Amygdaloidal Basalts Research Articles

The Evolution of Paleo-Porosity in Basalts: Reversing Pore-Filling Mechanisms Using X-Ray Computed Tomography

Often carrying a high-volume fraction of vesicles, basaltic rocks can be an important reservoir horizon in petroleum systems, and are considered an excellent candidate for CO2 storage by in situ mineral trapping. The frequency of amygdaloidal basalts in many sequences highlights the prevalence of mineralisation, but when the vesicle network has been filled, the basalts can act as impermeable seals and traps. Characterising the spatial and temporal evolution of the porosity and permeability is critical to understanding the petro-physical properties and CO2 storage potential of basalts. We exploit X-ray computed tomography (XCT) to investigate the precipitation history of an amygdaloidal basalt containing a pore-connecting micro fracture network now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. The fracture network likely represents a preferential pathway for CO2-rich fluids during mineralisation. We investigate and quantify the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We provide a semi-quantitative technique for defining reservoir potential and quality through time and understanding sub-surface flow and storage. We found that permeability evolution is dependent on the precipitation mechanism and rates, as well as on the presence of micro fracture networks, and that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations.

Intracontinental basaltic magmatism in the sedimentary succession of the Tucuruí Group: An important record of the final evolution of the Araguaia Belt

The region of Tucuruí, northeastern Pará, is located in the transition between the Amazon Craton and the Araguaia Belt. In this region, there is an extensive graben filled by a volcano-sedimentary succession, 35 km wide that extends over 100 km in the NNW-SSE direction and which is represented by the Tucuruí Group. This group consists of a succession of basaltic flows and diabase sills interbedded with sub-arkose and siltstones. Together with the metasedimentary rocks of the Tocantins Group, these rocks represent the latest records of the Araguaia Orogen in the extreme North of Brazil. The volcanic deposits consist of basalt flows and volcanic breccias associated with diabase sills. The flows are predominantly composed of massive and locally amygdaloidal basalts and basaltic breccias. The basalts are aphanitic, fine-grained or glassy, holo-to hypocrystalline, and are made up of labradorite, augite, Fe–Ti minerals oxides, sulfides, and apatite. The massive basalts have intergranular or intersertal texture, whereas the amygdaloidal basalts are characterized by fibro-radial zeolites, epidote, and chlorite. The basaltic breccias are composed of elongated, hypovitreous basalt fragments encompassed by a volcanoclastic matrix with siltstone films and anastomosed siliceous-zeolitic materials. The basalts are subalkaline with tholeiitic Ocean Island Basalts affinity, intraplate, and are interpreted as products of fissural volcanism in a subaerial environment. Deformation is predominantly ruptile, and fluid remobilization occurs along faults and fractures, generating veins of quartz, carbonate, chlorite, and epidote by hydrothermal processes at low temperatures (anki-metamorphism). The sedimentary succession of the Tucuruí Group represents a paleo-segment formed in a shallow marine environment influenced by storm waves. The evolution of the succession is related to the installation and fill of a rift basin during the final stage of evolution of the Araguaia Belt at the Neoproterozoic/Paleozoic transition. This event is related to the extensional tectonics that marked the transition from a ductile to ruptile regime in the late stage of evolution of the Araguaia Orogen, and it represents a rare record of the end of the Brasiliano Cycle on the Brazilian Platform.

