Tectonic evolution of Cretaceous extensional basins in Zhejiang Province,eastern South China: structural and geochronological constraints

Widespread Cretaceous volcanic basins are common in eastern South China and are crucial to understanding how the Circum-Pacific and Tethyan plate boundaries evolved and interacted with one another in controlling the tectonic evolution of South China. Lithostratigraphic units in these basins are grouped, in ascending order, into the Early Cretaceous volcanic suite (K_1V), the Yongkang Group (K_1-2), and the Jinqu Group (K₂). SHRIMP U-Pb zircon geochronological results indicate that (1) the Early Cretaceous volcanic suite (K_1V) erupted at 136–129 Ma, (2) the Yongkang Group (K_1-2) was deposited from 129 Ma to 91 Ma, and (3) the deposition of the Jinqu Group (K₂) post-dated 91 Ma. Structural analyses of fault-slip data from these rock units delineate a four-stage tectonic evolution of the basins during Cretaceous to Palaeogene time. The first stage (Early to middle Cretaceous time, 136–91 Ma) was dominated by NW–SE extension, as manifested by voluminous volcanism, initial opening of NE-trending basins, and deposition of the Yongkang Group. This extension was followed during Late Cretaceous time by NW–SE compression that inverted previous rift basins. During the third stage in Late Cretaceous time, possibly since 78.5 Ma, the tectonic stress changed to N–S extension, which led to basin opening and deposition of the Jinqu Group along E-trending faults. This extension probably lasted until early Palaeogene time and was terminated by the latest NE–SW compressional deformation that caused basin inversion again. Geodynamically, the NW–SE-oriented stress fields were associated with plate kinematics along the Circum-Pacific plate boundary, and the extension–compression alternation is interpreted as resulting from variations of the subducted slab dynamics. A drastic change in the tectonic stress field from NW–SE to N–S implies that the Pacific subduction-dominated back-arc extension and shortening were completed in the Late Cretaceous, and simultaneously, that Neo-Tethyan subduction became dominant and exerted a new force on South China. The ongoing Neo-Tethyan subduction might provide plausible geodynamic interpretations for the Late Cretaceous N–S extension-dominated basin rifting, and the subsequent Cenozoic India–Asia collision might explain the early Palaeogene NE–SW compression-dominated basin inversion.     

Subduction cycling of volatiles and trace elements through the Central American volcanic arc: evidence from melt inclusions

Compositions of melt inclusions in olivine (Fo_90-64) from 11 localities in Guatemala, Nicaragua and Cost Rica along the Central American Volcanic Arc are used to constrain combined systematics of major and trace elements and volatile components (H₂O, S, Cl, F) in parental melts and to estimate volcanic fluxes of volatile elements. The melt inclusions cover the entire range of compositions reported for whole rocks from Central America. They point to large heterogeneity of magma sources on local and regional scales, related to variable contributions of diverse crustal (from the subducting and overriding plates) and mantle (from the wedge and incoming plate) components involved in magma genesis. Water in parental melts correlates inversely with Ti, Y and Na and positively with Ba/La and B/La (with the exception of Irazú Volcano), which indicates mantle melting fluxed by Ba-, B- and H₂O-rich, possibly, serpentinite-derived fluid beneath most parts of the arc. Different components with melt-like characteristics (high LREE, La/Nb and probably also Cl, S and F and low Ba/La) control the geochemical peculiarities of Guatemalan and Costa Rican magmas. The composition of parental magmas together with published data on volcanic volumes and total SO₂ flux from satellite measurements are used to constrain fluxes of volatile components and to estimate total magmatic flux in Central America. We found that volcanic flux accounts for only 13% of total magmatic and volatile fluxes. The remaining 87% of magmas remained in the lithosphere to form cumulates (∼39%) and intrusives (∼48%). The intrusive fraction of magmatic flux may be significantly larger beneath Nicaragua compared to Costa Rica. Interestingly, total fluxes of magmas and volatiles in Central America are quite similar to the global average estimates. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Granitoid emplacement by multiple sheeting during Variscan dextral transpression: The Saint-Laurent – La Jonquera pluton (Eastern Pyrenees)

