Bimodal Volcanic Suite Research Articles

Late Paleozoic anorogenic magmatism of the Gobi Altai (SW Mongolia): Tectonic position, geochronology and correlation with igneous activity of the Central Asian Orogenic Belt

Geological and geochronological U–Pb (zircon ID-TIMS) and 40Ar/39Ar (amphibole) data were obtained for six anorogenic magmatic associations in the Gobi Altai, southwestern Mongolia, in order to reveal its relationships with Late Paleozoic tectonothermal events in the southern part of the Central Asian Orogenic Belt (CAOB). In the Gobi Altai massifs of alkaline granites and grabens filled with bimodal volcanic suites occur as a chain 450km long east–southeast direction. The massifs and volcanic fields are controlled by contacts of pre-Ediacaran continental crustal blocks with Late Neoproterozoic–Cambrian juvenile ophiolite and arc-type complexes of the Lake zone. Gobi Altai alkaline granites bear sodic-rich mafic silicate minerals and possess typical properties of ferroan A-type granites. Our geochronological data constrain emplacement time of the Gobi Altai alkaline granites and bimodal volcanic rocks at 293±1–279±1Ma. This age interval coincides with timing of the emplacement of I-type granites (with a few S-type granites) and gabbroic intrusions in the Mongolian and Chinese Altai as well as with numerous massifs of A-type granites, layered mafic–ultramafic intrusions, and eruption of bimodal volcanic rocks between the southern edge of the Siberia craton and the northern margins of the Tarim and North China cratons. Hence, the Gobi Altai anorogenic magmatic complexes were produced by a large-scale Late Paleozoic magmatic event (or events) that encompassed a wide spectrum of terranes in CAOB. The most realistic models explaining these magmatic processes are: (i) the effect of the Tarim mantle plume, (ii) slab (or slabs) break-off after subduction blocking due to accretion of terranes of various types.

A combined fluid inclusion and S–Pb isotope study of the Neoproterozoic Pingshui volcanogenic massive sulfide Cu–Zn deposit, Southeast China

The Pingshui Cu–Zn deposit is located in the Jiangshan–Shaoxing fault zone, which marks the Neoproterozoic suture zone between the Yangtze block and Cathaysia block in South China. It contains 0.45million tons of proven ore reserves with grades of 1.03wt.% Cu and 1.83wt.% Zn. This deposit is composed of stratiform, massive sulfide ore bodies, which contain more than 60vol.% sulfide minerals. These ore bodies are hosted in altered mafic and felsic rocks (spilites and keratophyres) of the bimodal volcanic suite that makes up the Neoproterozoic Pingshui Formation. Metallic minerals include pyrite, chalcopyrite, sphalerite, tennantite, tetrahedrite and magnetite, with minor galena. Gangue minerals are quartz, sericite, chlorite, calcite, gypsum, barite and jasper. Three distinct mineralogical zones are recognized in these massive sulfide ore bodies: a distal zone composed of sphalerite+pyrite+barite (zone I); an intermediate zone characterized by a pyrite+sphalerite+chalcopyrite assemblages (zone II); and a proximal zone containing chalcopyrite+pyrite+magnetite (zone III). A thin, layer of exhalative jaspilite overlies the sulfide ore bodies except in the proximal zone. The volcanic rocks of the Pingshui Formation are all highly altered spilites and keratophyres, but their trace element geochemistry suggests that they were generated by partial melting of the depleted mantle in an island arc setting. Homogenization temperatures of the primary fluid inclusions in quartz from massive sulfide ores are between 217 and 328°C, and their salinities range from 3.2 to 5.7wt.% NaCl equivalent. Raman spectroscopy of the fluid inclusions showed that water is the dominant component, with no other volatile components. Fluid inclusion data suggest that the ore-forming fluids were derived from circulating seawater. The δ34S values of pyrite from the massive sulfide ores range from −3.6‰ to +3.4‰, indicating that the sulfur was primarily leached from the arc volcanic rocks of the Pingshui Formation. Both pyrite from the massive sulfide ores and plagioclase from the spilites have similar lead isotope compositions, implying that the lead was also derived from the Pingshui Formation. The low lead contents of the massive sulfide ores and the geochemistry of their host rocks are similar to many VMS Cu–Zn deposits in Canada (e.g., Noranda) and thus can be classified as belonging to the bimodal-mafic subtype. The presence of magnetite and the absence of jaspilite and barite at the −505m level in the Pingshui deposit suggest that this level is most likely the central zone of the original lateral massive sulfide ore bodies. If this interpretation is correct, the deep part of the Pingshui Cu–Zn deposit may have significant exploration potential.