ЛЮМИНЕСЦЕНЦИЯ КВАРЦ-ХАЛЦЕДОНОВЫХ ОБРАЗОВАНИЙ КУЗБАССА И ЕЁ СВЯЗЬ С ГЕОХИМИЧЕСКИМИ ОСОБЕННОСТЯМИ АГАТОВОЙ МИНЕРАЛИЗАЦИИ

Актуальность. Кварц-халцедоновые образования Кузбасса являются потенциально ювелирно-поделочным материалом, при этом степень их минералогической изученности недостаточна. Кроме того, требуют дальнейшего изучения некоторые аспекты формирования агатов. Цель: изучение люминесцентных характеристик кварц-халцедоновых образований в пределах проявлений Южно-Кузбасского магматического ареала, их изменений по зонам в исследуемых образцах, вариаций в поведении отдельных центров свечения, связанных с собственными и примесными дефектами, выяснение возможного влияния геохимических особенностей исследуемых минералов на их люминесцентные свойства. Объекты: кварц-халцедоновые образования с проявлений Южно-Кузбасского магматического ареала (проявление россыпного типа у д. Ключи (Крапивинский район), миндалекаменные базальты с агатовой минерализацией проявления участка Терсюк (Новокузнецкий район), миндалекаменные базальты с агатовой минерализацией проявлений Салтымаковского хребта (Крапивинский район, Кемеровская область). Методы: рентгеноструктурный анализ, рентгенолюминесцентный анализ, масс-спектрометрия с индуктивно связанной плазмой. Результаты. Методом рентгеноструктурного анализа изучен минеральный состав. Главным образом представлены кварц и халцедон, при этом наибольшее содержание халцедона наблюдается в миндалинах с концентрически-зональным строением. В числе других минералов встречаются кристобалит, доломит, кальцит, цеолиты. Посредством проведения рентгенолюминесцентного анализа получены данные о содержании собственных и примесных дефектов различной природы. Распределение центров люминесценции в слоях агата проявления Терсюк может свидетельствовать о смене окислительного потенциала (во внешней зоне) на восстановительный (во внутренних зонах), что подтверждается геохимическими данными. Для оникса Салтымаковского хребта характерна идентичность спектров РЛ для всех исследуемых слоев, что свидетельствует о постоянстве условий минералообразования. В целом сделан вывод о преобладании в миндалинах участка Терсюк восстановительной среды, а в миндалинах проявлений Салтымаковского хребта и у д. Ключи – окислительной.

Magmatism of the Weddell Sea rift system in Antarctica: Implications for the age and mechanism of rifting and early stage Gondwana breakup

Thick (∼800 m) basaltic successions from the eastern Antarctic Peninsula have been dated in the interval 180–177 Ma and preserve a transition from a continental margin arc to a back-arc extensional setting. Amygdaloidal basalts from the Black Coast region of the eastern margin of the Antarctic Peninsula represent a rare onshore example of magmatism associated with back-arc extension that defines the early phase of Weddell Sea rifting and magmatism, and Gondwana breakup. The early phase of extension in the Weddell Sea rift system has previously been interpreted to be related to back-arc basin development with associated magnetic anomalies attributed to mafic-intermediate magmatism, but with no clearly defined evidence of back-arc magmatism. The analysis provided here identifies the first geochemical evidence of a transition from arc-like basalts to the development of depleted back-arc basin basalts in the interval 180–177 Ma. The exposed Black Coast basaltic successions are interpreted to form a minor component of magmatism that is also defined by onshore sub-ice magnetic anomalies, as well as the extensive magnetic anomalies of the southern Weddell Sea. Back-arc magmatism is also preserved on the Falkland Plateau where intrusions postdating 180 Ma are associated with early phase rifting in the Weddell Sea rift system. Back-arc extension was probably short-lived and had ceased by the time the northern Weddell Sea magmatism was emplaced (<175 Ma) and certainly by 171 Ma, when an episode of silicic magmatism was widespread along the eastern Antarctic Peninsula. Previous attempts to correlate mafic magmatism from the eastern Antarctic Peninsula to the Ferrar large igneous province, or, as part of a bimodal association with the Chon Aike silicic province are both dismissed based on age and geochemical criteria.

Accretionary Tectonics of Rock Complexes in the Western Margin of the Siberian Craton

