Acidic Dikes Research Articles

Petrology and geochemistry of multiphase post-granitic dikes: A case study from the Gabal Serbal area, Southwestern Sinai, Egypt

Abstract Variable single and/or swarms of post-granitic dikes are widespread at Gabal Serbal, Southwestern Sinai, Egypt. The present article aims to identify and discriminate these multiphase dikes through detailed geological, petrographical, and geochemical examinations. These dikes are classified into two subphases: (1) acidic dikes (porphyritic dacite, microgranite, granophyre, and alkaline granophyre dikes); and (2) basic dikes (basalt and dolerite dikes). They range from vertical or steeply inclined bodies, 0.5–15 m wide, pink to black color, and NE–SW to N–S directions. Acidic dikes with different mineralogical constituents have medium to high k-characters, originating from calc-alkaline magma and extruded in a volcanic arc environment. In contrast, basic dikes have medium k-characters, originating from tholeiitic magma and developing within a plate environment. Basic dikes are enriched with opaque minerals, where the basaltic dike contains iron oxides (magnetite and hematite), such as apatite in addition to copper minerals. Dolerite dike comprises magnetite, titanomagnetite, and pyrite.

Au mineralization-related magmatism in the giant Jiapigou mining district of Northeast China

The Jiapigou mining district (JMD) in the northeastern margin of the North China Craton mainly contains quartz vein- and altered rock-type gold deposits that have been prospected and mined for over 200 years. However, the ore-formation mechanism and geodynamic setting are still limited understood. Here we present new geological, mineralogical, gochronological, geochemical and Hf isotopic evidences of the ore-related intrusions from the deposits in the JMD to reveal the tectonic setting and genetic relationship between the Au mineralizitions and synchronic magmatism. The results show the following: (1) zircon U–Pb dating of the ore-related intrusions, such as basic dikes (gabbro and porphyritic gabbro), granites, and acid dikes (granite aplite dike and rhyolite porphyry) bracket their emplacement in the range of 177–171 Ma; (2) The basic dikes are characterized by enrichment in large ion lithophile elements (LILEs; i.e., Rb, Ba, and U), moderately depletion of high field strength elements (HFSEs; i.e., Nb and Ta), and high εHf (t) values (–13 to +12.8), whereas granites are characterized by high SiO2 (73.24–75.12 wt%) and total alkali (K2O + Na2O = 5.74–8.58 wt%) concentrations, low TFe2O3 (0.78–1.43 wt%) and CaO (0.98–1.36 wt%) concentrations, enrichment of LILEs (i.e., Rb, Ba, Th, and K) and depletion of HFSEs (i.e., Nb, Ta, Ti, and P), and low high εHf (t) values (–13.6 to –11.0), which features as well as those of the acid dikes. These results indicate that the ore-related intrusions, such as basic dikes, granites, and acid dikes were emplaced during the Middle Jurassic (177–171 Ma), constrained that the giant Au mineraliziton in the JMD occurred in the Middle Jurassic. And the basic dikes were likely derived from an enriched lithospheric mantle source that has been influenced by fluids expelled from a subducted slab, whereas granites and acid dikes were likely derived from partial melting of Archean crustal materials. Combined with regional geological observations, we conclude that the giant Au mineralization and ore-related magmatism occurred in an extensional setting associated with the subduction of the Paleo-Pacific Plate. The basic dikes and granites can be regarded as a precursor for the Au mineraliziton in the JMD. And mantle- and crustal-fluids and metals substantially contributed to the formation of the giant Au mineralization in the JMD.

Paleomagnetism of Permo–Triassic volcanic units in northern Patagonia: are we tracking the final stages of collision of Patagonia?