The structural study of the Saint-Laurent – La Jonquera pluton (Eastern Pyrenees), a Variscan composite laccolithic intrusion emplaced in metasedimentary and gneissic rocks of the Roc de Frausa dome, by means of the anisotropy of magnetic susceptibility (AMS) technique has allowed the determination of the nature and orientation of its magmatic fabrics. The magmatic foliation has a predominant NE–SW strike and the mean lineation is also NE–SW trending with a shallow plunge. A strain gradient is measured so that the tonalites to granodiorites that form the basal parts of the pluton, and are intruded into amphibolite-facies metamorphic rocks, recorded the highest anisotropies, whereas the monzogranites and leucogranites, emplaced into upper crustal, low-grade metamorphic rocks, are weakly deformed. These results point to the synkinematic sequential emplacement of multiple granitoid sheets, from less to more differentiated magmatic stages, during the Late Carboniferous D₂ event characterized by an E–W-trending dextral transpression. The magmatic foliation appears locally disturbed by the effects of two tectonic events. The first of them (D₃) produced mylonitization of granitoids along NW–SE retrograding shear zones and open folds in the host Ediacaran metasediments of the Roc de Frausa massif, likely during late Variscan times. Interference between D₂ and D₃ structures was responsible for the dome geometry of the whole Roc de Frausa massif. The second and last perturbation consisted of local southward tilting of the granitoids coupled to the Mesozoic–Cenozoic cover during the Alpine.     

Mineralogy,geochemistry and petrogenesis of Kurile island-arc basalts

Whole-rock (major- and trace-element) and mineral chemical data are presented for basaltic rocks from the main evolutionary stages of the Kurile island arc, NW Pacific. An outer, inactive arc contains a Cretaceous-Lower Tertiary sequence of tholeiitic, calcalkaline and shoshonitic basalts. The main arc (Miocene-Quaternary) is dominated by weakly tholeiitic, with lesser, alkalic basalts. The mineralogy of Kuriles basalts is characterised by An-rich plagioclases, a continuous transition from chromites to titanomagnetites, pyroxenes with low Fe³⁺ contents and without strong Fe-enrichment, abundance of groundmass pigeonites and the absence of amphiboles. There is an increase in K₂O contents both along-arc (northwards) and towards the reararc side. The basalts show an exceptionally wide but continuous range of K₂O contents (0.1–4.7%) which correlate with other LIL element contents. Tholeiitic basalts with low LIL element contents, La/Yb and Th/U, but high K/ Rb, P₂O₅/La and Zr/Nb were derived from depleted, lherzolitic mantle which had suffered fluid metasomatism by K, Rb, Cs, Sr, Ba, Pb and H₂O only. Alkali basalts are also thought to be derived from depleted mantle but melt metasomatism involved addition of all LIL elements to a garnet lherzolite mantle. The Kuriles basalts and their mantle sources range continuously between these two end-member compositions. The metasomatic fluids/melts were probably released by early dehydration and later melting within subducted oceanic lithosphere though the process is not adequately constrained.     

Petrology and geochemistry of greywackes from Goa-Dharwar sector,western Dharwar Craton: Implications for volcanoclastic origin

Late Archaean Supracrustals of the Goa-Dharwar sector (GDS) are composed of a thick sequence of greywacke sequence with narrow intercalations of quartzite, BIF and carbonates. Mafic volcanics occupy the base of the sequence. The greywackes are predominantly tuffacious containing chlorite-sericite and hornblende. Arkosic variations containing biotite dominate the western part of the sector. Fine-grained variations occur as isolated narrow lenses within other types of greywackes. The conglomeratic greywackes are localized along the western and the eastern margins of the sector. All of the greywackes are all typically immature containing coarser clasts of mostly plagioclase (18–23%) and quartz (32–34%). Lithic fragments of felsic volcanic rocks are common. The matrix is dominated by mafic material. Biotite and amphibole are related to metamorphic recrystallization. Chlorite, sericite, epidote, carbonate and chert are products of the interplay of diagenesis and low-grade metamorphism. Fe-Ti oxide, sphene, apatite and zircon are usual accessories. But for slight enrichment in K₂O, the major element chemistry of the GDS greywackes is similar to the chemistry of Late Archaean greywackes. They also compare in respect of V, Co, Hf contents, K₂O/Na₂O, SiO₂/Al₂O₃, Na₂O/Al₂O₃, Ba/Rb, Th/U, La/Th, Sm/Nd ratios, steep REE patterns with distinct LREE enrichment and HREE depletion. The GDS greywackes however are distinctly enriched in Rb, Ba, Sr, Th, U, Cu, Zr, Ce/Ce* and depleted in Cr, Ni, and Zn. The conglomeritic and biotite bearing verities contain considerable proportions of clasts derived from the basement tonalitic/granitic terrain. The common tuffacious greywackes containing hornblende and biotite-sericite however include only volcanic clasts and bear evidence of derivation from submarine weathering of predominantly felsic volcanics erupted on a large scale to form a magmatic arc in the later stages of geosynclinal deposition. Geochemical data suggest that the GDS greywackes were laid down in progressively changing basin geometry from a passive to active continental margin and island arc setting.     