The geological, geochemical, and isotope-geochronological evidence of the events at the final stage of the Neoproterozoic history of the Yenisei Range is considered (beginning from the formation of fragments of the oceanic crust in the region and their accretion to the Siberian Craton until the postaccretionary stage of crustal tension and onset of the Caledonian orogeny). Based on an analysis of new data on the petrogeochemical composition, age, and geodynamic nature of the formation of contrasting rocks in the composition of tectonic melange of the Near-Yenisei (Prieniseiskaya) regional shear zone, we have found the chronological sequence of events that marks the early stages of the Paleoasian Ocean evolution in the zone of its junction with the Siberian Craton. These events are documented by the continental marginal, ophiolitic, and island-arc geological complexes, each of which has different geochemical features. The most ancient structures are represented by fragments of oceanic crust and island arcs from the Isakovka terrane (700–620 Ma). The age of glaucophane-schist metamorphic units that formed in the paleosubduction zone corresponds to the time interval of 640–620 Ma. The formation of high-pressure tectonites in the suture zone, about 600 Ma in age, marks the finishing stage of accretion of the Isakovka block to the western margin of the Siberian Craton. The final events in the early history of the Asian Paleoocean were related to the formation of Late Vendian riftogenic amygdaloidal basalts (572 ± 6.5 Ma) and intrusion of postcollisional leucogranites of the Osinovka massif (550–540 Ma), which intruded earlier fragments of the oceanic crust in the Isakovka terrane. These data allow us to refine the Late Precambrian stratigraphic scheme in the northwestern Trans-Angarian part of the Yenisei Range and the evolutionary features of the Sayan–Yenisei accretionary belt. The revealed Late Neoproterozoic landmarks of the evolution of the Isakovka terrane are attributed to the terminal phase of the breakup of Rodinia, separation of the Siberian Craton, and opening of the Paleoasian Ocean.

Effect of the mineralogical composition on the petrophysical behavior of the amygdaloidal and vesicular basalt of Wadi Wizr, Eastern Desert, Egypt

This paper gives an account of the petrophysical characteristics and the petrographical descriptions of Tertiary vesicular and amygdaloidal olivine basalt flows from Wadi Wizr in the central Eastern Desert of Egypt. The petrographical studies indicated that the studied vesicular basalts are rich in calcic-plagioclase, augite and olivine in addition to numerous amounts of fine opaque minerals and vesicles filled with carbonate and quartz amygdales. The degree of oxidation and alteration of magnetite and ilmenite are discussed in detail. Petrophysically, the studied samples can be grouped into two main groups; the first group includes amygdaloidal basalts and the second group consists of vesicular basalts. The vesicular group (the permeable one) is characterized by fair to very good porosity (∅), good permeability (k), very low true formation resistivity factor (F) and contain micro to ultra micropores. On the other hand, the amygdaloidal basalt group (impermeable group) is characterized by very low storage capacity properties, fair porosity, negligible permeability, medium to high true formation resistivity factor and ultra micropores. The mercury injection capillary pressure technique (MICP) indicates that the pore throats of the studied vesicular samples have a binomial distribution (rank IV), while that of the amygdaloidal samples have a trinomial distribution (rank V). It has been found in this study that the petrophysical behavior of basalts is dependent on the degree of oxidation and alteration; and in particular on the rate of cooling and oxidation of the opaque minerals which caused filling in the primarily produced vesicles by low temperature secondary minerals.

Petrogenesis of Gunbarrel magmatic rocks: Homogeneous continental tholeiites associated with extension and rifting of Neoproterozoic Laurentia

The ca. 780Ma Gunbarrel Igneous Event of northwest Laurentia consists of spatially discrete suites of sills, dykes and lavas distributed over a vast area extending from Wyoming in the south to the Wopmay Orogen and the Mackenzie Mountains of Northwest Canada. Thick (≤100m) sills and rare dykes in Wopmay orogen and thinner (≤30m) sills, dykes and rare lavas in the Mackenzie Mountains are moderately evolved, augite+oligoclase–labradorite+ilmenite–magnetite gabbros and amygdaloidal basalts. Systematic petrochemical differences between units reveals that each is likely derived from subtly distinct parental magmas collectively exhibiting mutually consistent element variations. The dataset is remarkably homogenous, in particular, the incompatible trace elements and the Sm–Nd isotopes. All rocks preserve petrochemical evidence of an enriched MORB-like mantle source, but a small lithospheric component in the primary magmas resulted in elevated LILE, minor negative HFSE anomalies and sub-depleted mantle but supra-bulk earth ɛNd values. The lithospheric component was slightly older, modestly fractionated, Sr-depleted, garnet-free (pyroxenitic?) lower crust or, similar material that was previously recycled into the lithospheric mantle. Mineral chemical data for plagioclase and clinopyroxene in chill margin samples from a Hottah sheet in Wopmay orogen, indicates rapid and repeated turbulent mixing of geochemically and thermally similar magmas. These were staged from large, lowermost crust(?) magma chambers centred over an asthenospheric thermochemical anomaly thought to lie to the west of present-day North America. These magmas were then rapidly emplaced across western Laurentia.