Structural and stratigraphical significance of U–Pb ages from the Mora and Saldanha volcanic complexes (NE Portugal, Iberian Variscides)

Abstract Geological mapping and structural investigations were carried out in the eastern rim of the Morais Allochthonous Complex in NE Portugal (NW Iberian Massif) along the first-order tectonic boundary between the lower part of the Galicia–Trás-os-Montes Zone (the parautochthonous Schistose Domain) and the autochthonous Central Iberian Zone. The aim was to correlate the Palaeozoic stratigraphic pile of both domains, and to establish their pre- and synorogenic Variscan evolution. An Early Palaeozoic volcanic complex forms part of the pre-orogenic record of the parautochthonous Schistose Domain. It includes two volcanic units, the oldest of which is described for the first time in this work and is named the Mora Volcanics. It is a bimodal volcanic suite with basic to acid rocks which has yielded a late Cambrian age ( c. 493 Ma). Above the Mora Volcanics, the Saldanha Volcanics show petrographic, geochemical and age ( c. 483 Ma) similarities to the Ollo de Sapo Formation, a volcano-detritic ensemble found in the autochthonous domain of NW and Central Iberia. The stratigraphic record and the geochemical signatures of both volcanic units indicate that they were originated by fast partial melting induced by extension along the northern Gondwana margin at the Cambro-Ordovician boundary, coeval with the opening of the Rheic Ocean.

Compositional diversity of ca. 110 Ma magmatism in the northern Lhasa Terrane, Tibet: Implications for the magmatic origin and crustal growth in a continent–continent collision zone

The Yanhu area in the northern Lhasa subterrane exposes diverse rock types including basalts, rhyolites, quartz dioritic porphyries, and associated dioritic enclaves. The basalts and rhyolites occur as a bimodal volcanic suite, and the quartz dioritic porphyries intrude into the older basalts as a small apophysis. In this paper, we report for the first time the zircon LA-ICP-MS U–Pb age and Hf isotopic composition data, whole-rock major and trace element composition data, and Sr–Nd isotopic data from the diverse Early Cretaceous magmatic rocks from Yanhu. The three basalt samples that we dated yielded zircon U–Pb ages of 110±0.7Ma, 108.9±1.1Ma, and 111.8±3.2Ma. The zircons from one quartz diorite porphyry yielded an age of 109.7±0.8Ma, which is coeval with the dioritic enclave (110.4±1.4Ma). The basalts show a high-K calc-alkaline signature, enriched in Rb, Th, U, and light rare earth elements (REEs) and depleted in Nb, Ta, Ti, Zr, and Hf. These rocks possess varying whole-rock εNd(t) (−0.6 to +2.1) and zircon εHf(t) (+0.6 to +8.9) values. The rhyolite samples are high-K calc-alkaline and are metaluminous to slightly peraluminous. These rocks yielded whole-rock εNd(t) of +0.1 to +0.9 and zircon εHf(t) of +5.1 to +12.4. The quartz dioritic porphyries are characterized by high Al2O3 content (15.9–16.1wt.%), high Sr (466–556ppm), low Yb (1.36–1.41ppm), and low Y (13.8ppm) abundances. Similar geochemical signatures are also present in the dioritic enclaves, revealing that both the quartz dioritic porphyries and the dioritic enclaves have an affinity with adakitic rocks. Moreover, the host rocks and the enclaves display homogeneous εNd(t) (+3.5 to +3.6 and +3.4 to +4.0, respectively) and zircon εHf(t) values (+12.4 to +16.9 and +11.5 to +15.7, respectively). Our geochemical data indicate that the magmatic rocks from Yanhu were derived from the partial melting of distinct source regions, i.e., a heterogeneous metasomatized mantle wedge source for basalts, a juvenile crust source for rhyolites, and a thickened mafic lower crust source that mixed with basaltic magmas for the adakitic rocks (including quartz dioritic porphyries and dioritic enclaves). Compared to typical arc basalts, the basalts from Yanhu are relatively enriched with high field strength elements (HFSEs) (e.g., Zr, Nb), resembling those of within-plate basalts elsewhere. In combination with the presence of a coeval bimodal volcanic rock suite, the ca. 110Ma magmatism in Yanhu is inferred to have occurred in an extensional setting. Our new data, together with recently published data, enable us to correlate the generation of the compositional diversity of the ca. 110Ma Yanhu magmatic rocks that formed in an extensional setting to the slab break-off during the southward subduction of the Bangong–Nujiang Ocean lithosphere. The presence of basaltic magmatism and coeval silicic magmatic rocks with positive zircon εHf(t) indicate that the extensive magmatism at ca. 110Ma have contributed significantly to the crustal growth of the northern Lhasa subterrane.

