Olivine Basalt Flows Research Articles

Chemical composition of magnetite and chlorite from the stringer zone of the Nudeh volcanogenic massive sulfide (VMS) deposit, Iran: geological implications

The southwest Sabzevar basin situated in the Sabzevar zone is considered to be an attractive metallogenic province in Iran that hosts both volcanogenic massive sulfide (VMS) and stratiform manganese deposits. The Nudeh Besshi-type VMS deposit is located in the Lower Late Cretaceous volcano-sedimentary sequence. The ore mineralization in this deposit is hosted in the alkali olivine basalt flow and tuffaceous silty sandstone rocks. The Nudeh VMS deposit consists of 2 million metric tons of Cu-Zn massive sulfide overlying a Cu-Fe-rich stringer. The massive sulfide orebody consists dominantly of pyrite, chalcopyrite, friedrichite, magnetite, and sphalerite, together with minor quartz, chlorite, and sericite. Chloritization, silicification, and sericitization are the main wall-rock alteration types; alteration intensity increases towards the stringer zone. Chloritized footwall rocks extend up to 20 m below the stringer zone. The quartz-bearing stringer veins also contain pyrite, chalcopyrite, magnetite, and bornite. Magnetite crystals from the stringer ores show variable contents of many elements, such as MgO (0.05 wt%), Al2O3 (0.63 wt%), TiO2 (0.07 wt%), V2O3 (0.045 wt%), SiO2 (0.65 wt%), CoO (0.10 wt%), NiO (0.009 wt%), ZnO (0.023 wt%), and CaO (0.03 wt%). The moderate to high V contents are interpreted to result from relatively reduced, seafloor hydrothermal activiy. Compositional variations of magnetite are possibly related to variations in oxygen fugacity, temperature, and water/rock interaction. Within the stringer zone, chlorite 2 (Chl-2) in the vein-veinlets and chlorite 1 (Chl-1) in the chloritized alkali olivine basalt rock are chemically indistinguishable, with 26.92–34.67 wt% FeO and 5.99–14.01 wt% MgO. Chlorite geothermometer studies indicate crystallization formation temperatures of 414 °C (Chl-1) and 303 °C (Chl-2), respectively.

Mineralogy of a New Occurrence of Lamprophyre Dyke from the Saurashtra Peninsula of Gujarat, Northwest Deccan Trap, India

ABSTRACT A new occurrence of lamprophyre dyke is being reported from Saurashtra peninsula of Gujarat, north-western Deccan trap. The paper presents and discusses the detail mineralogy, field setting and whole rock major and trace element geochemistry. The newly discovered lamprophyre dyke intruded into the porphyritic olivine basalt flow. It is E-W trending dyke paralleling with Narmada rift trend, 200 m long and ~1 m in width. Petrographic observations suggest the lamprophyre dyke is fine grained, porphyritic and shows typical panidiomoprphic texture. It contains phenocrysts of euhedral to subhedral amphibole (kaersutite), subhedral clinopyroxenes (diposide augite) and olivine pseudomorph enveloped in the groundmass composed of plagioclase, clinopyroxene and amphibole with carbonate, Ti-magnetite, zeolite and apatite occurring as accessory phases. Few clinopyroxenes show reaction and embayed margins thus indicating xenocrystic orgin. The normative chemistry of the lamprophyre dyke suggests alkaline nature with significant amount of olivine and nepheline. Whole rock geochemical data shows comparatively higher SiO2 than the other lamprophyres of the Deccan and similar to the MgO depleted lamprophyres of the Chhota Udepur alkaline province. The primitive mantle normalized multi-elements pattern shows absence of any high field strength element (HFSE) anomaly. Studied dyke displays more similarity with the earlier reported pre- Deccan alkaline lamprophyres and more particular with the Chhota Udepur lamprophyre. Occurrence of this new lamprophyre dyke from the Saurashtra region extends and confirms the presence of lamprophyre magmatism in the far north-western part of the Deccan.