Lithologic and structural controls on fluvial knickzones in basalts of the Paraná Basin, Brazil

Knickzones are common features along rivers on the basaltic plateaus of the Paraná Basin. According to current conceptual models, knickpoints are formed in massive basalts that have a high density of vertical joints. Vesicular–amygdaloidal basalts and those with horizontal joints tend to form reaches of low slope due to their lower resistance to erosion. However, field surveys revealed complexities in this general relationship. The research presented here sought to verify the controls on the genesis of knickzones in this type of geological environment. We studied a 61 km-long mixed bedrock–alluvial river. The longitudinal profile of the river was surveyed on a topographic map with 5 m contour intervals. Tectonic lineaments oriented transverse to the channel and longitudinal lineaments in which the river lies were identified from maps. A detailed field survey of the lithologic characteristics of the riverbed was also performed. The results show that knickzones may form in any litho-structural zone in the flood basalts. On the other hand, low slope zones are predominantly sculpted into vesicular–amygdaloidal basalts, which are less resistant to erosion. The fracture densities of vesicular–amygdaloidal basalts are similar in low slope zones and in knickzones (4.86 and 4.93 m/m2, respectively). This indicates that knickzones in this type of basalt are not caused by higher resistance to erosion. Approximately 60% of the 18 knickzones identified are associated with tectonic lineaments, irrespective of the structural characteristics of the basalts. Vesicular–amygdaloidal basalt and/or basalt with horizontal joints allow the fastest knickzone migration and aid in the formation of convexities. Knickpoints in these basalts do not migrate, but erosion in the pools advances downstream and breaks the bedrock steps, thus increasing the slope. Massive basalt with vertical joints causes slower migration, and its presence at convexities indicates local uplift. Convex segments are only formed upstream of faults.

Siderophile and chalcophile metal variations in basalts: Implications for the sulfide saturation history and Ni–Cu–PGE mineralization potential of the Tarim continental flood basalt province, Xinjiang Province, China

It is well known that a genetic link exists between the formation of ultramafic-hosted Ni–Cu–PGE sulfide deposits and the eruption of associated chalcophile- and siderophile-depleted continental flood basalts. However, until recently little research has been undertaken on the Permian Tarim Basin continental flood basalt province in Xinjiang province, China. Here, we present new trace, siderophile and chalcophile element data from continental flood basalts that are well exposed and preserved in the Keping area of Xinjiang province and we discuss their sulfide saturation history and Ni–Cu mineralization potential. Ultramafic–mafic units dominated by basaltic rocks outcrop over an area of about 250,000km2 in the Tarim Basin; these Lower Permian basaltic rocks are hosted by the Kupukuziman and Kaipaizileike Formations, a mixture of basalts, trachybasalts, amygdaloidal basalts and basaltic pyroclastics.Major, trace and rare earth element (REE) data indicate that basalts within both formations are cogenetic, whilst low concentrations of Ni, Cu and platinum-group elements (PGE) indicate that they are chalcophile element depleted. The basalts have very low (Cu/Zr)PM and extremely low (Pd/Zr)PM and (Pd/Cu)PM ratios; both these ratios and the concentrations of Ni, Cu and the PGEs are invariant with changes in MgO concentrations. Furthermore, the magmas represented by the Keping basalts have very low calculated sulfur concentration at sulfide saturation (SCSS) capacities, and only needed to assimilate small amounts of sulfur-bearing crustal material to breach the SCSS and form immiscible sulfide liquids. This assimilation of crustal material appears to have occurred at depth; this is evidenced by the clear link between crustal contamination (as expressed by (Th/Nb)PM ratios) and the removal and depletion of chalcophile elements in the magmas that formed the basalts in the Keping area (as expressed by (Pd/Cu)PM and (Pd/Zr)PM ratios). The geochemistry of the basalts is consistent with an enriched mantle source; all of the basalts assimilated significant amounts of crustal material, became sulfur saturated and formed immiscible magmatic sulfides at depth; these magmatic Ni–Cu–PGE sulfides scavenged chalcophile elements from the magma and remained at depth whilst the chalcophile-depleted basalts were erupted.The identification of the segregation of magmatic sulfides from ultramafic–mafic magmas within the Tarim Basin indicates that the area should be considered highly prospective for mineral exploration for magmatic Ni–Cu–PGE deposits. Magmatic Ni–Cu–PGE sulfide enriched units may well exist in magma conduits or at the base of shallow magma chambers within the basin, and any ultramafic sills within the basin that are cogenetic and coeval with the Lower Permian Keping area basalts, for example the layered ultramafic–mafic bodies in the Bachu area, should be considered as prospective targets.

