New geochronological data on volcanic rocks from northeast Sudan and their implication for crustal evolution

The basement volcano-sedimentary rocks of northeast Sudan form part of the Nubian Shield of northeast Africa. Volcanic rocks from the Kadawēb area yield Rb—Sr wholerock isochron ages of 718 and 722 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7027 and 0.7029. In the Homogar area, 150 km to the south, volcanic rocks yield a Rb—Sr whole-rock isochron age of 671 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7034. Although all of these lavas have been altered by a low-grade greenschist facies event, isotopic and geochemical evidence indicates limited open system behaviour. Thus these dates most probably represent extrusive ages indicating two episodes of volcanic activity during the evolution of the Nubian Shield. These results place some important constraints on the nature of crustal evolution in northeast Africa.     

Paleoproterozoic (2155–1970 Ma) evolution of the Guiana Shield (Transamazonian event) in the light of new paleomagnetic data from French Guiana

We present a comprehensive paleomagnetic study on Paleoproterozoic (2173–2060 Ma) plutonic and metamorphic rocks from French Guiana, representative of the full range of the main Transamazonian tectonothermal steps. Twenty-seven groups of directions and poles were obtained from combination of 102 sites (613 samples) based on age constraint, similar lithology and/or geographical proximity. Paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases. This is also reinforced by positive field tests (baked contact and reversal tests). Recent U/Pb and Pb/Pb on zircon and complementary ⁴⁰Ar/³⁹Ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages. Estimated magnetic ages, based on amphibole dating as a proxy, enable us to construct a Guiana Shield apparent polar wander path for the 2155–1970 Ma period. It is also possible to present paleolatidudinal evolution and continental drift rates related to specific Transamazonian tectonic regimes.French Guiana and probably the Guiana Shield were located at the Equator from ca. 2155 to 2130 Ma during the Meso-Rhyacian D1 magmatic accretion phase, related to subduction of Eorhyacian oceanic crust. After closure of the Eorhyacian Ocean and collision of West African and Amazonian plates, the Guiana Shield moved. The first evolution towards 60° latitude, occurs after 2080 Ma, during the Neorhyacian D2a post collisional sinistral transcurrent phase. During the Late Rhyacian D2b phase, up to 2050 Ma, the Guiana Shield reaches the pole and starts to move to lower latitudes on an opposite meridian. By the Orosirian D2c phase, from ca. 2050 to 1970 Ma, the Guiana Shield reaches the Equator.Based on the amphibole ⁴⁰Ar/³⁹Ar dates, we estimate the continental drift between 12 and 16 cm/y for the Meso to Late Rhyacian period followed by a lower rate between 9 and 14 cm/y up to Orosirian time. This study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100–200 Ma time period. Our results confirm the possibility of APWP construction on Paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi-method radiometric dating.     

Precadomian relicts in the Armorican Massif: Their age and role in the evolution of the western and central European Cadomian-Hercynian belt

Well-dated Precambrian is mostly developed in the north of the Armorican Massif, in which area voluminous Cadomian magmatism is dated at between 650 and 550 Ma. Much older relicts occur at Cap de la Hague, in Guernsey and in the Tregor, and are also found in the northern continental margin of the Iberian Peninsula. In all these occurrences, whole-rock systems have been opened, so that the true ages cannot be determined by the Rb-Sr whole rock isochron method. Four U-Pb zircon ages are between 1.8 and 2 Ga (in Guernsey: Icart orthogneisses; in Tregor: Port Beni, Trebeurden, Morguignen orthogneisses).There is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 Ma to 270 Ma ago. On the contrary, the evolution of initial ⁸⁷Sr/⁸⁶Sr ratios with time shows that the observed mid- and west European continental crust is probably not older than 700 Ma. The increase of the initial ⁸⁷Sr/⁸⁶Sr ratios of the magmas with time suggests that, after its formation in Cadomian times, this segment of continental crust evolved virtually as a closed system and Hercynian magmatism arose principally from re-melting of relatively young sialic components.     