Fault geometries from the space distribution of the 1990–1997 Sannio–Benevento earthquakes: inferences on the active deformation in Southern Apennines

In this study, we analyze the recent (1990–1997) seismicity that affected the northern sector (Sannio–Benevento area) of the Southern Apennines chain. We applied the Best Estimate Method (BEM), which collapses hypocentral clouds, to the events of low energy (M_{d max}=4.1) seismic sequences in order to constrain the location and geometry of the seismogenetic structures. The results indicate that earthquakes aligned along three main structures: two sub-parallel structures striking NW–SE (1990–1992, Benevento sequence) and one structure striking NE–SW (1997, Sannio sequence). The southernmost NW–SE structure, which dips towards NE, overlies the fault that is likely to be responsible for a larger historical earthquake (I_{o max}=XI MCS, 1688 earthquake). The northernmost NW–SE striking structure dips towards SW. The NE–SW striking structure is sub-vertical and it is located at the northern tip of the fault segment supposed to be responsible for the 1688 earthquake. The spatio-temporal evolution of the 1990–1997 seismicity indicates a progressive migration from SE (Benevento) to NW (Sannio) associated to a deepening of hypocenters (i.e., from about 5 to 12 km). Hypocenters cluster at the interface between the major structural discontinuities (e.g., pre-existing thrust surfaces) or within higher rigidity layers (e.g., the Apulia carbonates). Available focal mechanisms from earthquakes occurred on the recognized NW–SE and NE–SW faults are consistent with dip-slip normal solutions. This evidences the occurrence of coexisting NW–SE and NE–SW extensions in Southern Apennines.     

Anisotropy of magnetic susceptibility (AMS) studies of Campanian–Maastrichtian sediments of Ariyalur Group,Cauvery Basin,Tamil Nadu,India: An appraisal to Paleocurrent directions

Oriented samples of sediments from Ariyalur Group, Cauvery Basin, south India, were studied for low field anisotropy of magnetic susceptibility (AMS) measurements to unravel the magnetic fabrics and paleocurrent directions. The results of AMS parameters of the sediments indicate primary depositional fabrics for Sillakkudi, Ottakovil and Kallamedu sandstone formations and secondary fabric for Kallankurichchi limestone formation. The obtained low degree of anisotropy (P _j ), oblate shape AMS ellipsoid and distribution of maximum (K ₁) and minimum (K ₃) susceptibility axes on equal area projection confirm the primary sedimentary fabric for Sillakkudi, Ottakovil and Kallamedu Formations. In the case of ferruginous, lower arenaceous, Gryphaea limestone and upper arenaceous limestone beds of Kallankurichchi Formation have recorded more than one fabric. The observed AMS parameters like shape factor (T) (prolate to oblate), q value and random distribution of minimum (K ₃) and maximum (K ₁) susceptibility axes are supported for secondary fabrics in Kallankurichchi Formation as a result of post-depositional processes. Based on petrographic studies, it can be established that K ₁ AMS axis of biotite mineral could represent the flow direction. The established paleocurrent direction for Sillakkudi is NW–SE direction while Ottakovil and Kallamedu Formations recorded NE–SW direction. Overall the paleoflow directions observed for Ariyalur Group is NE–SW to NW–SE.     