Mineralogy, microchemistry and fluid inclusion studies of the Besshi-type Nudeh Cu-Zn VMS deposit, Iran

The southwestern Sabzevar basin is the north of Central Iranian Microcontinent hosts abundant mineral deposits, including exhalative Mn mineralization and Cu-Zn volcanogenic massive sulfide (VMS) deposits. Amongst them, the Nudeh Besshi-type Cu–Zn volcanogenic massive sulfide (VMS) deposit is hosted within the lower part of a Late Cretaceous volcano-sedimentary sequence composed of alkali olivine basalt flows and tuffaceous silty sandstone. Based on investigations into the ore geometry, mineralogy, and texture, we recognized three different ore facies: (1) a stockwork of sulfide-bearing quartz veins cutting across the footwall volcano-sedimentary rocks and representing the stringer zone; (2) a massive ore type, displaying replacement texture with pyrite, chalcopyrite, sphalerite, friedrichite, and minor magnetite; and (3) a bedded ore type, with laminated to disseminated pyrite and chalcopyrite. EPMA studies indicate a distinctive minor element distribution between the different ore types of the Nudeh deposit. The Fe content in the sphalerite ranges from 0.65–1.80 wt.%, indicating the Fe-poor nature of the sphalerite. However, the Cd content in sphalerite ranged between 0.164–0.278 wt.%. According to the mineral compositions, Zn, Se, and Ag are found in bornite as minor elements. In the bedded ore facies, the pyrite contains higher levels of Se (up to 0.35 wt.%). The Zn content in the friedrichite in all of the ore samples is low. The Co/Ni ratios in pyrite from the Nudeh ore are lower than those of most magmatic deposits, but are similar to those from volcanogenic deposits, and hence support the proposed hydrothermal origin of the deposit. Two generations of quartz, Q1 and Q2 in the stockwork veins, contain primary fluid inclusions and these contain two phases (liquid and vapor). The lack of vapor-rich inclusions or variable liquid/vapor ratios indicate that the fluids did not boil at the site of trapping. Salinity for both Q1 and Q2 fluid inclusions ranges between 2.2–6.8 wt.% eq. NaCl. Homogenization temperatures for inclusions in the Q1 and Q2 veins average at about 296 °C and are similar to the temperatures of hydrothermal fluids discharged through vents in many modern seafloor VMS deposit. The Nudeh Besshi-type VMS deposit appears to have formed on the seafloor and based on the salinity and temperature constraints from the underlying stockwork, a buoyancy plume model is proposed as a mechanism for precipitation.

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.

Geology, ore facies and sulfur isotopes geochemistry of the Nudeh Besshi-type volcanogenic massive sulfide deposit, southwest Sabzevar basin, Iran

The southwest Sabzevar basin is placed in the southwestern part of a crustal domain known as the Sabzevar zone, at the north of Central Iranian microcontinent. This basin hosts abundant mineral deposits; particularly of the Mn exhalative and Cu-Zn volcanogenic massive sulfide (VMS) types. The evolution of this basin is governed by the Neo-tethys oceanic crust subduction beneath the Central Iranian microcontinent and by the resulting continental arc (Sanandaj-Sirjan) and back-arc (Sabzevar-Naien). This evolution followed two major sequences: (I) Lower Late Cretaceous Volcano-Sedimentary Sequence (LLCVSS), which is indicated by fine-grained siliciclastic sediments, gray basic coarse-grained different pyroclastic rocks and bimodal volcanism. During this stage, tuff-hosted stratiform, exhalative Mn deposits (Nudeh, Benesbourd, Ferizy and Goft), oxide Cu deposits (Garab and Ferizy) and Cu-Zn VMS (Nudeh, Chun and Lala) deposits formed. (II) Upper Late Cretaceous Sedimentary Dominated Sequence (ULCSS), including pelagic limestone, marly tuff, silty limestone and marl with minor andesitic tuff rocks. The economically most important Mn (Zakeri and Cheshmeh-sefid) deposits of Sabzevar zone occur within the marly tuff of this sequence. The Nudeh Cu–Zn volcanogenic massive sulfide (VMS) deposit is situated in the LLCVSS. The host-rock of deposits consists of alkali olivine basalt flow and tuffaceous silty sandstone. Mineralization occurs as stratiform blanket-like and tabular orebodies. Based on ore body structure, mineralogy, and ore fabric, we recognize three different ore facies in the Nudeh deposit: (1) a stringer zone, consisting of a discordant mineralization of sulfides forming a stockwork of sulfide-bearing quartz veins cutting the footwall volcano-sedimentary rocks; (2) a massive ore, consisting of massive replacement pyrite, chalcopyrite, sphalerite and Friedrichite with magnetite; (3) bedded ore, with laminated to disseminated pyrite, and chalcopyrite. Chloritization, silicification, sericitization and epidotization are the main wall-rock alterations; alteration intensity increases towards the stringer zone. The δ34S composition of the sulfides ranges from −1.5‰ to +3.69‰ with a general increase of δ34S ratios of massive ore facies to stockwork zone. The heavier values indicate that some of the sulfur was derived from seawater sulfate that was ultimately thermochemically reduced in deep hydrothermal reaction zones. Sulfur isotopes, along with sedimentological, textural, petrological, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a Besshi-type VMS ore deposit.