Tectonomagmatic setting and provenance of the Santa Marta Schists,northern Colombia: Insights on the growth and approach of Cretaceous Caribbean oceanic terranes to the South American continent

Metamorphosed volcano-sedimentary rocks accreted to the northern South American continental margin are major vestiges of the Caribbean oceanic plate evolution and its interactions with the continent. Selected whole rock geochemistry, Nd–Sr isotopes and detrital zircon geochronology were obtained in metabasic and metasedimentary rocks from the Santa Marta and San Lorenzo Schists in northernmost Colombia. Trace element patterns are characterized by primitive island arc and MORB signatures. Similarly initial ⁸⁷Sr/⁸⁶Sr-ε_Nd isotopic relations correlate with oceanic arcs and MORB reservoirs, suggesting that the protoliths were formed within a back-arc setting or at the transition between the inta-oceanic arc and the Caribbean oceanic crust. Trace element trends from associated metasedimentary rocks show that the provenance was controlled by a volcanic arc and a sialic continental domain, whereas detrital U/Pb zircons from the Santa Marta Schists and adjacent southeastern metamorphic units show Late Cretaceous and older Mesozoic, Late Paleozoic and Mesoproterozoic sources. Comparison with continental inland basins suggests that this arc-basin is allocthonous to its current position, and was still active by ca. 82 Ma. The geological features are comparable to other arc remnants found in northeastern Colombia and the Netherland Antilles. The geochemical and U/Pb detrital signatures from the metasedimentary rocks suggest that this tectonic domain was already in proximity to the continental margin, in a configuration similar to the modern Antilles or the Kermadec arc in the Pacific. The older continental detritus were derived from the ongoing Andean uplift feeding the intra-oceanic tectonic environment. Cross-cutting relations with granitoids and metamorphic ages suggest that metamorphism was completed by ca. 65 Ma.     

台湾玉里带变质岩LA-ICP-MS锆石U-Pb年龄及其地质意义

台湾玉里带位于台湾东部,其主体为变质砂岩、千枚状泥岩夹绿帘片岩、阳起片岩、浅变质枕状玄武岩等(原岩为海相火山-陆源沉积岩),其次为高压相的蓝闪片岩。火山岩属钙碱性系列,具低Ba、Nb、Sr和高Pb丰度,稀土元素总量普遍较低,具有轻稀土元素弱富集、Eu无亏损的稀土元素配分模式;(含火山碎屑)变质砂岩的LA-ICP-MS锆石U-Pb年龄表明,~(206)Pb/~(238)U表面年龄有6个峰值:最新年龄23.3Ma(古近纪)为火山碎屑锆石年龄,也是成岩年龄;中生代—古生代年龄有96.7Ma(晚白垩世)、124.2Ma及130.6Ma(早白垩世)、214.1~228Ma(晚三叠世)、283.7Ma和289Ma(早二叠世)等,锆石为次棱角状或次圆状,代表了源岩的岩浆年龄;~(207)Pb/~(206)Pb表面年龄出现古元古代峰值(1769~1852Ma)和新太古代峰值(2506Ma和2530Ma),均为磨圆的碎屑变质或岩浆锆石,代表了不明古元古代变质基底的年龄。玉里带变质岩原岩可能形成于古近纪(23.3Ma),与南海扩张的白云运动同时,属白云运动有关的火山作用的产物。     

Magmatic and tectonic history of Jurassic ophiolites and associated granitoids from the South Apuseni Mountains (Romania)