Petrology and geochemistry of the Juzzak Sill,western Chagai arc (Pakistan): implications for petrogenesis and emplacement

The Juzzak Sill occurs in the western part of the east-west trending, subduction-related magmatic belt known as the Chagai arc. The sill is concordantly emplaced in the Paleocene Juzzak Formation and locally cross-cuts the Early to Middle Eocene Robat Limestone and Eocene Saindak Formation. The sill is a porphyritic pyroxene diorite that grades into a porphyritic andesite (60.12–61.57 wt% SiO₂) along the chilled margins. It comprises phenocrysts of hypersthene and plagioclase (An_32–45) in a medium- to fine-grained groundmass of these minerals, opaque oxide, and apatite. The rocks are high-K (2.37–2.86 wt% K₂O) calc-alkaline with low Mg# (42–55), Cr (51–80 ppm), and Ni (22–30 ppm) contents. Mantle-normalized trace element patterns, exhibited by marked negative Nb anomalies and positive spikes for Sr, Rb, and Zr and are akin to island arc signatures. The relatively higher ratios of Zr/Y (3.57–6.58), Ti/V (46.05–54.36), Ta/Yb (0.14–0.15), and Th/Yb (2.56–2.65) and high ⁸⁷Sr/⁸⁶Sr ratio (0.70524) suggest the role of continental crust materials, thus implying continental margin-type arc affinity. The source diagnostic ratios including K/Ba, P/Zr, and La/Ce of Juzzak Sill andesite and Eocene andesite from the Chagai arc are more or less similar, but the former has a much higher K/Y and Ba/Y ratios, which suggests assimilations of the host sediments during intrusion.     

The Dir-Utror metavolcanic sequence,Kohistan arc terrane,northern Pakistan

The Dir-Utror volcanic series forms a NE–SW trending belt within the northwestern portion of the Kohistan island arc terrane in the western Himalayas of northern Pakistan. The Kohistan arc terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan and Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dominated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocryst in mafic to intermediate rocks, K-feldspar and quartz phenocrysts predominate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolite are the most common metamorphic phases; blue-green amphibole, andesine, muscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespread but less abundant. Phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote-amphibolite facies conditions attained locally.Whole rock major element compositions define a calc-alkaline trend: CaO, FeO, MgO, TiO₂, Al₂O₃, V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORB-normalized trace element concentrations show enrichment of the low-field strength incompatible elements (Ce, La, Ba, Rb, K) and deep negative Nb, P, and Ti anomalies—patterns typical of subduction related magmas. Mafic volcanic rocks plot in fields for calc-alkaline volcanics on trace element discrimination diagrams, showing that pre-existing oceanic crust is not preserved here. All rocks are LREE-enriched, with La=16–112×chondrite, La/Lu=2.6–9.8×chondrite, and Eu/Eu*=0.5–0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu* ratios, reflecting the dominant role of plagioclase fractionation in their formation. Some andesites have La/Lu ratios which are too high to result from fractionation of the more mafic lavas; chondrite-normalized REE patterns for these andesites cross those of the basaltic andesites, indicating that these lavas cannot be related to a common parent.The high proportion of mafic lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is mafic relative to normal continental margins. Andesites with high La, La/Lu, K₂O, and Rb may be crustal melts; we suggest that garnet-rich high-pressure granulites similar to those exposed in the Jijal complex may be restites formed during partial melting of the crust.     

Oligocene–Miocene thrusting in central Aegean: insights from the Cycladic island of Amorgos

In the central Aegean, the Cycladic island of Amorgos consists of two high‐pressure (HP) units, the marble‐rich Amorgos unit, which is correlated to the Mesozoic ‘cover’ sequence of the Menderes Massif, and the Cycladic Blueschist unit. New structural data show that the deformation history of the Amorgos HP‐rocks was principally governed by early Oligocene (or late Eocene)–early Miocene ductile to brittle thrusting (D₁–D₃) followed by middle–late Miocene oblique contractional movements (D₄–D₅). The D₁ phase caused syn‐blueschist‐facies ductile thrusting of the Cycladic Blueschist unit over the Amorgos unit, with ambiguous kinematics. Progressive deformation under continuous NW–SE compression produced a sequence of imbricate NW‐directed thrusts (D_2/3) characterized by a stratification of fault‐related rocks, with mylonitic zones (D₂) giving way downwards to cataclastic zones (D₃). Ductile D₂ thrusting synchronous to greenschist‐facies retrogression, was accompanied by mega‐sheath folding during constrictional and general shear deformation. Brittle D₃ thrusting was associated with NW‐verging F₃ folds trending at a high‐angle to the transport direction. Orthogonal contraction gave way to transpression during which the compression orientation changed from NW–SE (D₄) to NE–SW (D₅). Back‐arc related NW–SE pure extension (D₆) seems to have been established in post‐late Miocene times and related high‐angle normal faulting affected HP‐rocks only after they had already reached the uppermost crustal levels. Oligocene–early Miocene deformation history is interpreted to indicate syn‐compressional exhumation of HP‐rocks possibly in an extrusion wedge. In this case, Amorgos HP‐rocks should have occupied the base of the extrusion wedge. Copyright © 2011 John Wiley & Sons, Ltd.     