Permian volcanics in the Northern Gemericum and Bôrka Nappe system: U-Pb zircon dating and the implications for geodynamic evolution (Western Carpathians, Slovakia)

Permian volcanics in the Northern Gemericum and Bôrka Nappe system: U-Pb zircon dating and the implications for geodynamic evolution (Western Carpathians, Slovakia)U-Pb dating (SHRIMP) of magmatic zircon ages from the Northern Gemericum Permian volcanics (Petrová Hora Formation) yielded the Concordia age of 272.4 ± 7.3 Ma for basaltic andesite, as well as the Concordia age of 275.2 ± 4 Ma for rhyodacites. Both zircon ages correspond to the Cisuralian Epoch in the time span of the Kungurian Stage. Acquired206Pb/238U zircon age data support the nearly contemporaneous origin of the acid and basic volcanogenic members in the Northern Gemericum Permian strata. The bimodal volcanic suite proves the transtension/extension tectonic regime in the North Gemeric sedimentary basin during the Late Cisuralian. The magmatic zircon ages of rhyodacites, occurring in the lower thrust sheet of the Bôrka Nappe (Jasov Formation), gave a younger Concordia age of 266 ± 1.8 Ma proving the Guadalupian Epoch, in the time span of the Wordian/Capitanian. In comparison to the Northern Gemericum realm, this age refers to the relatively younger stage of rift-related extensional movements. In the wide Alpine-Dinarides realm the Middle Permian (Guadalupian) movements are related to the beginning of the Alpine sedimentary cycle. Thus, the Middle Permian rifting expresses the beginning of the formation of the future Meliata oceanic trough.

Cambrian bimodal volcanism in the Lhasa Terrane, southern Tibet: Record of an early Paleozoic Andean-type magmatic arc in the Australian proto-Tethyan margin

This paper reports new zircon LA-ICP-MS U–Pb age and Hf-isotope, whole-rock major and trace element, and Sr–Nd isotope data from Cambrian metavolcanic rocks in the central Lhasa subterrane of southern Tibet. These rocks form a bimodal volcanic suite consisting mainly of silicic metavolcanic rocks with subordinate metabasalts. Five silicic metavolcanic samples dated at ca. 492Ma and one metabasalt sample yielding a near-concordant 206Pb/238U age of 492±4Ma indicate that mafic and silicic eruptions were contemporaneous. The metabasalts are mostly high-K calc-alkaline, enriched in Th, U, and light rare earth elements (LREEs), and depleted in Nb, Ta, Ti, Zr, and Hf, geochemically resembling the Andean arc basalts. The silicic metavolcanic rocks are high-K calc-alkaline and low in Nb and Zr. The metabasaltic rocks have negative whole-rock εNd(t) values (−4.7 to −3.5) and varying zircon εHf(t) values (−0.7 to +7.5), differing significantly from those of the silicic metavolcanic rocks, which yield negative whole-rock εNd(t) values of −8.4 to −7.2 and varying zircon εHf(t) values (−13.9 to −4.6). The metabasaltic rocks are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components, whereas the silicic metavolcanic rocks were derived from basaltic melt-induced anatexis of the ancient Lhasa basement with inherited mantle melt signatures. The Western Qiangtang–Amdo–Tethyan Himalaya situated in the Indian proto-Tethyan margin and the Lhasa and other possible microcontinents or terranes (e.g., Gongshan, Baoshan, Tengchong, Burma, and Sibumasu) paleographically located in the Australian proto-Tethyan margin represent an early Paleozoic Andean-type magmatic arc facing the proto-Tethyan Ocean. The emplacement of the bimodal volcanic rocks and the development of the Cambro–Ordovician angular unconformity in the central Lhasa subterrane can be attributed to slab break-off of the proto-Tethyan Ocean lithosphere following the collisional accretion of microcontinents or terranes located outboard of the magmatic arc (possibly Eastern Qiangtang and South China).

Metallogenic Province and Large Scale Mineralization of Volcanogenic Massive Sulfide Deposits in China

Volcanogenic massive sulfide (VMS) deposits are one of the most important base–metal deposit types in China, are major sources of Zn, Cu, Pb, Ag, and Au, and significant sources for Co, Sn, Se, Mn, Cd, In, Bi, Te, Ga, and Ge. They typically occur at or near the seafloor in submarine volcanic environments, and are classified according to base metal content, gold content, or host-rock lithology. The spatial distribution of the deposits is determined by the different geological settings, with VMS deposits concentrated in the Sanjiang, Qilian and Altai metallogenic provinces. VMS deposits in China range in age from Archaean to Mesozoic, and have three epochs of large scale mineralization of Proterozoic, Palaeozoic and Mesozoic. Only Hongtoushan Cu–Zn deposit has been recognized so far in an Archaean greenstone belt, at the north margin of the North China Platform. The Proterozoic era was one of the important metallogenic periods for the formation of VMS mineralization, mainly in the Early and Late Proterozoic periods. VMS-type Cu–Fe and Cu–Zn deposits related to submarine volcanic-sedimentary rocks, were formed in the Aulacogens and rifts in the interior and along both sides of the North China Platform, and the southern margin of the Yangtze Platform. More than half of the VMS deposits formed in the Palaeozoic, and three important VMS–metallogenic provinces have been recognized, they are Altai–Junggar (i.e. Ashele Cu–Pb–Zn deposit), Sanjiang (i.e. Laochang Zn–Pb–Cu deposit) and Qilian (i.e. Baiyinchang Cu–Zn deposit). The Triassic is a significant tectonic and metallogenic period for China. In the Sanjiang Palaeo–Tethys, the Late Triassic Yidun arc is the latest arc–basin system, in which the Gacun-style VMS Pb–Zn–Cu–Ag deposits developed in the intra-arc rift basins, with bimodal volcanic suites at the northern segment of the arc.