The Jurassic ophiolites in the South Apuseni Mountains represent remnants of the Neotethys Ocean and belong to the East Vardar ophiolites that contain ophiolite fragments as well as granitoids and volcanics with island-arc affinity. New U–Pb zircon ages, and Sr and Nd isotope ratios give insights into their tectono-magmatic history. The ophiolite lithologies show tholeiitic MOR-type affinities, but are occasionally slightly enriched in Th and U, and depleted in Nb, which indicates that they probably formed in a marginal or back-arc basin. These ophiolites are associated with calc-alkaline granitoids and volcanics, which show trace element signatures characteristic for subduction-enrichment (high LILE, low HFSE). Low ⁸⁷Sr/⁸⁶Sr ratios (0.703836–0.704550) and high ¹⁴³Nd/¹⁴⁴Nd ratios (0.512599–0.512616) of the calc-alkaline series overlap with the ratios measured in the ophiolitic rocks (0.703863–0.704303 and 0.512496–0.512673), and hence show no contamination with continental crust. This excludes a collisional to post-collisional origin of the granitoids and is consistent with the previously proposed intra-oceanic island arc setting. The new U–Pb ages of the ophiolite lithologies (158.9–155.9 Ma, Oxfordian to Early Kimmeridgian) and granitoids (158.6–152.9 Ma, latest Oxfordian to Late Kimmeridgian) indicate that the two distinct magmatic series evolved within a narrow time range. It is proposed that the ophiolites and island arc granitoids formed above a long-lived NE-dipping subduction zone. A sudden flip in subduction polarity led to collision between island arc and continental margin, immediately followed by obduction of the ophiolites and granitoids on top of the continental margin of the Dacia Mega-Unit. Since the granitoids lack crustal input, they must have intruded the Apuseni ophiolites before both magmatic sequences were obducted onto the continental margin. The age of the youngest granitoid (~153 Ma, Late Kimmeridgian) yields an estimate for the maximum age of emplacement of the South Apuseni ophiolites and associated granitoids onto the Dacia Mega-Unit.     

Geochemistry and geochronology of Early Mesozoic tholeiites from Central Morocco

From Central Morocco (Central High Atlas, Middle Atlas, Haute Moulouya) continental tholeiites were investigated geochemically and geochronologically. These tholeiites are intercalated within continental redbeds of the Early Mesozoic (Triassic-Liassic). The major, trace and rare earth element contents classify these volcanic rocks as basaltic to andesitic-basaltic, quartz-normative tholeiites. Some trace element ratios (e.g. Zr/Nb, Zr/ Y, Y/Nb, Ti/V) suffer a heterogeneous source with a composition similar to MORB (P- to N-type). The enriched LILE contents, the negative Nb anomaly and the inital⁸⁷Sr/⁸⁶Sr ratios (0.7064–0.7069) reveal the presence of a crustal component up to 13–17 wt%.⁸⁷Sr/⁸⁶Sr ratios of carbonate mineral separates from different lava flows show different cycles of alteration; however, the major and trace element chemistry together with Sr isotope evidence, indicate that the alteration phases are not submarine in origin.⁴⁰Ar/³⁹Ar age determinations on translucent plagioclase phenocrysts yield extrusion ages which range between 210.4 ± 2.1 Ma and 196.3 ± 1.2 Ma. These ages correspond to a stratigraphic period between the Norian (Rhaetian?) and the Upper Sinemurian.     