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015 ± 4.4 Ma, 966.5 ± 3.5 and 808 ± 3.1 respectively. The plagiogranite shows high SiO₂, Na₂O and extremely low K₂O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO₂ and moderate Na₂O, together with high K₂O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO₂ moderately higher Na₂O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.     

5.9 Mw, 18th June 2010 earthquake and fault segment linkage at Andaman – A study based on macroseismic survey,GPS geodesy and Coulomb stress changes

18th June, 2010 5.9 M_w earthquake at North Andaman triggered along NW–SE pre-existing fault with reverse fault mechanism. Macroseismic survey and GPS geodesy reveal maximum damages following NE–SW trend due to normal fault mechanism. Coulomb stress modeling for post- and inter-seismic earthquakes after the 2004 mega-earthquake show different stages of fault segment linkage at North Andaman. The present earthquake has been explained as co-shock due to asiesmic soft linkage of fault propagation.     

Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input

The sedimentary section (at DSDP Site 495) on the subducting Cocos Plate has large stratigraphic changes in incompatible elements and element ratios, the result of early carbonate deposition followed by late hemipelagic deposition. Lavas from Central America define both local and regional geochemical trends that reflect the strong influence of the two Cocos Plate sediment units. Element ratios with large stratigraphic variations on the Cocos Plate (e.g. Ba/Th, U/La) define local variations within individual volcanic centers in Central America, indicating that marine stratigraphy controls some geochemical characteristics of the lavas. These local trends can be explained by changing the proportions of hemipelagic sediment input into the magma generation process. These local trends are observed in all the segments of the arc, regardless of the intensity of the slab signature. Regional variations are most clearly seen in element ratios that are nearly constant through the Cocos Plate sediment stratigraphy (e.g. Ba/La, U/Th), suggesting that regional variations reflect differences in the intensity of the flux from the subducting slab. The slab signal is strongest in Nicaragua and along the volcanic front. The signal decreases to the northwest and southeast of Nicaragua and toward the back arc. The large slab signature in the lavas from western Nicaragua occurs in the area with the thinnest continental crust and steepest dip of the slab. The mass flux of incompatible elements into the system is easily estimated, except for elements, like Pb, that have high and variable abundances in the basaltic oceanic crust section. The mass flux of elements out of the system depends on eruption rates, which are variable along the arc and only approximately known. Comparison of input and output fluxes for five different segments of the arc reveals that some elements (K, B, Cs, and Rb) are very efficiently delivered to the volcanoes from the subducted slab. Other elements (Sr, Ba, and U) are returned to the surface with moderate efficiency, whereas some elements (REEs) may come mostly from the mantle wedge with minor slab contribution. The relative order of recycling efficiencies of incompatible elements implies that a hydrous fluid dominates the transfer of material from the slab to mantle. Received: 7 September 1998 / Accepted: 30 September 1999     

Meso-Cenozoic tectonic evolution of the Dangyang Basin,north-central Yangtze craton,central China

Based on detailed structural data and available tectonic chronological data from the Dangyang Basin, the authors propose that the north-central Yangtze craton experienced three stages of tectonic evolution since Late Triassic time. In the Late Triassic to Early Jurassic (T₃–J₁), due to the Indosinian orogeny, nearly N–S compression and shortening occurred, which initiated the Dangyang Basin as a foreland basin of the Qinling–Dabie orogen. During the Late Jurassic–Early Cretaceous (J₃–K₁) period, the Yanshanian intracontinental orogeny caused contemporaneous NE–SW and NW–SE shortening, which resulted in intense folding of the foreland basin; contraction formed a brush structure diverging in a SE direction and strongly converging in a NW direction around the Huangling anticline. In the Late Cretaceous to Palaeogene, the Yuan'an and Hanshui grabens were separated from other parts of the Dangyang Basin due to post-orogenic ENE–WSW extension. Finally, at the end of the Palaeogene, ENE–WSW shortening led to inversion and deformation of the grabens.     