Extensive middle Miocene weathering interpreted from a well-preserved paleosol, Cricket Flat, Oregon, USA

The high-resolution marine isotope climate record indicates pronounced global cooling during the Langhian (16–13.8Ma), beginning with the warm middle Miocene climatic optimum and ending with significant Antarctic ice sheet expansion and the transition to “icehouse” conditions. However, the timing and rate of cooling during the Langhian are uncertain. Terrestrial paleoclimate data from this interval is sparse and sometimes conflicting; in particular, there are gaps in the terrestrial record in the Pacific Northwest during the late Langhian and early Serravallian between about 14.5 and 12.5Ma. New terrestrial paleoclimate data from this time and region could help reconcile these conflicting records. Paleosols are particularly useful for reconstructing paleoenvironment because the rate and style of pedogenesis are primarily a function of surface environmental conditions; however, complete and well-preserved paleosols are uncommon. Most soils form in erosive environments that are not preserved, or in environments such as floodplains that accumulate in small increments; the resulting cumulic soils are usually thin, weakly developed, and subject to diagenetic overprinting from subsequent soils. The paleosol at Cricket Flat in northeastern Oregon is an unusually complete and well-preserved paleosol from a middle Miocene volcanic sequence in the Powder River Volcanic Field. An olivine basalt flow buried the paleosol at approximately 13.8±0.6Ma, based on three 40Ar/39Ar dates on the basalt. We described the Cricket Flat paleosol and used its physical and chemical profile and micromorphology to assess pedogenesis. The Cricket Flat paleosol is an Ultisol-like paleosol, chemically consistent with a high degree of weathering. Temperature and rainfall proxies suggest that Cricket Flat received 1120±180mm precipitationyear−1 and experienced a mean annual temperature of 14.5±2.1°C during the formation of the paleosol, significantly warmer and wetter than today. This suggests slower cooling after the middle Miocene climatic optimum than is seen in the existing paleosol record.

Contribution to the Petrography, Geochemistry, and Petrogenesis of Zarqa-Ma’in Pleistocene Alkali Olivine Basalt Flow of Central Jordan

The Zarqa-Ma’in basalt (MB) occurs near a plateau basalt (wadi fills) covering about 15 km2 of Makawir, Ataruz, and Hammat um Hasana cone areas in central Jordan. The tectonic evolution occurred through intraplate volcanism and erupted through fissure systems along the Dead Sea, transforming the fault during Miocene to Pleistocene period. Three stages of eruption of MB have been recorded during Pleistocene from 6 to 0.6 Ma. The petrographic analyses data show that the MB rocks are composed of plagioclase, olivine, pyroxene, and magnetite, including secondary minerals calcite, iddingsite, serpentine, and zeolite. Furthermore, the MB rocks have narrow ranges of major and trace element concentrations, and are of under saturated silica type and belong to sodic alkaline magma series. The geochemical characteristics of MB indicate that MB was derived from a slightly fractionated magma as reflected by its high MgO (6.3 - 11.7 ppm) concentration with Mg number from 0.41 to 0.61, low silica content (40.83 - 47.55 wt%), and high Cr and Ni concentrations (115 - 475 and 105 - 553 ppm, respectively). This basalt exhibited low degree of partial melting (10%) for garnet peridotite mantle source. The model mineral fractionation showed that the MB could be fractionated to clinopyroxene, orthopyroxene, olivine, and plagioclase.