The Aguablanca Ni–(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex

The Aguablanca Ni–(Cu) sulfide deposit is hosted by a breccia pipe within a gabbro–diorite pluton. The deposit probably formed due to the disruption of a partially crystallized layered mafic complex at about 12–19 km depth and the subsequent emplacement of melts and breccias at shallow levels (<2 km). The ore-hosting breccias are interpreted as fragments of an ultramafic cumulate, which were transported to the near surface along with a molten sulfide melt. Phlogopite Ar–Ar ages are 341–332 Ma in the breccia pipe, and 338–334 Ma in the layered mafic complex, and are similar to recently reported U–Pb ages of the host Aguablanca Stock and other nearby calc-alkaline metaluminous intrusions (ca. 350–330 Ma). Ore deposition resulted from the combination of two critical factors, the emplacement of a layered mafic complex deep in the continental crust and the development of small dilational structures along transcrustal strike-slip faults that triggered the forceful intrusion of magmas to shallow levels. The emplacement of basaltic magmas in the lower middle crust was accompanied by major interaction with the host rocks, immiscibility of a sulfide melt, and the formation of a magma chamber with ultramafic cumulates and sulfide melt at the bottom and a vertically zoned mafic to intermediate magmas above. Dismembered bodies of mafic/ultramafic rocks thought to be parts of the complex crop out about 50 km southwest of the deposit in a tectonically uplifted block (Cortegana Igneous Complex, Aracena Massif). Reactivation of Variscan structures that merged at the depth of the mafic complex led to sequential extraction of melts, cumulates, and sulfide magma. Lithogeochemistry and Sr and Nd isotope data of the Aguablanca Stock reflect the mixing from two distinct reservoirs, i.e., an evolved siliciclastic middle-upper continental crust and a primitive tholeiitic melt. Crustal contamination in the deep magma chamber was so intense that orthopyroxene replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical evidence includes enrichment in SiO₂ and incompatible elements, and Sr and Nd isotope compositions (⁸⁷Sr/⁸⁶Sr_i 0.708–0.710; ¹⁴³Nd/¹⁴⁴Nd_i 0.512–0.513). However, rocks of the Cortegana Igneous Complex have low initial ⁸⁷Sr/⁸⁶Sr and high initial ¹⁴³Nd/¹⁴⁴Nd values suggesting contamination by lower crustal rocks. Comparison of the geochemical and geological features of igneous rocks in the Aguablanca deposit and the Cortegana Igneous Complex indicates that, although probably part of the same magmatic system, they are rather different and the rocks of the Cortegana Igneous Complex were not the direct source of the Aguablanca deposit. Crust–magma interaction was a complex process, and the generation of orebodies was controlled by local but highly variable factors. The model for the formation of the Aguablanca deposit presented in this study implies that dense sulfide melts can effectively travel long distances through the continental crust and that dilational zones within compressional belts can effectively focus such melt transport into shallow environments.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

SOVETSKAYA GEOLOGIYA (SOVIET GEOLOGY) IN COVER-TO-COVER TRANSLATION, 1960, NOS. 11 AND 12

The Río Negro-Juruena Province (RNJP) occupies a large portion of the western part of the Amazonian Craton and is a zone of complex granitization and migmatization. Regional metamorphism, in general, occurred in the upper amphibolite facies. The granites and gneisses of the RNJP yield Rb-Sr and Pb-Pb whole-rock isochron dates ranging from 1.8 Ga to 1.55 Ga, with initial ⁸⁷Sr/⁸⁶Sr ratios of ～ 0.703 and a single-stage model μ₁ value of ～ 8.1. In order to improve the geochronological control, SHRIMP U-Pb zircon ages, conventional U-Pb zircon ages, and additional Pb-Pb whole-rock isochron ages were determined for samples of granitoids and gneisses from the Papuri-Uaupés and Guaviare-Orinoco rivers areas (northern part of the province) and Jamari-Machado rivers and Pontes de Lacerda areas (southern part). The granitoids from the northern part of the province yield conventional U-Pb zircon ages of 1709 ± 17 Ma and 1521 ± 31 Ma, and SHRIMP U-Pb concordant zircon results of 1800 ± 18 Ma. Samples of gneissic rocks from the southern part of the RNJP yielded SHRIMP U-Pb concordant ages of 1750 ± 24 Ma and 1570 ± 17 Ma and a Pb-Pb whole-rock isochron age of 1717 ± 120 Ma. These new U-Pb and Pb-Pb results confirm the previous Rb-Sr and Pb-Pb geochronological evidence that the main magmatic episodes within the RNJP occurred between 1.8 and 1.55 Ga, and suggest that this crustal province constitutes a segment of continental crust newly added to the Amazonian Craton at the end of the Early Proterozoic. In the area of the RNJP, there are several anorogenic rapakivi-type granite plutons. Because of the absence of recognized Archean material within the basement rocks, it is reasonable to consider the Early to Middle Proterozoic continental crust as the magmatic source for the rapakivi granite intrusions.     