A paleomagnetic and geochronologic study was carried out on Late Permian-to-Early Triassic magmatic units exposed in the North Patagonian Massif, near the locality of Estancia La Esperanza (Rio Negro Province, Argentina), to provide better paleogeographic and tectonic constraints on the evolution of Patagonia and its relations with Gondwana in the Late Paleozoic–Early Mesozoic. The study included the Late Permian (264 ± 2 Ma) Rhyolite Dome, the Collinao Dacite, for which we obtained a U–Pb zircon crystallization age of 253 ± 2 Ma, several basic dikes, that intrude older granite units (255 ± 2 Ma), and a swarm of acidic dikes for which an Early Triassic age had been previously assigned that we confirmed with a U–Pb zircon age of 244 ± 2 Ma. A paleomagnetic pole (C), computed on the basis of seven VGPs, was obtained for the Collinao Dacite and associated units (48.3°S, 349.9°E, N = 7, A95 = 15.1°). A single VGP for the somewhat older Rhyolite Dome falls close to that pole. A second paleomagnetic pole (A) which strongly disagrees with the C position was computed based on 12 VGPs from the same number of acidic dikes (87°S, 51.5°E, N = 12, A95 = 8.1°). The basic dikes, with ages bracketed between the dacitic and the rhyolitic dikes, yielded four VGPs, two of which are consistent with the C pole and the other two are close to the A pole. The C pole falls on a much older position (Late Carboniferous) on different Gondwana/South America reference paths, which is clearly anomalous based on its Late Permian age. The A pole, in turn, is consistent with the 250/240 Ma paleomagnetic poles in most reference paths for Gondwana/South America. Either regional tilting or rotation around a vertical axis to explain the anomalous C pole is unlikely considering the local geological evidence. A working model is presented to explain the discrepancy as due to around 30° CCW rotation of the whole North Patagonian Massif during the latest Permian and earliest Triassic, closing a V-shaped basin that separated it from southern Gondwana, as the final stages of collision of an allochthonous or para-autochthonous Patagonia terrane. This hypothesis is consistent with Late Permian-to-Early Triassic evidence of deformation in the Hesperides Basin as well as in northern Patagonia. Scarce previous paleomagnetic data from this region can be reconciled with this model, considering that they are very loosely constrained in age.

New evidence for Jurassic continental rifting in the northern Sanandaj Sirjan Zone, western Iran: the Ghalaylan seamount, southwest Ghorveh

ABSTRACT We address the growing controversy about the tectonic setting in which Jurassic magmatism of Iran occurred: arc or continental rift. In the Ghorveh area of the northern Sanandaj Sirjan zone (SaSZ), the Ghalayan metabasites are interlayered with marble and schist and locally cut by acidic dikes. Zircon U-Pb dating of the metabasitic rocks shows that these crystallized at ca. 145–144 Ma ago in the Late Jurassic (Tithonian). This complex was metamorphosed in the lower greenschist facies, however, some protolithic structures such as pillow lava and primary minerals are preserved. The metabasites are tholeiites with low SiO2 (45.6–50.5 wt.%), moderate Al2O3 (11.3–17.0 wt.%), and high TiO2 (0.7–2.9 wt.%) and Fe2O3 (9.4–14.1 wt.%). The Ghalayan metabasites are enriched in Light rare earth elements (LREEs) without significant Nb, Ta, Pb, Sr and Ba anomalies, similar to modern continental intra-plate tholeiitic basalts such as Afar and East African rifts. The Ghalaylan metabasites show wide ranges for 87Sr/86Sr(i) (0.7039–0.7077) and positive εNd(t) values (+0.1 to +4.6). These isotopic compositions are similar to those expected for slightly depleted subcontinental lithospheric mantle sources. Independently built discrimination diagrams indicate an intra-continental rifting regime for the source of Jurassic metabasites in the northern SaSZ. Geochemical and tectonic evidence suggests that rifting or a mantle plume was responsible for volcanic activity in the Upper Jurassic SaSZ. Considering the variation of ages of basaltic volcanism along the SaSZ, we suggest that Ghalayan basaltic magmatism reflected a submarine volcano that formed as part of the late stage continental rift, similar to Afar in the East African Rift system. Our results indicate that an extensional tectonic regime dominated SaSZ tectonics in the Middle to Late Jurassic.