Equilibrium between Clinopyroxene and Host Rocks: Implication for the Magmatic Source and Evolution of Alkali Basalts of the Taohekou Formation in the Northern Daba Mountains, China

Abstract:The Taohekou Formation is a volcanic‐sedimentary terrane formed in the early Silurian in the northern Daba Mountains, China. The volcanic rocks, with dominant alkali basalts and minor mantle xenoliths, are enriched in clinopyroxene phenocrysts. Geochemical analysis shows that the composition of clinopyroxenes from different lithofacies has a close affinity. There is a liner correlation present in composition of clinopyroxenes (including phenocryst, microcrystal and xenocryst) from coarse porphyritic basalts, pillow or fine porphyritic basalts to amygdaloidal basalts. All the clinopyroxenes, except the clinopyroxenes in mantle xenoliths, show a similar pattern of trace elements and REE, which indicates that they are likely products of successive fractional crystallization from cognate magma. Clinopyroxenes in mantle xenoliths, however, are mantle xenocrysts. The crystallization pressure of clinopyroxenes gradually decreases from mantle xenolith, deep‐seated xenocryst, coarse porphritic basalts, pillow or fine porphritic basalts, to amygdaloidal basalts, which are 1.92–4.41 GPa, 1.18–2.36 GPa, 1.13–2.05 GPa, 0.44–0.62 GPa and 0.14–0.28 GPa respectively. Calculation results suggest that the primary magma originated from a mantle region deeper than 68 km and stagnates in intervals of 37–68 km, 15–20 km and 5–9 km during its ascent. The alkali basalts are characterized by increasing concentrations of Si and alkaline with the magmatic evolution. Meanwhile, they are markedly enriched in LREE, and the patterns of trace elements and REE are similar to those of oceanic island basalts.

Chronology of hydrothermal and magmatic activity in the Dukat gold-silver ore field

The previously published and newly obtained geological and geochronological (Rb-Sr and Ar-Ar) data show that the igneous rocks and products of hydrothermal alteration in the Dukat ore field pertain to two ore-forming magmatic-hydrothermal systems (OMHSs). The igneous rocks of the Early Cretaceous rift-related OMHS are represented by potassium rhyolites of the Askol’d Formation with Rb-Sr ages of 124 ± 3 and 119.3 ± 3.4 Ma and intercalating amygdaloidal basalts. The products of the hydrothermal activity of this OMHS are the metasomatic anatase-chlorite assemblage of the root zone, which replaces potassium rhyolites, and shallow-seated quartz-adularia and quartz-carbonate-feldspar veinlets retained in rhyolite fragments in Late Cretaceous conglomerate and breccia. The Late Cretaceous OMHS was related to the origination of the Okhotsk-Chukotka volcanic belt and consists of calc-alkaline basaltic andesites of the Tavvatum Formation and moderately silicic K-Na rhyolites of the Nayakhan Formation with a Rb-Sr age of 84 ± 4 Ma. The Late Cretaceous postmagmatic hydrothermal activity in the Dukat ore field resulted in the formation of preore metasomatic rocks and orebodies of the unique Dukat Au-Ag deposit. The first stage of the Late Cretaceous hydrothermal activity gave birth to preore propylites with a Rb-Sr isochron age of adularia samples estimated at 85 ± 1 Ma and quartz-chlorite-sulfide and Ag-bearing quartz-chlorite-adularia orebodies with Rb-Sr isochron ages of adularia estimated at 84 ± 1 and 86.1 ± 4 Ma. The second stage was marked by the formation of garnet-bearing propylites and quartz-rhodonite orebodies with a Rb-Sr age of 73 ± 3 Ma. Further hydrothermal activity occurred after a break related to structural rearrangement of the ore field and was expressed in the replacement of propylites by products of argillin alteration and Ag-bearing Mn hydroxides. Paleogene basaltic dikes and related subeconomic mineralization concluded magmatic and hydrothermal processes in the Dukat ore field.