Geology of the Ore Fault Ni-Cu Deposit, Bird River Sill Complex, Manitoba

The geology of the Ore Fault Ni-Cu deposit has been reinterpreted in the light of recent studies of the geology of the Neoarchean Bird River Sill Complex and the occurrence of economic Ni-Cu and Zn-Cu-Ag mineralization. The Ni-Cu sulfides are hosted by the lower of two parallel NNW-trending and moderately W-dipping mafic–ultramafic intrusions related to the Bird River Sill Complex. The bodies intruded a bimodal volcanic suite consisting of weakly deformed mafic flows and felsic pyroclastic rocks. The mafic volcanic rocks are geochemically related to the MORB-type rocks of the Lamprey Falls Formation, and the felsic volcanic rocks are related to arc-type rocks of the Peterson Creek Formation. The original pyroxene- and olivine-dominated mineralogy of the sills has been replaced by variable serpentine-amphibole-talc-carbonate assemblages. The Ni-Cu sulfide assemblage exhibits textures and geochemical signatures typical of orthomagmatic sulfide mineralization. The sulfide minerals are associated with cumulate layers of magnetic ferrochromite that range in thickness from a few to tens of centimeters. A later event of Zn-Cu-Ag sulfide mineralization associated with quartz veining and garnet-chlorite alteration along a fault that strikes north, dips vertically, and cuts all rock types. The mineralized fault is cut by the NE-trending Peterson Creek shear zone. Where the Ni-Cu and Zn-Cu-Ag mineralized zones intersect, remnant magmatic features such as amphibolite or cumulate ferrochromite bands occur within quartz veins or local chlorite alteration. The mixed zones contain a very unusual polymetallic assemblage of Ni, Cu, Zn, Ag, and platinum group elements (PGE).

Records of Precambrian–Early Palaeozoic volcanic and sedimentary processes in the Central European Variscides: A review of SHRIMP zircon data from the Kaczawa succession (Sudetes, SW Poland)

The early, pre-orogenic stages of evolution in the Variscan belt, i.e. rifting processes, opening of sedimentary basins and associated igneous activities, are often obscure because many successions have yielded little or no biostratigraphic data, have a strong metamorphic overprint and are tectonically deformed and dislocated. The increasing application of SHRIMP zircon dating has provided useful constraints on magmatic and metamorphic processes, helped locate probable source areas for detritus within sedimentary successions and facilitated large-scale palaeogeographic correlations. This methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the Kaczawa Complex in the Polish West Sudetes. Thus, recent SHRIMP studies in the Kaczawa Mountains have yielded Early Ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the Kaczawa Succession: c. 503 Ma for metarhyodacites of crustal derivation, and c. 485 Ma for alkaline metatrachytes of mantle signature. These dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break-up of the northern peripheries of Gondwana. New SHRIMP data from metavolcaniclastic and metasedimentary rocks of the Kaczawa Complex have yielded results that have provided significantly changed interpretations on their age and relationships. For instance, a siliciclastic sequence interpreted as belonging to the lower part of the Kaczawa Complex (the Gackowa Sandstones) and seemingly sourced (using an array of geochemical and mineralogical evidence) from nearby early Ordovician volcanic rocks has, surprisingly, yielded zircon ages not younger than Precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the Neoproterozoic Lusatian Graywackes. Felsic metavolcaniclastic rocks embedded in the carbonate succession of the Wojcieszów Limestone have yielded SHRIMP zircon ages of c. 497 Ma, thus supporting a late Cambrian/early Ordovician age for this shallow-water limestone sequence (which had been proposed to be late Ordovician/Silurian or younger in age, based on controversial foraminifera evidence, and more recently assigned to the Mid Cambrian, based on newly found archaeocyathid fragments). And, age spectra of zircon populations in the metamudstone sequence of the Radzimowice Slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early Palaeozoic rifting, with source areas similar to those known from parts of Gondwana (e.g. NW Africa). Overall, the new SHRIMP zircon studies, both in metavolcanic and metasedimentary sequences, have considerably illuminated their affinities, and should provide encouragement for further application of this technique in this region. In general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along North Gondwana during latest Precambrian and early Palaeozoic times.