The age of young intrusions of Tsana Complex (Greater Caucasus) and isotope-geochemical evidence for their origin from hybrid magmas

This paper presents isotope-geochronological and petrological study of granitoids of the potentially ore-bearing (Au–As–Sb–Sn–Mo) Early Pliocene Tsana Complex, which are confined to the Main Caucasus fault zone (upthrow fault) in the central part of the Greater Caucasus Range. The Tsurungal and Karobi groups of magmatic bodies are distinguished based on spatial criterion. The Tsurungal group includes three small granite—granodiorite massifs (Tsurungal, Chorokhi, and Toteldash) and numerous acid and intermediate dikes in the upper reaches of the Tskhenistsqali River (Kvemo Svaneti, Georgia). The Karobi group comprises three subvolcanic rhyodacite bodies located in the upper reaches of the Chashuri River (Zemo Racha, Georgia) and numerous N–S-trending trachyandesite dikes near the axial zone of the Main Caucasus Range. The K-Ar and Rb-Sr isotope dating shows that the granitoid massifs and dike bodies of the Tsana Complex were formed in two different-age pulses of the Pliocene magmatism: phase I at 4.80 ± 0.15 and phase II at 4.15 ± 0.10 Ma. All hypabyssal rocks of the Karobi group, unlike those of the Tsurungal Group, were formed during the first pulse. Petrographic studies in combination with geochemical data indicate that most of the granitoids of the Tsana Complex are hybrid rocks (I-type post-collisional granites) and were derived through mixing of deep-seated mantle magmas with acid melts obtained by the upper crustal anatectic melting in the Main Caucasus fault zone. The granitoids of the Tsurungal Group define basic to acid evolution (diorite–granodiorite–granite–two-mica granite) possibly caused by both crystallization differentiation and increasing role of crustal contamination in the petrogenesis of the parental magmas of these rocks. This conclusion is also confirmed by the differences in the Sr isotope composition between granitoids of the early (87Sr/86Sr = 0.7053) and late (87Sr/86Sr = 0.7071) phases of the Tsana Complex. Main trends in spatiotemporal migration of magmatic activity in the central part of the Greater Caucasus in the Pliocene–Quaternary time were established using obtained and earlier published isotope-geochronological data.

Petrology and Geochemistry of Dikes in the North of Saveh in Iran

Dikes in the north of Saveh are located in a region with an area of approximately 200 square kilometers, in a distance of approximately 100 kilometers south of Tehran. Dikes mentioned in terms of petrological composition, are divided into two categories: alkaline and intermediate to acidic. Alkaline dikes include: andesitic basalt and andesite and intermediate to acidic dikes include: trachyte and trachyandesite. In terms of geochemical, dikes in the north of Saveh have a dual nature of alkaline and calc-alkaline. Both groups are derived from more enrichment source than primitive mantle. Despite similarity of pattern of both groups, varieties of alkaline having less silica, in the elements Sr, Ti, Nb and Ta show more enrichment and in the elements Hf, Rb, Th, K show less enrichment than varieties of calc-alkaline.

Geochemical signature of Eocene Kuh-e Dom shoshonitic dikes in NE Ardestan, Central Iran: implications for melt evolution and tectonic setting

*Corr esponding author The Late Eocene Kuh-e Dom composite intrusion forms a segment of the Urumieh-Dokhtar magmatic arc, which recorded syn- to post-collisional magmatism during the Alpine–Himalayan orogeny in central Iran. Numerous acid and intermediate–basic dikes intrude the composite intrusive complex of the arc segment and its host-rock assemblage. The silicic dikes of porphyric microgranite, porphyric microgranodiorite and aplite consist of quartz, K-feldspar, plagioclase (albite), biotite and rare amphibole. The dikes are of subaluminous composition with shoshonitic affinity. Trace-element patterns exhibit pronounced negative anomalies of Nb, Ta, Ti, P and Sr together with positive anomalies of Cs, Th, U and La suggesting partial melting of a quartzo-feldspathic crustal source. The intermediate–basic dikes with phonolite, basanite and trachyandesite chemical compositions typically contain pyroxene (diopside–augite) and plagioclase phenocrysts (An 30–60 and An 98 ), calcic amphiboles (magnesiohornblende– magnesiohastingsite), magnesian biotites and alkali-feldspars (Or 95 ). The rocks show shoshonitic geochemical affinities. Low Ba/Rb ratios and high Rb/Sr ratios suggest that the primary dike melt originated by partial melting of a phlogopite-bearing lithospheric mantle, whereas LILE and LREE enrichment along with low Nb/Zr and Hf/Sm ratios and high Ba/ Nb and Rb/Nb ratios imply that these rocks formed at a convergent continental margin. The acidic dikes were emplaced in a transitional syn-collisional subduction setting whereas the intermediate–basic dikes have developed from remnant melt batches after cessation of active subduction, mostly in the post-collisional setting.