Opaque Mineralogy as a Tracer of Magmatic History of Volcanic Rocks: an Example from the Neogene‐Quaternary Harrat Rahat Intercontinental Volcanic Field, North Western Saudi Arabia

Abstract:The Neogene‐Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I (olivine basalt and hawaiite) and Group II (mugearite, benmorite and trachyte). The maximum forsterite content (∼85) is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate (Fo=50), fayalitic (Fo=25) and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2+, Mn2+ and Na+ that indicates normal fractional crystallization. The common occurrence of coarse apatite with titanomagnetite in the benmorite indicates that P5+ becomes saturated in this rock variety and drops again in trachyte. Cr‐spinel is recorded in Group I varieties only and the Cr# (0.5) suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content (Fo) suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr‐spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high‐Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle‐crust boundary. The increase in fO2 causes a decrease in Cr2O3, and Al2O3, and a strong increase in the proportion of Fe3+ and Mg# of spinel crystallizing from the basaltic melt at T ∼1200°C. The olivine‐pyroxene and olivine‐spinel geothermometers yielded equilibrium temperature in the range of 935–1025°C, whereas the range of <500–850°C from single‐pyroxene thermometry indicates either post crystallization re‐equilibrium of the clinopyroxene, or the mineral is xenocrystic and re‐equilibrated in a cooling basaltic magma.

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field, Saudi Arabia: Implications for duration and migration of volcanic activity

New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7Ma–Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (∼10Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.

K-Ar ages of the Quaternary basalts in the Jeongok area, the central part of Korean Peninsula

The Quaternary basalts along the Chugaryeong Fault zone in central part of the Korean Peninsula are composed of two stratigraphic units in the Jeongok area, calling the Chatan and Jeongok basalts. K-Ar analyses of the mixture of the fine-grained plagioclase and anorthoclase from the 18 basalt samples were carried out to reveal the volcanic activity of the area. The results show that the Chatan and Jeongok basalts give a uniform age, giving the weighted average ages of 0.15 ± 0.01 and 0.51 ± 0.01 Ma, respectively. This reveals the age gap between the Chatan and Jeongok basalts was 360,000 years, providing a unique volcanic history for the Quaternary volcanism. The volcanism formed the two large amounts of alkali olivine basalt flows that are similar to each other in primitive chemical composition, phenocryst composition and viscosity with an interval of 360,000 years. The results also make a time constraint on the Jeongok Paleolithic Site where the Jeongok basalt occurs below the sediments containing the Paleolithic artifacts. The age (0.51 Ma) of the Jeongok basalt provides the lower limit for the age of Paleolithic artifacts. Olivine phenocrysts from both the Chatan and Jeongok basalts have significant amount of excess argon, giving apparent older ages on the whole rock K-Ar dating by previous researchers.

Shipboard paleomagnetic age estimates for an acoustic basement emplacement in Marion Plateau, off northeast Australia

Ocean Drilling Program Leg 194 was carried out in the Marion Plateau, located to the northeast of Australia. Drilling penetrated few metres of altered olivine basalt flows, underlying the sediments. Shipboard paleomagnetic investigations on these basalts resulted in well-defined paleomagnetic directions. The directions from the archive-half long-core measurements at hole 1193C between 540.97 and 543.97 m below seafloor, together with the archive-half long-core and discrete measurements from the working-half of hole 1198B between 513.85 and 517.25 m below seafloor, yielded an overall mean inclination value of 52.7° (α 95 = 4.0°, K = 537, n = 129, N = 4). These give the paleolatitude of the Marion Plateau to be 33.3°S with upper and lower 95% confidence limit 37.3°S and 29.6°S, respectively. Comparison of the corresponding paleo-colatitude result with the Australian apparent polar wander path indicates a possible emplacement time for the basalt of either 130–110 Ma or 190–165 Ma. The latter result is in better agreement with the available 40Ar/39Ar age of 162 ± 0.9 Ma reported for the basalt.