Generation of Late Cretaceous silicic rocks in SE China: Age, major element and numerical simulation constraints

Rhyolite-dominating bimodal volcanic suites (rhyolite/basalt), mafic dikes and A-type granites distribute from N Zhejiang to S Fujian over 800 km in the Southeast Coast Magmatic Belt (SCMB) – the Late Yanshanian (LY) orogenic belt in SE China. Data of 40Ar/ 39Ar and K–Ar whole-rock ages and LA-ICPMS U–Pb zircon ages indicate that rhyolitic volcanism (101–72 Ma) is contemporaneous with the A-type granitic intrusions (100–90 Ma) and mafic dike injections (94–77 Ma). This time span is used to define the upper volcanic series in Zhejiang–Fujian areas. One striking feature of rhyolites in the SCMB is that many are strongly peraluminous (SP) and others, mostly restrict in Fujian, are peralkaline to mildly peraluminous (P-MP) and chemically resemble A-type granites. The SP character is unique among well-known large rhyolite provinces worldwide. Based on experimental works for a common thermal regime and inherited zircon age information, we suggest that SP and P-MP rhyolites represent low pressure melting of the felsic (quartzofeldspathic) granite (±metapelite) and the accompanied granodioritic, tonalitic and trondhjemitic member of the core complex assemblage, respectively, to account for the decreasing aluminosity. They could have also been contaminated by young igneous rocks, and ancient crust to a lesser degree, during ascent to the surface. Plate subduction and lithosphere extension processes, respectively, are numerically simulated for the magma generation of these rhyolites using the mafic underplating model. Results suggest that the most effective controlling factor to generate SP and associated P-MP (A-type) magmas during 95–80 Ma is thinning of the lithosphere thickness with a high exhumation rate. Under this circumstance, the core complex assemblage can be uplifted to lower level of the crust and match the constraint of experimental works.

Zeolites and zeolitization of acid pyroclastic rocks from paroxysmal Paleogene volcanism, Eastern Rhodopes, Bulgaria

An extensive bimodal volcanic suite developed in the Late Paleogene in the Eastern Rhodopes Mountains, Southern Bulgaria. Most of the widespread and thick rhyolitic tuffs and ignimbrites, erupted during two major Early Oligocene acid phases, were emplaced in a shallow marine environment and subsequently altered. K-rich Ca-clinoptilolite (in some places K-clinoptilolite, mordenite or analcime), clay minerals (celadonite or/and smectite), adularia (or albite) and opal-CT replace the volcanic glass. The zeolitized pyroclastic rocks are strongly depleted in MnO, Na2O, K2O, and enriched in CaO, Fe2O3, TiO2 and MgO. Regional zoning in distribution of the major glass-replacing minerals has been established: quartz + adularia (or albite) ± zeolites in the proximal to the vent area; zeolites in the medial and clay minerals in the distal area. Vertical zoning has been also recognized because only the basal pyroclastic layer is transformed into adularia + quartz whereas clinoptilolite, accompanied by opal-CT and clay minerals, prevails in the rest of the section. The zeolitized glass shards are characteristically zoned: (1) a thin rim (<5μm thick) of celadonite coats the shard surface retaining their original shape; (2) a 30–50μm-thick zone, made of contiguous platy zeolite crystals, oriented perpendicular to the rim and (3) a central zone of large euhedral crystals, surrounding a central hollow area. Phenocrysts are not affected by the zeolitization and some relict glass is found in the top of the pyroclastic section. According to the semiquantitative X-ray analysis and heat of immersion test of Culfaz et al. (1973) the clinoptilolite content in the zeolitized pyroclastic rocks varies from 38 to 72 wt.%. The cation exchange capacity values range between 40 and 153 meq/100 g and show a significant positive correlation with clinoptilolite content obtained by the heat of immersion test (r = 0.71). The lateral zoning and presence of typical hydrothermal minerals such as celadotite indicate that zeolitization of the thick Eastern Rhodopes pyroclastic series results from the operation of a large low temperature hydrothermal system. The hydrothermal solutions consist of marine water heated by the anomalous geothermal gradient of the active volcanic areas or/and by the hot pyroclastic deposits.

Zircon U–Pb age, Hf and O isotope constraints on protolith origin of ultrahigh-pressure eclogite and gneiss in the Dabie orogen