The Cobres Plutonic Complex, eastern Puna (NW Argentina): Petrological and structural constraints for Lower Paleozoic magmatism

The southern portion of the Cobres Range is studied with an interdisciplinary, petrological–geochemical and structural-cartographic approach. The Cobres Plutonic Complex (CPC) consists of two main plutons, a granodiorite and a monzogranite, as well as small bodies of gabbros, acidic dikes, and episyenite bands. Their mineralogy and geochemistry suggest a comagmatic character for both plutons, which are strongly peraluminous and define a typical calc–alkaline trend. Harker diagrams indicate a fractionated crystallization trend. The REEs have similar patterns for enclaves, granodiorites, monzogranites, aplites, and acidic dikes, also similar to those from the upper crust rocks. Gabbros included in the CPC show geochemical signatures compatible with the mantle. The CPC is syntectonic, though the granodioritic and monzogranitic plutons represent different stages in the tectomagmatic history of the region. The geology of the CPC and its regional framework show characteristics—thermal anomalies, alternation of contractional and extensional deformational events, magmatic rocks with crustal and mantle affiliations, and contemporaneous extensional basins—that indicate intrusion of the CPC in an extensional accretionary orogen.

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol + hy-normative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative sequence is exposed in the Dents de Bertol area (center of intrusion). P– T calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 °C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg# cpx = 83–89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment.

STRATIGRAPHY AND TECTONIC AND METAMORPHIC EVOLUTION OF THE PORTO AZZURRO UNIT IN THE MONTE CALAMITA PROMONTORY (SOUTHEASTERN ELBA ISLAND, TUSCANY)

The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and of the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the SE part of the Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piedmontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dikes. Moreover, in this part of the Island, the relationships between the uplift of the plutonic bodies and the final deformations of the tectonic stack are well exposed. To improve the geological knowledge of SE Elba, the authors carried out a 1:10.000 geological survey of the Calamita Promontory (mostly made up of the Porto Azzurro Unit) and performed petrographic and meso-/micro-structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, likely pre-Carboniferous basement (Mt. Calamita Fm.), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt.Calamita Fm., five main lithofacies were recognized and mapped. In particular, garnet-bearing, albite micaschist (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); Porphyroids-like rocks (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of the lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello Geothermal Field, whereas the other lithofacies can be probably correletable with the ?Ordovician formations of the Tuscan Metamorphic Units. The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (Dx), recognized in the Mt.Calamita Fm., which is defined by pre- Alpine schistosity and mineralogic relics (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the Greenschists facies, which deformed also the Mesozoic covers; c) a following folding event (D3) which probably occurred during or immediately after the strong thermometamorphic imprint (including the magnetite-rich skarn bodies), due to the Neogene magmatic intrusions; d) Subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt.Calamita Fm. and the Mesozoic cover) and between the latter and the overlying tectonic Units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch). A final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and N-S trending high –angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphic, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that defined for the Larderello geothermal region. Thus, the Mt.Calamita area can be considered as a little older, but similar geological model for all the future interpretations of the deep structure of southern Tuscany crossed by the Crop 18 profile.

Radioelement distributions in the Proterozoic granites and associated pegmatites of Gabal El Fereyid area, Southeastern Desert, Egypt

Lithologically, the rock types in the Gabal El Fereyid area are dominantly granites with minor amounts of pegmatites. The granites range in composition from tonalite to granite-adamallite with minor acidic dikes, quartz and pegmatite veins. The granite-adamallite is peraluminous and formed as a result of partial melting of amphibole-bearing rocks at depths of ∼24–30 km and at temperatures of 800–950°C. Among the different rock types, the muscovite-rich pegmatites had the highest U and Th contents (66 and 38 ppm on average, respectively). The high level of radioactivity in pegmatites is attributed to the presence of the radioactive minerals thorianite, uranophane and allanite as confirmed by XRD analysis. Binary relations of Zr/U, Zr/Th, Ce/U and Ce/Th against either U or Th in the granite-adamellite exhibit significant negative correlations indicating that both elements are not preferentially hosted in the accessory minerals phases such as zircon and monazite, but could be associated with major forming minerals such as biotite, muscovite, plagioclase and quartz, or U is situated within labile sites within granite. The uranium and thorium enrichment in the pegmatites is a two-stage process. The primary stage is magmatic whereas the secondary enrichment is from hydrothermal concentration. The magmatic U and Th are indicated by the presence of thorianite and allanite, whereas evidence of hydrothermal mineralization is the alteration of rock-forming minerals such as feldspar and the formation of secondary minerals such as uranophane and pyrite.

GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

Geochemical characteristics of halogen-bearing hastingsite, scapolite and phlogopite from the Sangan iron skarn deposits, northeastern Iran.

Mesozoic shale, siltstone, sandstone and carbonate rocks and Eocene volcanic, volcaniclastic and pyroclastic rocks are intruded by late Eocene granitic batholith and related acidic dikes. As a result of hydrothermal activity associated with the granitoid, two distinct skarn zones of calcic skarn zone and magnesian skarn zone were formed in the carbonate rocks. Cl-bearing amphiboles of ferroactinolite to potassium rich ferrohastingsite composition are restricted to the calcic skarn zone and associating with salite, grandite, magnetite, calcite and sometimes with scapolite. The Cl content of amphiboles shows positive relationships with Fe2+/(Fe2++Mg) and K/(K+Na) and a negative relation with Si/(Si+AlIV). In contrast, F bearing phiogopite is a major mineral in the magnesian skarn. F content exhibits positive relationship with wt% of SiO2 and MgO. The estimated log(fH2O/fHF) equilibrating with the phlogopite ranges from 4.6 to 5.5 at 400°C. Cl rich scapolite is widespread in both calcic and magnesian skarn zones. The scapolite has a compositional range of 23 to 40 mole% meionite in marialite-meionite solid solution series. XC1 of the scapolite ranges from 0.44 to 0.98 (mostly more than 0.75). Scapolite of XC1=0.98 from the hornfels is the most Cl-rich scapolite ever reported. Cl content of the scapolite positively correlate with Na/(Na+Ca+K). Calculated XNaCl values for fluid in equilibrium with scapolite from the Sangan deposits reach up to more than 55 wt% equivalent NaCl. The Cl-rich hastingsite, Cl-rich scapolite, F-rich phiogopite, and fluid inclusions with high salinity (up to 70 wt% eq. NaCl) in granitic rocks and in skarn zones suggest that hydrothermal fluids were derived from the granitoid magma. The ore elements such as Fe and Cu could be transported by solutions as metal-chloride complexes. Large amounts of cations such as Al, Si, and Ca incorporated in the fluids should be derived from the country rocks and from the breakdown products of early skarn minerals.

丹波帯中央部の火成岩岩脈群の産状と岩石記載

Many dikes of various rock types are observed in the central area of the Tamba belt, They intruded into the Tamba Group of age from late Carboniferous to late Jurassic. Whereas basic to intermediate dikes are uniformly distributed in a relative sense, acidic dikes are unevenly spread. Many dikes are distributed around the Shuzan syncline, however, no dikes are found in sandstone area. These observations are suggestive that the dikes were formed under the structural control of the Tamba belt. The rocks from the dikes are classified into seven groups with two subgroups as follows: (1), Granite-porphyry; (2), Felsite; (3), Diorite-porphyry; (4), A, Hornblende-porphyrite with plagioclase phenocryst; B, Hornblende-porphyrite with no plagioclase phenocryst; (5), Clinopyroxene-brown hornblende-porphyrite; (6), Diabase; (7), Diabase-porphyrite. The groups, (1) to (4) A, are acid to intermediate rocks, and the others are intermediate to basic rocks. Petrographic descriptions for these rocks have been carried out.