Formation of juvenile continental crust in the Arabian–Nubian shield: evidence from granitic rocks of the Nakasib suture, NE Sudan

Major and trace element data, U–Pb zircon ages, and initial isotopic compositions of Sr, Nd, and Pb are reported for ten granitic and one rhyolitic rock sample from the neo-Proterozoic Nakasib suture in NE Sudan. Chemical data indicate that the samples are medium- to high-K, "I-type" granitic rocks that mostly plot as "volcanic arc granites" on discriminant diagrams. Geochronologic data indicate that rifting occurred 790±2?Ma and constrain the time of deformation associated with suturing of the Gebeit and Haya terranes to have ended by approximately 740?Ma. Isotopic data show a limited range, with initial ⁸⁷Sr/⁸⁶Sr=0.7021 to 0.7032 (mean=0.7025), εNd(t) =+5.5 to +7.0 (mean=+6.4), and ²⁰⁶Pb/²⁰⁴Pb = 17.50–17.62. Neodymium model ages (T_DM; 0.69–0.85?Ga; mean = 0.76?Ga) are indistinguishable from crystallization ages (0.79–0.71?Ga; mean=0.76?Ga), and the isotopic data considered together indicate derivation from homogeneously depleted mantle. The geochronologic data indicate that the terrane accretion to form the Arabian–Nubian shield began just prior to 750?Ma. The isotopic data reinforces models for the generation of large volumes of juvenile continental crust during neo-Proterozoic time, probably at intra-oceanic convergent margins. The data also indicate that crust formation was associated with two cycles of incompatible element enrichment in granitic rocks, with an earlier cycle beginning approximately 870?Ma and culminating approximately 740?Ma, and the second cycle beginning after pervasive high-degree melts – possibly hot-spot related – were emplaced approximately 690–720?Ma.     

Hydrothermal alteration associated with gold mineralization in the southern Apuseni Mountains, Romania: preliminary Sr isotopic data

Precious-metal mineralization in the southern Apuseni Mountains of western Romania is hosted by mid-Miocene (∼14 Ma) andesitic stocks and lava flows. The mineralized veins are surrounded by aureoles of hydrothermal alteration, consisting of quartz, sericite, K-feldspar, pyrite and calcite. The alteration process caused a total homogenization of initial ⁸⁷Sr/⁸⁶Sr in the rocks. Ages determined for the hydrothermal alteration are 13.7–15.7 Ma, indicating that hydrothermal alteration immediately followed igneous activity. Furthermore, a large influx of radiogenic Sr took place during alteration, this Sr probably being derived from the hydrothermal leaching of continental meta-sedimentary rocks in the basement. Received: 5 December 1997 / Accepted: 26 February 1998     

Origin of two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton: implications for early Mesozoic lithosphere thinning

New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian–Early Triassic at ca. 255–245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO₂ and K₂O, moderate initial ⁸⁷Sr/⁸⁶Sr, low negative ε_Nd(t) and ε_Hf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust–mantle boundary in syncollisional to post-collisional/post-orogenic tectonic settings. The late rock suite, erupted during the Middle–Late Triassic at ca. 238–228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high Al₂O₃, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial ⁸⁷Sr/⁸⁶Sr; high negative ε_Nd(t) and ε_Hf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle–Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.     

Multi-proxy isotopic tracing of magmatic sources and crustal recycling in the Palaeozoic to Early Jurassic active margin of North-Western Gondwana