Zircon U–Pb dating, Hf and O isotope analyses were carried out for ultrahigh-pressure eclogite and felsic gneiss from the Dabie orogen in China. The results provide constraints on their protolith origin, with significance for continental growth by rift magmatism during splitting of the Yangtze Block from the supercontinent Rodinia. Mafic and felsic protoliths formed as a bimodal volcanic suite at about 750 Ma, with incorporation of ca. 2.15 Ga crust into the felsic protolith. δ18O values of −4.1‰ to 4.4‰ suggest differential involvement of meteoric water in protolith magmas. Initial Hf isotope ratios are subdivided into two groups, with positive εHf(t) values of 12.9±0.7 to 5.9±0.9 and neutral εHf(t) values of 2.3±0.3 to −2.7±0.6, respectively. The positive εHf(t) values correspond to depleted mantle Hf model ages of 0.82 to 1.24 Ga, whereas the negative εHf(t) values correspond to crust Hf model ages of 1.82±0.07 Ga. A few zircons from the gneiss have strongly negative εHf(t) values of −22.6±0.6, which are associated not only with consistent depleted mantle and crust Hf model ages of 2.2 Ga but also with the old U–Pb ages falling on a 2147±22 Ma discordia line. Thus the felsic protolith contains Paleoproterozoic crustal relicts, some of which were originally derived from coeval depleted mantle. On the other hand, strongly positive εHf(t) values are associated with young U–Pb ages falling on the ∼750 Ma discordia lines. This suggests that both eclogite and gneiss protoliths principally formed by immediate reworking of mid-Neoproterozoic juvenile crust, with contrasting features only in petrochemistry. The highest εHf(t) values of 12.7 to 14.4 for the eclogite correspond to the youngest Hf model ages of 0.75 to 0.82 Ga relative to the depleted mantle reservoir. These Hf model ages are not only close to the timing of zircon growth from the mafic magma, but also similar to ages for bimodal magmatism in the periphery of the Yangtze Block. This demonstrates new addition of the depleted mantle material to the continental crust by rift magmatism in the northern margin of the Yangtze Block, with coeval crust–mantle interaction in the mid-Neoproterozoic rifting tectonic zone. Therefore, the bimodal magmatism at about 750 Ma transports both heat and material from the mantle to the crust, resulting in reworking of both the Paleoproterozoic old crust and the meteoric-hydrothermally altered juvenile crust along the active rifting zone.

Mesozoic Bimodal Volcanic Suite in Zhalantun of the Da Hinggan Range and Its Geological Significance: Zircon U-Pb Age and Hf Isotopic Constraints

Mesozoic bimodal volcanic rocks of basaltic andesite and rhyolite are widely distributed in the Da Hinggan Range, but their petrogenetic relationships and geodynamic implications are rarely constrained. Detailed studies on doleritic and porphyry dikes in the Zhalantun area indicate that they display features of magma mixing, suggesting their coeval formation. In situ zircon U-Pb dating shows that the porphyry was emplaced in the Early Cretaceous with a 206Pb/238U age of 130±1 Ma. Zircons from the dolerite also yield an Early Cretaceous emplacement age of 124±2 Ma although some inherited zircons have been identified. These age results indicate that the Early Cretaceous was an important period of magmatism in the Da Hinggan Range. Zircons from porphyry are characterized by positive value of eHf(t) as high as 10.3±0.5 with Hf depleted mantle model age of 349–568 Ma, whereas magmatic zircons from the dolerite have eHf(t) value of 11.0±1.4 with Hf depleted mantel model age of 342–657 Ma, consistent with those from the porphyry. Considering other data on the geological evolution of this area, it is concluded that the mafic magma originated from the partial melting of Paleozoic enriched lithospheric mantle, whereas the felsic magma came from recycling of juvenile crust formed during the Paleozoic. Both of the protoliths are closely related to the subduction of the Paleo-Asian Ocean during the Paleozoic, indicating that the Paleozoic is an important period of large-scale crustal growth in the area.

Early Palaeozoic crustal melting in an extensional setting: petrological and Sm–Nd evidence from the Izera granite-gneisses, Polish Sudetes

The Izera Block in the West Sudetes, which is composed of granites, gneisses (and transitional granite-gneisses) and minor mica schists, is one of the largest outcrops of Early Palaeozoic (ca. 500 Ma) metagranitoid rocks in the basement units of the Variscides of Central Europe. The Izera granites show S-type features: magmatic cordierite, relict garnet and sillimanite, lack of mafic enclaves, and absence of coexisting tonalites and diorites. The paucity of pegmatites indicates that the granitic magma was relatively dry. The S-type character of these granites is further supported by their peraluminous character (A/CNK 1.0–1.63), high content of normative corundum (up to 3.5%) and relatively high 87Sr /86Sr initial ratio. The chemical variation of these rocks was controlled by the fractional crystallization of plagioclase (CaO, Sr, Eu/Eu*), biotite and cordierite (Al2O3, MgO, FeO), zircon (Zr, Hf) and monazite (REE). Initial eNd values range from −5.2 to −6.9 (mean: −5.9, SD=0.6). These largely negative eNd values imply that the granitic magmas emplaced ca. 500 Ma were extracted from a source reservoir that was strongly enriched in LREE (i.e., with low Sm/Nd ratio) on a time-integrated basis. The relatively consistent depleted mantle model ages (1,730–2,175 Ma; mean: 1,890 Ma) is in agreement with the earlier reported presence of ca. 2.1 Ga old inherited Pb component in zircon from the closely related Rumburk granite. This points to an old (Early Proterozoic) crustal residence age of the inferred metasedimentary protoliths of the Izera granitoids, with only minor contribution to their protoliths of juvenile components of Late Proterozoic/Early Palaeozoic age. Although the Izera granites show some trace element features reminiscent of syn-collisional or post-collisional granitoids, they more likely belong to the broad anorogenic class. Our data corroborate some previous interpretations that granite generation was connected with the Early Palaeozoic rifting of the passive margin of the Saxothuringian block, well documented in the region by bimodal volcanic suites of similar age (Kaczawa Unit, eastern and southern envelope of the Karkonosze–Izera Block). In this scenario, granite magmatism and bimodal volcanism would represent two broadly concomitant effects of a single major event of lithospheric break-up at the northern edge of Gondwana.