Geochemistry,petrogenesis and tectonic significance of the Newer Dolerites from the Singhbhum Orissa craton,eastern Indian shield

The Singhbhum Orissa craton, eastern India contains rocks as old as 3.6 Ga. The Newer Dolerites occur in two distinct orientations (NE/SW and NW/SE) in the Singhbhum Granitoid Complex (SBGC). These dikes are mostly tholeiites and quartz-normative dolerites associated with subordinate norites. We recognize three geochemical groups of the Newer Dolerites that were emplaced in the SBGC. Group I dikes contain lower SiO₂ ( < 53.29%) and higher Mg #, Ni and Cr than group II dikes. Group III dikes have higher SiO₂ than groups I and II. A few investigated samples show boninitic geochemical features. They have high-MgO (>8%), high-SiO₂ (>52%) and low-TiO₂ ( ≤ 0.5%) bulk-rock compositions. The main feature of the Newer Dolerite spidergrams is enrichment in the large-ion lithophile elements (LILE, e.g. Rb, K and Ba) relative to high field-strength elements (HFSE), resulting in high LILE/HFSE ratios. These geochemical characteristics suggest that the Newer Dolerites are subduction related. High La/Ta ratios (21–66) support a non-plume source. Therefore, we conclude that the Newer Dolerites show geochemical signatures similar to those of back-arc basalts.     

Structural evolution of the Irtysh Shear Zone (northwestern China) and implications for the amalgamation of arc systems in the Central Asian Orogenic Belt

The NW–SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D₁ is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D₂ is represented by a shallowly dipping fabric and related ∼ NW–SE stretching lineations oriented sub-parallel to the strike of the orogen. D₂ foliations are folded by ∼ NW–SE folds (F₃) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D₃). Two samples of mica schists yielded youngest detrital zircon peaks at ∼322 Ma, placing a maximum constraint on the timing of D₁–D₃ deformation. A ∼ NE–SW granitic dyke swarm (∼252 Ma) crosscuts D₃ fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D₃ transpressional deformation took place prior to ∼252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D₁–D₃ at ∼322–252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D₁), collapse (D₂), and transpressional deformation (D₃) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D₃), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic.     

3-D Mohr circle construction using vein orientation data from Gadag (southern India) – Implications to recognize fluid pressure fluctuation

In this paper orientations of quartz veins from the Archaean age lode-gold bearing region of Gadag (southern India) are used to determine the relative stress and fluid pressure (P_f) conditions by constructing 3-D Mohr circle. Anisotropy of magnetic susceptibility (AMS) analysis of the host massive metabasalt reveals that the magnetic foliation is NW–SE striking, which is related to early NE–SW compression (D1/D2 deformation) that affected the region. The quartz veins have a wide range of orientations, with NW–SE striking veins (steep northeasterly dips) being the most prominent. Vein emplacement is inferred to have taken place under NW–SE compression that is known to have caused late deformation (D3) in the region. It is argued that the NW–SE fabric defined the pre-existing anisotropy and channelized fluid flow during D3. The permeability was initially low, which resulted in high P_f (>σ₂). 3-D Mohr circle analysis indicates that the driving pressure ratio (R′) was 0.94, a condition that favoured fracturing and reactivation of fabric elements (foliations and fractures) having a wide range of orientations. This led to an increase in permeability and fluid flowed (burped) into the fractures. Resulting vein emplacement and sealing of fractures led to a reduction of P_f (<σ₂). It is argued that at this low P_f, NW–SE oriented structures continued to remain susceptible to reactivation and vein emplacement, while fractures of all other orientations were inactive and remained sealed. As a consequence, the study area has a cluster of NW–SE oriented veins. R′ is calculated to be 0.07 from 3-D Mohr circle analysis at low P_f, when fractures with NW–SE orientation only were susceptible to dilation. However, it is envisaged that any emplacement of veins in these fractures would have sealed them, thus reducing the permeability and initiating the next cycle of rise in P_f (>σ₂). Thus, it is concluded that the quartz veins in the Gadag region are a consequence of an interplay between conditions that fluctuated from P_f > σ₂ to P_f < σ₂.     