We trace source variations of active margin granitoids which crystallised intermittently over ~300 Ma in varying kinematic regimes, by combining zircon Lu-Hf isotopic data from Early Palaeozoic to Early Jurassic igneous and metaigneous rocks in the Mérida Andes, Venezuela and the Santander Massif, Colombia, with new whole rock Rb/Sr and Sm-Nd isotopic data, and quartz O isotopic data. These new data are unique in South America because they were obtained from discrete magmatic and metamorphic zircon populations, providing a high temporal resolution dataset, and compare several isotopic systems on the same samples. Collectively, these data provide valuable insight into the evolution of the isotopic structure of the continental crust in long-lived active margins.Phanerozoic active margin-related granitoids in the Mérida Andes and the Santander Massif yield zircon Lu-Hf model ages ranging between 0.77 Ga and 1.57 Ga which clearly define temporal trends that can be correlated with changes in tectonic regimes. The oldest Lu-Hf model ages of >1.3 Ga are restricted to granitoids which formed during Barrovian metamorphism and crustal thickening between ~499 Ma and ~473 Ma. These granitoids yield high initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that evolved, Rb-rich middle to upper crust was the major source of melt. Granitoids and rhyolites which crystallised during subsequent extension between ~472 Ma and ~452 Ma yield younger Lu-Hf model ages of 0.80 Ga–1.3 Ga and low initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that they were derived from much more juvenile, Rb-poor sources such as mafic lower crust and mantle-derived melts. The rapid change in magmatic sources at ~472 Ma can be attributed either to reduced crustal assimilation during extension, or a short pulse of crustal growth by addition of juvenile material to the continental crust. Between ~472 Ma and ~196 Ma Lu-Hf model ages remain mostly constant between ~1.0 and ~1.2 Ga. The large scatter and the absence of definite trends in initial ⁸⁷Sr/⁸⁶Sr ratios suggest that both mafic, Rb-poor, and evolved Rb-rich sources were important precursors of active margin magmas in Colombia and Venezuela throughout the Palaeozoic to the Early Jurassic.Previous studies have shown that the genesis of arc magmas may be stimulated by heat advection to the crust during the underplating of mantle derived melt, but the absence of permanent younging trends in Lu-Hf model ages from ~472 Ma to ~196 Ma suggests that very little new crust was generated during this period in the studied region. An overwhelming majority of the analysed igneous rocks yield zircon Lu-Hf model ages of >1 Ga which may be accounted for by documented local crustal end members of 1 Ga–1.6 Ga, and do not require contributions from the depleted mantle. Therefore, recycling of ~1 Ga and older crust was a dominant process in the north-western corner of Gondwana between ~472 Ma and ~196 Ma.This study shows that whole rock Sm-Nd and zircon Lu-Hf data can be interpreted similarly regarding the age of the source regions, whereas Rb-Sr and O isotope data from the same rocks yield valuable information regarding the geochemical nature of the source.     

The strontium and oxygen isotopic record of hydrothermal alteration of syenites from the Abu Khruq complex,Egypt

Hydrothermal convection initiated by emplacement of the gabbro-syenite complex of Abu Khruq into the Egyptian basement 89 Ma ago systematically altered the trace element and isotopic compositions of the syenites. The scale of Sr transport in migrating solutions was far larger than the scale of Sr isotopic equilibration within rocks. As a result, Sr exchange was heterogeneous in the syenites, an effect which can be observed on three different scales. Within grains of a single mineral species, heterogeneities are related to grain boundaries and microfractures through which fluids migrated. Among minerals within rock samples, heterogeneities are related to differences in susceptibility to Sr alteration. Among samples within a single unit, heterogeneous alteration is apparently related to differences in permeability close to fracture zones.During the early stages of alteration radiogenic Sr derived from the country rocks was added to the syenites, causing small net changes in concentration (5 ppm ave.). Some Rb-Sr mineral isochrons from single rock samples yield the emplacement age because isotopic equilibration of this added Sr sometimes occurred within rock specimens. However, regressions of the whole-rock Rb-Sr data yield apparent ages that are about 10 Ma too old. Later stage alteration involved larger changes in whole-rock Sr concentration (45 ppm ave.) but had little further effect on the isotopic relationships because the Sr was derived from cogenetic gabbros rather than the country rocks.Alterations of Rb, Sr, and Sr isotopic compositions are not well correlated with changes in ¹⁸O/¹⁶O ratio because mineralogy played an important role in decoupling trace element and oxygen isotopic alteration. In general, the absence of such correlations for whole-rock data is not diagnostic of rocks with unaltered trace element and isotopic compositions. Mineral-scale Sr isotopic heterogeneities associated with grain boundaries and microfractures may be the most unambiguous evidence of trace element mobility.Deceased on 9/81     

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO₂ contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO₂ contents and high Mg# (Mg# = 100 Mg/(Mg + Fe²⁺)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (⁸⁷Sr/⁸⁶Sr)_i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (⁸⁷Sr/⁸⁶Sr)_i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO₂ (77.63–82.52 wt.%) and low total Fe₂O₃, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.     