Petrology and geochemistry of mafic rocks from mélange and flysch units adjacent to the Yarlung Zangbo Suture Zone, southern Tibet

The Yarlung Zangbo Suture Zone (YZSZ), Southern Tibet, is marked by a continuous but tectonically disturbed ophiolitic belt. Three main mélange-type units lie immediately south of it:the Early Cretaceous ophiolitic mélange, the Mesozoic Yamdrock mélange and the Triassic flysch.The petrography, mineral chemistry and whole-rock chemistry of mafic rocks from these three geological units are reported in order to assess their tectonic settings. mafic rocks from the ophiolitic mélange are mainly gabbros and composed of primary plagioclase, clinopyroxene and locally brown amphibole. Except for a few coarse-grained gabbros that have plagioclase compositions ranging from An 80 to An 50, most plagioclase grains are albite pseudomorphs. Clinopyroxenes are augite to diopside. Reddish brown tschermakite to magnesiohornblende are probably of magmatic origin. Ilmenite and titanite are common. REE patterns are LREE-depleted with average (La/Yb) N of 0.5. Slight negative Nb–Ta and Ti anomalies suggest that these rocks formed in a back-arc basin. They show geochemical characteristics similar to mafic rocks of the Lau Basin. This study confirms that mafic rocks from the ophiolitic mélange are derived from tectonic disruption of the YZSZ ophiolites. mafic rocks from the Yamdrock mélange are mostly porphyritic and amygdaloidal basalts. Their primary composition consists of prismatic plagioclase, augite and ilmenite. A few samples also contain small euhedral crystals of brown magnesiohornblende, probably of magmatic origin. These rocks have been affected by low but pervasive hydrothermal metamorphism. REE patterns are LREE-enriched with average (La/Yb) N of 5.3.These rocks are of intraplate affinity, and are similar to volcanic rocks of the Indian Réunion hotspot. mafic rocks from the flysch are mostly gabbros. They are clinopyroxene-rich. Interstitial quartz is common. Ilmenite and titanite are abundant and relatively well preserved. REE patterns are the most LREE-enriched of any analyzed samples from the sedimentary units, with average (La/Yb) N of 6.9, while multi-element patterns show slight negative Nb–Ta and Ti anomalies. These last patterns are similar to patterns in volcanic rocks from the western Deccan Traps of India, whose geochemical behavior reflects mixing between an enriched mantle end-member of Réunion island-type and Indian continental lithospheric material. Hence, these rocks are interpreted to have been derived from an enriched intraplate mantle source, with additional crustal assimilation resulting from disaggregation of the Indian Plate during the opening of the Neo-Tethys ocean. AFC processes have been modelled and strongly suggest a felsic component analogous to Cambro-Ordovician granites as the major contaminant.

Carboniferous-Triassic subduction and accretion in the western Kunlun, China: Implications for the collisional and accretionary tectonics of the northern Tibetan Plateau

A newly defined, 250 km by 500 km, Carboniferous-Triassic subduction-accretion complex, the Mazar accretionary prism in the western Kunlun, comprises two subduction complexes and a forearc-basin succession. (1) The Bazar Dara subduction complex contains imbricated blocks of sandstone, arenite, limestone, and metavolcanic rocks in a matrix of weakly metamorphosed Triassic deep-sea turbidites. The metavolcanic rocks include basalt, diabase, spilite, and andesitic porphyry. Trace element geochemistry shows that pillow and amygdaloidal basalts are oceanic-island tholeiites. This zone has Ordovician to Permian fossils and is situated on the older, more highly deformed and metamorphosed side of the prism adjacent to the Sailiyak magmatic arc. (2) The Heweitan subduction complex is composed of blocks of limestone, turbidite, and radiolarite in a slate-phyllite matrix intercalated with calc-alkalic volcanic rocks. This complex has Permian to Triassic fossils and is situated on the younger, less deformed, and metamorphosed side of the prism adjacent to the suture zone. (3) The Qitai forearc basins are infilled with turbidites (Late Triassic) intercalated with carbonates; these rocks overlie the accretionary prism. The accretion-related structure is dominated by large-scale northeast-dipping thrusts and subvertical cleavage stitched by 215‐190 Ma granites. There is an overall decrease in metamorphic grade and deformation intensity from the arc to the suture zone across the Bazar Dara and Heweitan subduction complexes. The Mazar accretionary prism formed by subduction-accretion processes during closure of the Paleotethyan Ocean and the final docking of the Gondwanan Karakoram-Qiangtang block to the Cathaysian (Eurasian) Kunlun block.