Paleomagnetic study of some of the dikes from the Esh El Mellaha Range, Eastern Desert, Egypt

Two oppositely directed secondary directions of magnetization were isolated from some of the dikes in the Esh El Mellaha Range. Radiometric and paleomagnetic age considerations suggest that these were acquired during an interval of low-grade metamorphism in late Cambrian time (approx. 530 m.y.). The paleopole position for the acidic dikes (three sites) is 87.3N 25.9E dp 9.6, dm 14.5, for the intermediate dikes (eight sites) 81.6N 120.8E dp 3.3 dm 5.2 and for the basic dikes (11 sites) 82.7N 86.5W dp 5.6 dm 9.1.

Structure and emplacement of the Alpine-type peridotites from Beni Bousera, Rif, Morocco: A polyphase tectonic interpretation

The Beni Bousera peridotite antiform, in the southern branch of the Gibraltar orocline exhibits a lherzolitic core, containing thin pyroxenite layers surrounded by harzburgites, then by dunites and garnet-bearing dunite. In the peripheral areas, the amount of pyroxenite greatly increases by the development of garnet-pyroxenite veins. These data indicate a gradient of partial-melting increasing upward. We conclude that a reversed geothermal gradient was temporarily established in the upper part of the peridotite, just under its migmatitic roof (kinzigite aureole, then sillimanite-gneiss). Study of ductile, penetrative structures and discussion of the later “cold” structures show that the foliation of the tectonites was originally flat, with a NW-SE-trending stretching lineation. Orthopyroxene crystals are used as tectonic markers. “Isostrain zones” indicate that simple shear and finite strain increase upward. We conclude that a ductile shear zone acted between peridotite and gneiss. Strain heating along this shear zone probably has been responsible for partial melting of the upper part of the peridotite. In our model, the peridotite emplacement into the mid-continental crust began during crustal extension. Then, as comparison with the Rondà massif suggests, an intracrustal thrusting stage was followed by high-temperature and relatively low-pressure metamorphism and by intrusion of acidic dikes. The uplift of the mantle slab continued by compressive upthrusting and isostatic doming. This complex, polyphase evolution probably extended beyond the Alpine orogeny, as suggested by Kornprobst.

Geology of Mount Gran Area, Antarctica: ABSTRACT

The Mount Gran area, encompassing Mount Gran and the ice-free Gran Valley (unofficial name) adjoining on the north, is located 77°S and 161°E in the rugged, glaciated mountains of South Victoria Land, Antarctica. Most of the area is underlain by sedimentary rocks, intruded by, and sandwiched between, thick diabase sills. However, a basement complex of metamorphic and igneous rocks crops out in a small, isolated exposure at the northwest corner of Gran Valley. This complex consists of granitic rocks and gneisses which are cut by acid and basic dikes. The gneisses show foliation striking northwest with a nearly vertical dip. The age of the gneisses may be Precambrian or possibly Early Paleozoic. The sedimentary rocks, all referred to the Beacon Sandstone Group, which ranges from Silurian to Jurassic in age, crop out on the southeast face of Mount Gran and in a thick belt rimming most of the 8-mile long, northeasterly trending Gran Valley. Dips are 3°-8° northwestward. In Gran Valley, the dominant lithologic type consists of light gray to white, well sorted, fine- to medium-grained, cross-bedded, quartzose sandstone between 1,000 and 1,300 feet in thickness. The lower half of this section includes ferruginous concretions as much as one foot in diameter. A few thin beds of green silicified siltstone occur in the upper half of the section. At Mount Gran 130 feet of the quartzose sandstone is overlain by 470 feet of a nearly cyclic sequence of carbonaceous shale, coal, conglomerate, arkosic sandstone with interbedded shaly siltstone, and sandstone. Correlation between the quartzose sandstone at Mount Gran and Gran Valley is not clear because of lack of diagnostic fossils and discontinuity of exposures, but it is believed that the coal-bearing section stratigraphically overlies the Gran Valley section. The youngest formation, the Ferrar Dolerite, probably of Jurassic or Cretaceous age, occupies the largest part of the Mount Gran area. Two large sills are present in Gran Valley. One, which probably overlies the basement complex and generally underlies the Beacon Group, forms the floor of the valley. The second overlies the Beacon Group in Gran Valley, forming the escarpment and skyline of the Mount Gran area. To the south, at Mount Gran, the two sills join, leaving only a remnant of Beacon sandstone which is dissected by diabase sills and dikes. End_of_Article - Last_Page 352------------