Recognition of Early Paleozoic Magmatisms in the Supposed Proterozoic Basements of Zhalantun, Great Xing’an Range, NE China

The Zhalantun terrane from the Xing'an massif, northeast China, was used to be considered as Proterozoic basements. However, amounts of detrital zircon ages from the meta-sedimentary rocks deny the existence of Precambrian basements recently. Notably, magmatic rocks were barely reported to limit the exact ages of the Zhalantun basements. In this study, we collected rhyolite, gabbro and quartz diorite for zircon in-situ U-Pb isotopic dating, which yield crystallization ages of ～505 Ma, ～447 Ma and ～125 Ma, respectively. Muscovite schist and siltstone define maximum depositional ages of ～499 Ma and ～489 Ma, respectively. Additionally, these dated supracrustal rocks and plutons also yield ancient detrital/xenocryst zircon ages of ～600–1000 Ma, ～1600–2220 Ma, ～2400 Ma, ～2600–2860 Ma. Based on the whole-rock major and trace element compositions, the ～505 Ma rhyolites display high SiO_2 and alkaline contents, low Fe_2O_3T, TiO_2 and Al_2O_3, and relatively high Mg O and Mg#, which exhibit calc-alkaline characteristics. These rhyolites yield fractionated REE patterns and negative Nb, Ta, Ti, Sr, P and Eu anomalies and positive Zr anomalies. The geochemistry, petrology and Lu-Hf isotopes imply that rhyolites were derived from the partial melting of continental basalt induced by upwelling of sub-arc mantle magmas, and then experienced fractional crystallization of plagioclase, which points to a continental arc regime. The ～447 Ma gabbros exhibit low Si O2 and alkaline contents, high Fe2 O3 T, Ti O2, Mg O and Mg#. They show minor depletions of La and Ce, flat MREE and HREE patterns, and negative Nb, Ta, Zr and Hf anomalies. Both sub-arc mantle and N-MORB-like mantle were involved in the formation of the gabbros, indicative of a probable back-arc basin tectonic setting. Given that, the previously believed Proterozoic supracrustal rocks and several plutons from the Zhalantun Precambrian basements were proved to be Paleozoic to Mesozoic rocks, among which these Paleozoic magmatic rocks were generally related to subduction regime. So far, none Proterozoic rocks have been identified from the Zhalantun Precambrian basement, though some ～600–3210 Ma ancient detrital/xenocryst zircons were reported. Combined with ancient zircon ages and newly reported ～2.5 Ga and ～1.8 Ga granites from the south of the Zhalantun, therefore, the Precambrian rocks probably once exposed in the Zhalantun while they were re-worked and consumed during later long tectonic evolutionary history, resulting in absence of Precambrian rocks in the Zhalantun.     

Zircon U–Pb geochronology and geochemistry of the post-collisional volcanic rocks in eastern Xinjiang Province,NW China: implications for the tectonic evolution of the Junggar terrane

We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial ⁸⁷Sr/⁸⁶Sr values (0.7034–0.7042) and positive ε_Nd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive ε_Nd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).     

Early Neoproterozoic evolution of Southeast Pakistan: evidence from geochemistry,geochronology, and isotopic composition of the Nagarparkar Igneous Complex

