suggesting that the BSC formed in a supra-subduction setting on the South Pamir terrane prior to the collision between Central and South Pamir blocks. It was later underthrust and then obducted onto the southern margin of the closing Rushan-Pshart Ocean, which separated these two Gondwanan terranes during the Permian–Triassic. The obduction of the Bashgumbaz ophiolites can be considered as a time constraint for the collision between the South and Central Pamir terranes, which occurred within the framework of the Cimmerian orogeny that shaped the southern margin of Eurasia.

Immature intra-oceanic arc-type volcanism on the Izanagi Plate revealed by the geochemistry of the Daimaruyama greenstones in the Hiroo Complex, southern Hidaka Belt, central Hokkaido, Japan

The Izanagi Plate is assumed to have underlain the western Panthalassa Ocean to the east of Eurasia, and to have been subducting under the Eurasian continent. Although the Izanagi Plate has been lost to subduction, the subduction complexes of the circum-Panthalassa continental margins provide evidence that subduction-related volcanism occurred within the Panthalassa Ocean, and not just along its margins. The Daimaruyama mass is a kilometer-sized allochthonous greenstone body in the Hiroo Complex in the southeastern part of the Nakanogawa Group in the southern Hidaka Belt, northern Japan. The Hiroo Complex is a subduction complex that formed within the Paleo-Kuril arc–trench system at 57–48 Ma. The Daimaruyama greenstones consist mainly of coarse volcaniclastic rocks with lesser amount of lava. Red bedded chert, red shale, and micritic limestone are also observed as blocks associated with the greenstones. The presence of Early Cretaceous (Aptian–Albian) radiolaria in red bedded cherts within the greenstones indicates that the Daimaruyama greenstones formed after this time. An integrated major and trace element geochemical dataset for whole-rocks and clinopyroxenes of the greenstones indicates a calc-alkaline magmatic trend with low TiO2 contents and increases in SiO2 and decreases in FeO* with increasing differentiation. Negative anomalies of Nb, Ta, and Ti in normal mid-ocean-ridge basalt type normalized patterns are interpreted as “arc-signatures”. Using “rhyolite-MELTS”, we conducted a numerical simulation of magmatic differentiation under conditions of 1.5 kbar and H2O = 3 wt% to reproduce the liquid line of descent of the Daimaruyama greenstones. Back-calculations of the equilibrium melt compositions from the trace element chemistry of the clinopyroxenes generally agree with the whole-rock rare earth element compositions of the Daimaruyama greenstones, therefore providing support for the conditions used for the rhyolite-MELTS calculations as well as the actual results. Clinopyroxene trace element compositions indicate substantial enrichment of Ba in the magma, reflecting the participation of shallow subduction components such as aqueous fluids. Geochemical investigations reveal that the Daimaruyama greenstones were probably submarine volcanic rocks that formed as a result of the subduction of the Izanagi Plate within the Thalassa Ocean (the Thalassa Ocean was the eastern realm of Panthalassa, and represents the proto-Pacific Ocean) after the Early Cretaceous (Aptian–Albian; 125–101 Ma), and they were eventually accreted onto the Paleo-Kuril arc–trench system at 57–48 Ma to form an allochthonous block as part of the mélange facies of the Hiroo Complex on the landward slope of the trench.