Geology and mineralization of the San Antonio mine, Santa Eulalia district, Chihuahua, Mexico

The mineral deposits of the San Antonio mine, in the Santa Eulalia district of Chihuahua, Mexico, occur in Upper Cretaceous and possibly Lower Cretaceous limestones. They lie within a downfaulted limestone block, are closely associated with a zone of acid dikes, and form replacement deposits of the contact-metamorphic type. The chief yield has been lead and silver ore, but tin and vanadium ores have been produced commercially. The bodies are unusual in that lead and vanadium are important products from a contact-metamorphic area and in that tin occurs in replacement deposits in limestone. Detailed descriptions of tin and vanadium ore bodies are given. The writer concludes that tin was the last ore element to be introduced and suggests it may have been pneumatolytic ; that the vanadium ore has been derived in large measure from the breakdown of primary vanadium-bearing silicates; and that the deposits are best classed in the pyrometasomatic group, of contact origin.

Geology of southeastern Venezuela

Research Article| June 01, 1942 Geology of southeastern Venezuela V. M. LÓPEZ; V. M. LÓPEZ Search for other works by this author on: GSW Google Scholar E. MENCHER; E. MENCHER Search for other works by this author on: GSW Google Scholar J. H. BRINEMAN, JR. J. H. BRINEMAN, JR. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1942) 53 (6): 849–872. https://doi.org/10.1130/GSAB-53-849 Article history received: 29 May 1941 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation V. M. LÓPEZ, E. MENCHER, J. H. BRINEMAN; Geology of southeastern Venezuela. GSA Bulletin 1942;; 53 (6): 849–872. doi: https://doi.org/10.1130/GSAB-53-849 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In southeastern Venezuela, in the region of the Gran Sabana, there outcrops the Roraima formation of conglomerates, sandstones, and shales, with interbedded thin tuffs. The greatest thickness, 2400 meters, at Mount Auyán-tepui, represents merely an erosion remnant. Predominant reddish tints, cross-bedding, arkosic materials, lateral and vertical variations, and the complete lack of fossils indicate a terrestrial origin. Thick dikes, sills, and laccoliths of gabbroitic rock intrude the Roraima, as well as a few acid dikes probably related to the same magma. The sediments are highly silicified. The age of the Roraima is undetermined.The Roraima formation was deposited on the peneplaned surface of the Archean basement of the Guayana Shield. In the Gran Sabana area, these old rocks outcrop only along the Brazilian border where they consist of red and gray silicified porphyries.The Roraima beds are essentially horizontal, except for gentle arching due to the gabbroitic intrusions. The present topography of mesas and cuestas is controlled by an original northward dip of the sediments off the old basement combined with the structures resulting from intrusion. The drainage is controlled by stream piracy due to a gentle southward tilting of the older northward-dipping beds.Gold and diamonds in small quantities are associated with the basal conglomerate of the Roraima series and are concentrated in the Recent alluvial deposits eroded from the conglomerate. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonics of the Maracaibo Basin, Venezuela

Ancient land masses of northern South America and the Caribbean Sea region have had an important influence on the position and shape of the Maracaibo Basin in northwestern Venezuela. Gondwana and Antillia, the two major land masses involved, lie southeast and northwest, respectively, of the basin. Minor land masses of the Venezuelan Andes, Sierra de Perija, and Island of Toas are located on the south, west, and north edges of the basin, and have a close structural relation to secondary folding. Maracaibo Basin is a flask-shaped structural geosyncline 150 miles long, is flanked on the east, south, and west by high mountains, and is connected by a narrow neck with the Caribbean Sea on the north. Igneous, metamorphic, and sedimentary rocks within the basin range in age from t least Ordovician to Recent. Acidic and basic dikes and batholiths cut igneous and metamorphic rock. In places, sediments are intruded. Structural influence of igneous intrusions on surrounding rocks is believed to be negligible. En echelon arrangement of structures in the Maracaibo Basin is suggested. The more highly inclined flank of the folds is generally toward the axis of the basin. Marginal faults locally are present, but are considered to be incidental to folding. Rotational movement of orogenic forces in the basin is suggested by a northward plunge of structures west of the lake, and a southward plunge of structures on the east side of the basin.