Petrologic evolution of Pre-Unzen and Unzen magma chambers beneath the Shimabara Peninsula, Kyushu, Japan: Evidence from petrography and bulk rock chemistry

The volcanic history of the Shimabara Peninsula, Kyushu, Japan, is divided into two stages; Pre-Unzen volcano stage (4 Ma-500 ka) and Unzen volcano stage (500 ka-present). Pre-Unzen volcanic rocks comprise olivine basalt and two-pyroxene andesite lava flows and pyroclastics. The similarity of trace elements chemistry indicates that Pre-Unzen basalts evolved from different primary magmas originated in the same mantle source. They were differentiated by olivine-dominant fractional crystallization and crustal assimilation. This process produced parental magmas for Pre-Unzen andesite. The evolution of Pre-Unzen andesites can be explained by the combination of plagioclase + pyroxenes + magnetite fractional crystallization and crustal assimilation. Unzen volcanic rocks are composed of hornblende andesite to dacite lava domes, lava flows and pyroclastics. They have petrographical and geochemical features indicating that they were produced by magma mixing. Mafic inclusions are commonly included in Unzen volcanic rocks and show evidence of hybridization between aphyric basalt and phenocryst-rich dacite magma in various ratios. The existence of mafic inclusions with positive Nb anomalies in spidergram indicates an injection of ocean-island type basaltic magmas. The estimated mafic endmember for the dacite magma is an evolved basalt with MgO = 5.5 wt.%. This corresponds to the most evolved composition of the Pre-Unzen basalt and suggests continuous basaltic magma plumbing system throughout the eruptive history of the Shimabara peninsula.

Debris avalanches on the western flank of Piton des Neiges shield volcano (Reunion Island)

The Saint-Gilles breccias, on the western flank of Piton des Neiges volcano, are clearly identified as debris avalanche deposits. A petrographic, textural and structural analysis of the breccias and inter-bedded autochthonous lava flows enables us to distinguish at least four successive flank slides. The oldest deposit sampled the hydrothermally-altered inner parts of the volcano, and has a large volume. Failure was favored by the presence of a deep intensely-weathered layer. The younger deposits are from superficial sources, as their products are rarely hydrothermalized and are more vesicular. The breccia formation, and especially the progressive breaking up occurring during the debris avalanche displacement, indicates the existence of high speed transport. In the Cap La Houssaye coastal area, abrasion and striation of the underlying lava formation, as well as the packing features observed in the breccia, are considered to be deceleration structures.

Geochronology of Neogene alkaline volcanic rocks (Miles Canyon basalt), southern Yukon Territory, Canada: the relative effectiveness of laser 40Ar/39Ar and K-Ar geochronology

Miles Canyon basalt is an informal term used to describe numerous exposures of young alkaline olivine basalt flows in southern Yukon. The volcanic rocks are part of the Northern Cordilleran volcanic province. K-Ar and Ar-Ar whole-rock dates indicate that the Miles Canyon succession of flows at the Whitehorse Rapids are clearly Late Miocene in age (ca. 8.4 Ma). The largest exposure of the Miles Canyon basalt occurs at the Alligator Lake volcanic complex where two nearly concordant Pliocene Ar-Ar dates indicate eruption at ca. 3.2 Ma. K-Ar analyses from other sites yield dates of 2.4 and 7.1 Ma and indicate an episodic Neogene volcanic history for the region. There is no evidence of Quaternary or postglacial volcanism. The dates are older than assumed by previous workers, and in some cases the K-Ar dates are strongly discordant from Ar-Ar determinations. More accurate Ar-Ar determinations may result from the method's ability to select smaller amounts of better material for analysis. Excess 40Ar was not encountered. As a result, the accuracy of any single or several discordant K-Ar determinations for Neogene subaerial volcanic rocks, particularly low-K rocks such as basalts, should be questioned and resulting interpretations made with caution. Models accounting for the eruption of the Northern Cordilleran volcanic province lavas have typically relied upon extension along north-trending faults that were generated by stresses along the continental margin. However, we consider a slab window model which better accounts for the initiation and distribution of northern Cordilleran Neogene volcanic activity.