Geochemistry and Origin of Early Proterozoic Dongargarh Rapakivi Granite Complex, Central India - An Example for Magma Mixing and Differentiation

Early Proterozoic Dongargarh granite complex of Central India, intruding the tonalitic to granodioritic Amgaon gneisses and the Nandgaon Group bimodal volcanic suite, comprises three different textural and compositional types, viz., porphyritic granodiorite (PG), coarse equigranular granite (EG) and microgranite (MG). Synplutonic mafic dykes are common in the granite complex. The PG is characterised by rapakivi texture and the EG is the dominant facies and exhibits sporadically developed rapakivi texture. Microgranular enclaves are common in the EG while they are rare in PG. Major and trace element geochemistry of PG shows marked I- type and some occasional A-type granite characters unusual for a rapakivi granite while the EG shows A-type granite signatures. The field, petrographic, chemical and isotopic data of these granites suggest their derivation by mixing of mantle derived basic magma with a crustal-derived partly crystalline granitic magma. Episodic mafic magma underplating caused the anatexis of the Archaean lower continental crust in a continental margin tectonic setting resulting first in the formation of the I-type granodiorite followed by A-type granite. The I-type granodiorite is mixed with the basic magma (synplutonic dykes) while the EG is formed by mingling of A- type granite magma and the intruding basic magma.

Geology and mineralisation in the Meso- to Neoproterozoic Ghanzi-Chobe belt of northwest Botswana

The Ghanzi-Chobe Belt defines a linear zone of volcano-sedimentary rocks that form inliers through the Kalahari Desert of western and northern Botswana. Geological investigations along the Ghanzi Ridge have revealed a sequence composed of a basal bimodal volcanic suite with minor intercalated sedimentary rocks, termed the Kgwebe Formation, and an upper siliciclastic sedimentary unit with subordinate carbonates, called the Ghanzi Group. To the northeast in the Chobe District, basal volcanics informally termed the Goha Hills Formation are overlain by carbonate-bearing siliciclastics assigned to the Chinamba Hills Formation. Geochemical analysis and field relations of the Kgwebe Formation volcanics indicate that the sequences were accumulated in a continental rift basin. Rifting was initiated by extensional tectonics associated with a continental collision along the Namaqua-Natal Belt. The Kgwebe Formation volcanics started to accumulate during Mesoproterozoic times about 1 106 Ma ago. The depositional basin developed from fluvial and lacustrine systems during the accumulation of the Kgwebe Formation, into an extended shallow marine basin that accommodated the Kuke, Ngwako Pan, D'kar, and Mamuno Formations of the Ghanzi Group. Stratabound copper-sulfide mineralisation was developed at a redox interface within the Ghanzi Group. Mineralisation is confined to the basal part of the grey-green, argillitic facies of the D'kar Formation and is either disseminated or is found along cleavage planes and fractures. Minor Cu mineralisation also occurs in carbonates of the Chinamba Hills Formation. Tectonic deformation during the Pan African Damaran Orogeny resulted in a fold and thrust belt known as the Ghanzi-Chobe Belt.

Twin magma source geneses of bimodal volcanic rocks in continental rifts, Se China: Constraints on elemental and Nd-Pb-isotopic studies

twin magma source model of bimodal volcanic suites could be a common model of volcanic lithogeneses in continental rifts where are located plateform margins.

A Neoproterozoic supra-subduction terrane in northern Eritrea, NE Africa

The western part of a tectonic block in the complex Pan-African architecture of northern Eritrea contains partly re-equilibrated low- T , high- P assemblages (550°C, 14.5 kbar), basalts with oceanic geochemistry and a major olistostrome containing lenses of serpentinite. These features demonstrate subduction zone products. East of this accretionary wedge the block is dominated by island-arc volcanic and volcaniclastic rocks, and various calc-alkaline plutons. A bimodal volcanic suite of primitive andesitic basalts and evolved dacites and ignimbrites represents early submarine arc development and emergent volcanism, respectively. Intense, polyphase deformation culminated in sinistral transpression with crustal-scale imbrication between several east-verging thrusts. A tectonically bounded unit in the internal part of this arc association also contains relict high- P assemblages (650°C, 10–13 kbar) in oceanic basalts, which are possibly products of tectonic underplating as the arc developed. Oceanwards collapse of the tectonically inflating arc may explain formation of evolved, subaerial volcanic rocks within small extensional basins. Down-to-west extensional structures may have been inverted during terrane accretion to act as east-verging shear zones during late thickening. The area remains to be correlated with those areas in Arabia that were juxtaposed with it before Red Sea opening.