Deformation correlations,stress field switches and evolution of an orogenic intersection: The Pan-African Kaoko-Damara orogenic junction,Namibia

Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers are correlated across all tectono-metamorphic provinces in the Damara Orogenic System.Correlations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing,and switches between different deformation events.Horizontal principle compressive stress rotated clockwise ~180°in total during Kaoko Belt evolution,and~135° during Damara Belt evolution.At most stages,stress field variation is progressive and can be attributed to events within the Damara Orogenic System,caused by changes in relative trajectories of the interacting Rio De La Plata,Congo,and Kalahari Cratons.Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ~590 Ma,involving E-W directed shortening,progressing through different transpressional states with ~45° rotation of the stress field to strike-slip shear under NW-SE shortening at ~550-530 Ma.Damaran orogenesis evolved from collision at ~555-550 Ma with NW-SE directed shortening in common with the Kaoko Belt,and subsequently evolved through ~90°rotation of the stress field to NE-SW shortening at ~512-508 Ma.Both Kaoko and Damara orogenic fronts were operating at the same time,with all three cratons being coaxially convergent during the 550-530 Ma period;Rio De La Plata directed SE against the Congo Craton margin,and both together over-riding the Kalahari Craton margin also towards the SE.Progressive stress field rotation was punctuated by rapid and significant switches at ~530-525 Ma,~508 Ma and ~505 Ma.These three events included:(1)Culmination of main phase orogenesis in the Damara Belt,coinciding with maximum burial and peak metamorphism at 530-525 Ma.This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt.Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time.This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field,and from this time forward contraction across the Damara Belt generated the stress field governing subsequent low-strain events in the Kaoko Belt.(2)A sudden switch to E-W directed shortening at ~508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System,most plausibly from arc obduction along the orogenic margin of Gondwana(Ross-Delamerian Orogen).(3)This imposed stress field established a N-S extension direction exploited by decompression melts,switch to vertical shortening,and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ~505 Ma,producing an extensional metamorphic core complex across the Central Zone.     

钦-杭结合带南段新元古代俯冲作用:来自粤西贵子混杂岩变基性岩年代学和地球化学的证据

新元古代以来,扬子地块和华夏地块拼贴形成钦-杭结合带。粤西贵子混杂岩为揭示钦-杭结合带南段地质演化提供了一个重要的窗口。贵子混杂岩构造岩块为强烈变形-变质的变基性岩、深海硅质岩,基质为石英岩、云母片岩以及具鲍马序列富锰质硅泥质岩等。对混杂岩中变基性岩年代学和元素地球化学分析显示,锆石U-Pb年龄为948±11Ma,除少部分属低钾(拉斑)外,其余均为钙碱性系列,CIPW标准矿物成分属石英拉斑玄武岩-橄榄拉斑玄武岩组合。变基性岩具有高TiO_2(平均1.85%)、P_2O_5(平均0.21%)的特点;无Eu异常或轻微负异常,配分曲线右倾(球粒陨石标准化(La/Yb)N=2.65~3.89),大离子亲石元素(Rb、Ba、Th、U)富集,高场强元素(Nb、Zr、Hf)轻微亏损,表现为洋壳消减作用下岛弧岩浆岩特征。高的Nb(7.56×10~(-6)~16.11×10~(-6),均大于7×10~(-6))、Nb/U(18.39~25.65)、(Nb/La)N(0.76~0.94,原始地幔标准化)显示为岛弧环境富Nb玄武岩。本研究认为,云开地块出露的这套构造混杂岩,是新元古代古华南洋俯冲在钦-杭结合带南段的记录,可为该带的南东边界提供线索。     

Tectonic Imprints of the Hazara Kashmir Syntaxis on the Mesozoic Rocks Exposed in Munda,Mohmand Agency,Northwest Pakistan

Two well-developed mesoscopic folds, D_2 and D_3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D_2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D_3 folds, which developed during SE–NW shortening. Micro- to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D_3. The newly established NW–SE trending micro- to mesoscopic structures in Munda termed D_2, which postdated F_1/F_2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D_2 and D_3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D_2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D_2 and D_3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D_2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.