Lithology, chemistry, age, and origin of the Proterozoic Cardenas Basalt, Grand Canyon, Arizona

Abstract The Cardenas Basalt, a 300-m-thick flow sequence, occurs in the middle of the 4000-m-thick Proterozoic sedimentary sequence in the Grand Canyon. It consists of a 100-m-thick lower member composed of about six, poorly exposed, coarsely ophitic pahoehoe flows of olivine basalt. In contrast, the upper member, 200 m thick, comprises four to six, resistant, aphyric, intersertal to intergranular flows that range from quartz tholeiite to tholeiitic andesite (icelandite). Petrographically, mafic intrusions exposed throughout the Inner Gorge of the Grand Canyon are similar to the lower member. Newly acquired RbSr isotopic data, in combination with those previously available, define a well-constrained isochron (10 points) at 1103±66 Ma. This date is concordant with a RbSr date from one of the mafic intrusions, supporting the conclusion that the Grand Canyon mafic magmatic episode occurred ∼ 1.1 Ga. The magma that fed both the intrusions and the lower member appears to have been principally derived from a slightly enriched mantle source. Subsequently, this magma underwent ∼ 20% fractionation of olivine and, possibly, minor assimilation of continental crust. Differences in SiO2, A12O3, TiO2, and P2O5 contents between the lower member and intrusions, apparently reflect heterogeneities in the mantle source, or different degrees of melting and/or crustal assimilation. The quartz-tholeiite magma of the upper member apparently resulted from more-extensive fractionation and crustal contamination of a mantle-derived magma. After its formation, this magma underwent further fractionation to produce, tholeiitic andesite (icelandite).

Magmatism in the South China Basin: 2. Post-spreading Quaternary basalts from Hainan Island, south China

Cenozoic magmatism on northern Hainan Island was part of a regional episode post-dating opening of the South China Basin (SCB). Tholeiite lava flows emanated from WSW-ENE-trending extensional fissures and are interlayered with mid-Miocene to Quaternary sediments, forming a 200–1000-m-thick sequence. Subsidiary alkali basalts were erupted from central volcanoes aligned on NW-SE-trending strike-slip faults, and in some cases contain ultramafic xenoliths, and megacrysts of augite, sapphire (corundum), zircon and anorthoclase. Cumulative thickness of the lavas rarely exceeds 200 m although sills of unknown age intrude early Oligocene to Cretaceous sediments at depths up to 3000 m. Lava compositions include quartz tholeiite, olivine tholeiite, alkali olivine basalt and basanite, and are subdivided into chemical types on the basis of temporal-spatial association. With time the basalts change from relatively thin intercalated quartz tholeiite and alkali olivine basalt flows to more massive olivine tholeiites. In common with other SCB eruptives, Hainan basalts resemble Dupal-type oceanic island basalt and reflect mixing of EM 2 and N-MORB mantle reservoirs. Decoupled systematics of Sm/Nd and 143 Nd 144 Nd are at least partly due to differentiation during partial melting, possibly by decompression melting of plagioclase/spinel- and spinel-lherzolite. It is proposed that EM 2 derives from sub-continental lithosphere or asthenosphere reservoirs, enriched by subduction prior to crustal extension.

Geology and geochronology of Inaccessible Island, South Atlantic

AbstractInaccessible Island is the eroded remnant of an extinct, comparatively small intraplate volcano dominated by flows of alkaline olivine basalt. The oldest stratigraphie unit is a hydrothermally altered basement of somewhat questionable early Pliocene (6.5 Ma) age. This is unconformably overlain by a volcanic superstructure built up during the last three million years. The two formations have different trace element signatures that may be attributed to different mantle sources. Boulders of gabbro are common but the presence of an in situ plutonic intrusion could not be confirmed. Their K-Ar age of 12.8 Ma may be spurious and their possible relationship with the volcano is uncertain. Reliable age determinations of 0.95–0.72 Ma were obtained on lava flows of the second volcanic stage, subdivided into four units or stratigraphie members. The latest unit consists of plugs, sills and flows of an evolved magma fraction (benmoreite and trachyte) of which benmoreite is considered to be the more voluminous. Several dyke swarms of different ages reveal the internal structure of the volcano. It is concluded that the main volcanic centre was located immediately offshore to the northwest and that the edifice was attached to an east–west volcanic rift zone. Apart from marine erosion, massive land-sliding probably took part in shaping the island and its submarine platform.