Carbonate Botryoids in Lower Devonian Amygdaloidal Basalts: Evidence for Precipitation of High-Magnesium Calcite from Heated and Volcanic CO2-Buffered Marine Waters

ABSTRACT Amygdaloidal basalts are found in the Lower Devonian Upper Gaspe Limestones in the Gaspe Peninsula. These basalts were extruded on a distal outer shelf, as evidenced by field relationships, embedded fragments of metazoans, and high vacuole content. The lava flows were fractured and brecciated; the network of fractures allowed connections of some vesicles with seawater. Vesicles in the nonfractured basalt are filled by silicates, whereas those opened to seawater are filled by carbonates. The latter comprise, in stratigraphic order, geopetal micrite, botryoidal calcite, and coarse anhedral calcite. Petrographic attributes, including microdolomite inclusions and trace-element distribution in the botryoids, indicate former high-magnesium calcite mineralogy. The d18OPDB (-9.2 to -10.0 and d13CPBD (-3.0 to -5.3) ratios support botryoid precipitation from heated marine waters that were contaminated by magmatic CO2 volatiles. High radiogenic Sr values originated from compaction waters expelled from the host succession. Submarine high-magnesium calcite agrees with the aragonite-inhibiting stage for the Early Devonian. The botryoid fabric should be the norm for early calcite in such an odd diagenetic setting.

Paleomagnetism of the latest Precambrian/Cambrian Unicoi basalts from the Blue Ridge, northeast Tennessee and southwest Virginia: evidence for Taconic deformation

Three components of magnetization have been observed in ninety-six samples (twelve sites) of amygdaloidal basalts and “sedimentary greenstones” of the Unicoi Formation in the Blue Ridge Province of northeast Tennessee and southwest Virginia. These components could be isolated by alternating field as well as thermal demagnetization. One component, with a direction close to that of the present-day geomagnetic field is ascribed to recent viscous remanent magnetizations; another component, with intermediate blocking temperatures and coercivities, gives a mean direction of D = 132°, I = +43°,α 95 = 9° for N = 10 sites before correction for tilt of the strata. This direction and the corresponding pole position are close to Ordovician/Silurian data from the North American craton and we infer this magnetization to be due to a thermal(?) remagnetization during or after the Taconic orogeny. This magnetization is of post-folding origin, which indicates that the Blue Ridge in our area was structurally affected by the Taconic deformation. The third component, with the highest blocking temperatures and coercivities, appears to reside in hematite. Its mean direction, D = 276°, I = −17°,α 95 = 13.8° for N = 6 sites (after tilt correction) corresponds to a pole close to Latest Precambrian and Cambrian poles for North America. The fold test is inconclusive for this magnetization at the 95% confidence level because of the near-coincidence of the strike and the declinations. We infer this direction to be due to early high-temperature oxidation of the basalts, and argue that its magnetization may have survived the later thermal events because of its intrinsic high blocking temperatures. A detailed examination of the paleomagnetic directions from this study reveals that the Blue Ridge in this area may have undergone a small counterclockwise rotation of about 15°.