ABSTRACT

Rocks of the early Neoproterozoic age of the world have remained in discussion for their unique identity and evolutionary history. The rocks are also present in Sindh province of Pakistan and have been in debate for a couple of years. Yet, these igneous rocks have been studied very poorly regarding U-Pb and Lu-Hf age dating. The early Neoproterozoic rocks located in Nagarparkar town of Sindh have been considered as shoulder of Malani Igneous Suite (MIS) discovered in Southwest of India. The Nagarparkar Igneous Complex (NPIC) rocks are low-grade metamorphosed, mafic and silicic rocks. These rocks are accompanied by felsic and mafic dikes. Two types of granite from NPIC have been identified as peraluminous I-type biotite granites (Bt-granites), of medium-K calc-alkaline rocks series and A-type potash granites (Kfs-granites) of high-K calc-alkaline rocks series. Geochemical study shows that these Kfs-granites are relatively high in K and Na contents and low MgO and CaO. The Bt-granites have positive Rb, Ba, and Sr with negative Eu anomalies rich with HFSEs Zr, Hf, and slightly depleted HREEs, whereas Kfs-granites have positive Rb with negative Ba, Sr, and Eu anomalies and have positive anomalies of Zr and Hf with HREEs. In addition, these rocks possess crustal material, which leads to the enrichment of some incompatible trace elements and depletion of Sr and Ba in Kfs-granites and relatively high Sr and Ba in Bt-granites, indicating a juvenile lower continental crust affinity. Zircon LA-ICP-MS U-Pb dating of these granites yielded weighted mean ²⁰⁶Pb/²³⁸U ages ranging from 812.3 ± 14.1 Ma (N = 18; MSWD = 3.7); and 810 ± 7.4 Ma (N = 16; MSDW = 0.36) for the Bt-granites, and 755.3 ± 7.1 Ma (N = 21; MSDW = 2.0); NP-GG-01 and 736.3 ± 4.3 Ma (N = 24; MSWD = 1.05) for Kfs-granites, respectively. The Bt-granites and Kfs-granites have positive zircon ε_Hf^(t) values, which specify that they are derived from a juvenile upper and lower continental crust. Based on the geochemical and geochronological data, we suggest that the Bt-granites were formed through lower continental crust earlier to the rifting time, whereas the Kfs-granites were formed via upper continental crust, during crustal thinning caused by Rodinia rifting. These zircon U-Pb ages 812 to 736 Ma, petrographic, and geochemical characteristics match with those of the adjacent Siwana, Jalore, Mount Abu, and Sirohi granites of MIS. Thus, we can suggest that NPIC granites and adjacent MIS can possibly be assumed as a missing link of the supercontinent Rodinia remnants.     

Late Neoproterozoic strongly peraluminous leucogranites, South Eastern Desert, Egypt – petrogenesis and geodynamic significance

Summary Late Neoproterozoic garnet-bearing leucogranites are developed locally along thrust faults in the South Eastern Desert, Egypt. This work presents field observations, whole rock major and trace element abundances, Rb–Sr isotope data and mineral chemistry for three occurrences in the Sikait-Nugrus area. Field observations show that the leucogranites cut the faults and their contact with the country rocks is sharp with no indication of contact metamorphism. They were intruded into a low-grade metamorphosed ophiolitic melange and a high-grade metamorphosed metasedimentary succession of biotite schist composition. Numerous biotite schist enclaves, having irregular and diffuse contacts, are recorded within the leucogranites. Whole rock Rb–Sr ages of the leucogranites from two different localities are 610±20 and 594±12Ma respectively; they are interpreted as emplacement ages. The leucogranites contain more than 70% SiO₂, and they are strongly peraluminous (A/CNK>1.1) with low TiO₂, Fe₂O₃*, MgO, CaO, Ba, Sr, LREE, Eu/Eu* and Sr/Ba and high Rb, Rb/Zr, Rb/Sr and Rb/Ba. These geochemical parameters and the low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703) indicate crustal derivation by dehydration partial melting from a juvenile protolith similar to the exposed biotite-rich metasediments. Models for the tectonic setting of these leucogranites suggest their emplacement during an extensional tectonic stage that follows continental collision. It is proposed that crustal heating, caused by decompression along shear zones, is responsible for the production of these granitic melts. The results support previous hypotheses and further document a regional late Neoproterozoic extensional tectonic event, which is probably related to the initial break-up of Gondwana.     

Petrogenesis and tectonic implications of early Paleozoic granitoids in East Kunlun belt: Evidences from geochronology,geochemistry and isotopes

The East Kunlun Orogenic Belt(EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism.Here we report the petrology, bulk geochemistry, zircon Ue Pb dating and, Lue Hf and SreN d isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U-Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450 -430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their(~(87)Sr/~(86)Sr)ivalues range from 0.7059 to 0.7085, εNd(t) values from -1.6 to -6.0 and the zircon εHf(t) values show large variations from +9.1 to -8.6 with Hf model ages(T_(DM2)) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.