Metallogenic evolution of convergent margins: Selected ore deposit models

Metallogenic patterns in convergent margin arcs are related to thickness of the arc crust and its petrologic evolution, which are themselves linked by a positive correlation between average silica content of arc rocks and average thickness of arc crust. In general, tin, mercury, antimony and fluorine are most abundant in areas of thickest crust. Although the composition of ore deposits is strongly controlled by deep crustal and mantle features, the actual form and type of many ore deposits is controlled by upper crustal features, particularly the relative abundance of carbonate and silicate country rock. For instance, epithermal precious metal deposits are commonly associated with subaerial, silicic volcanism, which generally forms in the later stages of arc evolution. Deposits of this type will be absent if arc volcanism is terminated prematurely or removed if subduction of thickened or hot oceanic crust causes extreme uplift and consequent erosion of the upper parts of the arc. Similarly, chimney-manto limestone replacement lead–zinc–silver deposits are common only in terranes with extensive thick limestone units, which are commonly formed in shallow shelf environments. Several important types of convergent margin deposits remain poorly understood. Acid-sulfate deposits in primitive oceanic volcanic arcs are associated with bimodal volcanic suites that appear to have formed at an early stage in the evolution of the arc. Alkaline porphyry copper deposits are closely associated with calc-alkaline porphyry copper deposits, commonly in the same volcanic arc, although they appear to have formed farther back from the trench or during periods of oblique subduction or arc reversal. Massive iron oxide deposits and related base and precious metal mineralization are closely associated with silicic magmas that form during later stages of arc evolution. In addition to tectonic and petrologic factors, metallogenic patterns probably reflect global forcing mechanisms, the most important of which are global seawater anoxia and plate reorganizations. Global anoxia produces deep water environments that favor sedex mineralization and can promote subsurface ore deposition where anoxic bottom waters recharge underlying aquifers. Plate reorganizations can lead to ore deposition by providing extensional environments in the upper crust and possibly by facilitating partial melting of oceanic crust that has been modified by earlier subducting slabs.

Geochemistry of hydrothermal alteration at the Deri massive sulphide deposit, Sirohi district, Rajasthan, NW India

The 1.1 Ga volcanogenic massive sulphide deposit at Deri in the Sirohi district of south Rajasthan occurs within a bimodal volcanic suite of tholeiites and rhyolites, with minor amounts of andesites and tourmaline-bearing chert, interlayered with arkosic sediments. The ores and the enclosing rocks have undergone superposed deformation and polymetamorphism initially under amphibolite facies conditions and later under hornblende hornfels facies conditions. Metamorphism, however, has not affected the bulk composition of the rocks to any significant degree. Three distinct semiconformable alteration facies, characterized by their conspicuous magnesian mineralogy, are recognized in the host rocks: (1) hornblende-biotite-plagioclase-quartz schist (AMV); (2) cordierite-anthophyllite-chlorite hornfels (AFV); and (3) biotite-chlorite(-sericite) schist/hornfels (BCS). The first is derived from the mafic volcanics, whereas the other two represent progressive alteration of felsic volcanic protoliths. Fe, Mg and water were added and Na was removed from all the alteration facies in varying amounts. The maximum enrichment is noted in BCS for Mg and Fe, whereas the maximum depletion is seen in this facies for Si, an element which is also depleted significantly in AFV. AMV on the other hand, shows enrichment of Si, Ca and to some extent, in Al. Alumina is also enriched considerably in BCS, probably due to clayey alteration and extreme leaching of silica. Amongst the trace elements, Rb, Ba, Nb and Y are gained in most of the facies, except in BCS, where Ba and Y show distinct depletion. The LREE, from La to Sm, were enriched about 1.5- to 3.0-fold in all the facies with a maximum in AFV where the flux took place at constant inter-REE proportions: 1.0 La, 0.79 Ce, 0.48 Nd and 0.35 Sm. Eu was depleted from both felsic facies, 7-fold in BCS to 4-fold in AFV, during alteration. The HREE (Er to Lu) remained immobile in all the altered facies. The chemical and mineralogical zonation in the alteration facies are interpreted to be due to the progressive reaction of an evolving sea-water hydrothermal fluid with the bimodal volcanic protoliths during convective circulation. Fluid-rock interaction, guided by vertical and lateral thermal gradients, produced a sericite-quartz assemblage in the felsic volcanics at the expense of feldspar during the initial stages (175 °C) which formed a sericite-chlorite zone upon rising temperature (200–250 °C) by base-fixing reactions. A further temperature increase (to ~ 300 °C) and deeper circulation in the mafic pile introduced more Fe and Mg, thereby transforming the previously formed assemblage to a nearly pure chloritic zone and the most intensely altered biotite-chlorite(-sericite) facies.