Amygdaloidal basalts: Isotopic and petrographic evidence for non-diagenetic crustal source of carbonate inclusions

Even though carbonate amygdules in volcanics are generally assumed to be diagenetic in origin, the authors are of the opinion that almost all carbonate inclusions in the investigated amygdaloidal volcanic rocks from Sicily (Italy), Pindos (Greece) and Bohemia (Czechoslovakia), have a different origin. On the basis of a mineralogical, petrological and geochemical study these “amygdules” are interpreted as being remains ofmagmatic incorporation of carbonate. Therefore, the identification of carbonate globules in volcanics purely on field observations, is inadequate to distinguish true amygdules from carbonate assimilation remnants. A nomenclature of the various types of inclusions is proposed. The isotopic data suggest that the incorporation of carbonatic material by magma could have taken place without any significant modification of the isotopic composition of the carbonate. This study suggests that limited amounts of carbonate could be dissolved in alkali basalt magma; the further addition of moderate amounts of carbonate develops liquid immiscibility. Our inclusions, in agreement with the experimental evidence (Wyllie, 1974), are contrary to the limestone syntexis hypothesis for the generation of feldspathoidal magmas from subalkaline magmas. A possible model of interaction between magma and carbonate is also suggested.

Diamond drilling with surfactants in Upper Michigan amygdaloidal basalts using surface-set bits : TRANS. SME/AIME, V258, N3, SEPT. 1975, P185–P190

Diamond drilling with surfactants in Upper Michigan amygdaloidal basalts using surface-set bits : TRANS. SME/AIME, V258, N3, SEPT. 1975, P185–P190

Environment, vegetation and phytogeography of the high-altitude bogs of Lesotho

The high-altitude bogs of Lesotho or "Mokhoabo" are situated in the riverheads of the Orange River system in the eastern escarpment of Southern Africa (Fig. 1). The mountain ranges of Lesotho, the Maluti Mts and the Drakensberg range (highest mountain Thabana Ntlenyana 3484 m) are covered by up to 1500 m thick, early Jurassic, amygdaloidal basalts, in which the tribu taries of the Orange River have cut deep valleys.

Secondary minerals of the Tertiary basalts, Barrington, New South Wales

SummarySecondary minerals described from Tertiary amygdaloidal basalts in the Hunter Springs area, Barrington, New South Wales, are regarded as having been deposited from cold solutions some considerable time after consolidation of the basalts. The constituents in the solutions were evidently derived from the basalts by the action of circulating meteoric waters rather than by deuteric activity.

Zeólitas dos Basaltos do Brasil Meridional (Gênese e Pagênese)

The basalts and diabases w hich cover an extensive area of the southern States of Brazil are of ten bearers of varied species of zeolites (analcite, chabazite, thomsonite, ptilolie, natrolite, scolecite, mesolite, laum ontite, stilbite, stellerite, and heulandite) and as m any other related minerals (delessite, daphnite, celadonite, quartz, chalcedony, calcite, gyrolie, apophyllite, pyrite, and native copper). In the basalts the above m entioned m inerals occur in cavities of varied forms, w hile in the diabases they are located in the zones of shearing. They originated, mainly, from final residual solutions of the basaltic magm a itself, which were trapped inside the cavities or in the fractures of the diabases. The order of deposi'ion of the m inerals in the am ygdalae and fractures haphazard. On the contrary, there exists a definite sequence, alw ays repeated, in the material examined, no matter what its source. The minerals of the chlorite group and those of the silica group are always the first to be form ed. They are follow ed by the zeolites, then apophyllite, calcite and fin ally the sulphides. Among the zeolites there also a certain order of deposition. The heulandite precedes the rest. It follow ed by stilbite, which in is turn precedes chabazite. In the zones of shearing of the diabases calcite seems to go contrary to the above order, preceding the deposition of zeolites. There are zones of amygdaloidal basalts in which the cavities are fille d by only one kind of mineral — sometimes only m inerals of the silica group, odier times only analcite, mesolite, or chabazite. Phenomena of pseudomorphic substitutions among the minerals which fill the cavities and fractures of the basalts and diabases are not rare. The inclusions of sandstones and even sands of the Botucatu form ation in the basaltic lava flows did not at all influence the lava which reached the surface. Th ere are no signs of thermal metamorphism between the oldest amygdaloidal basalts and lavas and the most recent diabasic intrusions. The absence of numerous species of zeolites seems to be connected witi the lack of certain elements in the lava and basalts and also the non-existence of differentiated petrographie types. In the am ygdaloidal basalts, as well as in the homogenous ones and in the other types, as well as in the diabases there are found neither pyrogenic zeolites nor phenomena of analcitization