Chronology of volcanism and rift basin propagation: Rungwe Volcanic Province, East Africa

Continental rifts are segmented along their lengths into 50 to 100 km‐long extensional basins, suggesting a genetic relationship with regularly segmented oceanic rifts. To investigate the spatial and temporal development of along‐axis segmentation in youthful continental rifts, field, remote sensing, and K‐Ar geochronology studies were conducted in four Western rift (East Africa) basins. Volcanism within the Rungwe region began in late Miocene time prior to or concurrent with the development of high‐angle border faults bounding the then isolated Karonga and Rukwa basins. The Usangu and Songwe border fault segments located between the Rukwa and Karonga basins developed in mid‐Pliocene and Pleistocene time, respectively. Within the northern Karonga basin, approximately 2–4 km of WSW‐ENE directed “thick‐skinned” extension is estimated both from extrapolation of late Pleistocene slip rates to 5 Ma and from surface fault geometries. Differential strains between these extensional basins of different ages are transmitted to adjoining segments by NW and ENE striking oblique‐slip transfer faults within comparatively high‐strain accommodation zones. Throws along transfer faults also accommodate 2 km (minimum) variations in depth to basement beneath adjacent basins, whereas monoclines and ramps accommodate differential uplift along the flanks of basins. The observed geometry of en echelon border fault segments linked by shorter oblique‐slip transfer faults is similar to patterns predicted in numerical models of en echelon normal fault interactions [e.g., Aydin and Pollard, 1982]. Eruptive centers for alkali olivine basalt and phonolite flows within the Rungwe region coincide with the tips of en echelon border fault segments and with high‐angle transfer faults linking discrete border fault segments. The locations of eruptive centers appear to be controlled by faults, and these centers generally have propagated northward along transfer faults during Plio‐Pleistocene time. Historic eruptions near late Miocene centers located at the southern end of the transfer fault system, however, argue against a northward migration of the magma source. Thus, an along‐axis propagation of “thick‐skinned” lithospheric extension links once isolated basins and produces an along‐axis segmentation of this youthful rift. Propagation of magmatism along transfer faults linking en echelon border faults locally contributes to the regular segmentation.

Late Cenozoic volcanic rocks of the Clearwater – Wells Gray area, British Columbia

The Clearwater – Wells Gray area of east-central British Columbia includes a succession of late Cenozoic, alkali olivine basalt flows that lie east of the extensive Chilcotin lavas and define the eastern end of the Anahim Volcanic Belt. The rocks are petrographically similar to but less altered than the Chilcotin basalts. The volcanic activity spanned at least two episodes of glacial advance and produced both subaerial flows and a subaqueous facies comprising pillow lava, pillow breccia, and tuff breccia, locally intercalated with fluvial gravels and sand. Four morphological assemblages have been recognized. An early glacial assemblage, characterized by tuyalike forms, gives K – Ar dates of 0.27 – 3.5 Ma. These circular features are surrounded by a deeply dissected valley-filling assemblage of subaerial and minor subaqueous flows and tuff breccia that rest locally on lag gravel and till. Subaerial flows in this assemblage give K – Ar dates of 0.15 – 0.56 Ma. Whitehorse Bluffs, a volcanic centre composed of crudely laminated tuff cut by high-level dykes, may be a source of some of these valley-filling flows. A late interglacial assemblage is composed of subaerial pyroclastic material, transitional deposits, and deposits that are clearly subaqueous. Volcanic activity in the area culminated with the formation of pyroclastic cones, blocky lava flows, and pit craters that postdate the last Cordilleran glaciation.

Mesozoic and Cenozoic Stratigraphy and Tectonic Events of Puno-Santa Lucia Area, Department of Puno, Peru

The Puno-Santa Lucia area is in the higher parts of the Andes in southern Peru. The oldest rocks in the area are quartzite and shale of the lower Paleozoic Cabanillas Group, more than 2,000 m thick. The Jurassic Lagunillas Group is divided into three units with an aggregate thickness of 1,505 m. The Cretaceous rocks are included in the Huancane Group and the Moho Group, the combined thickness of which is 410 m. The Ayavacas Limestone, upper member of the Moho Group, contains fossils of Cenomanian age. Trachytes of Cretaceous age are present near Juliaca and on the Capachica Peninsula. Redbeds, Cotacucho, Vilquechico, and Munani Formations, mapped by Newell within the Tertiary Puno Group, are considered here to be Late Cretaceous on the basis of their stratigraphic positio . Apparently these formations were deposited in a closed basin between the Cordillera Occidental and Cordillera Oriental. The Puno Group of probable Tertiary age consists of two units: the Saracocha Formation below, consisting mainly of conglomerate, is widely distributed in the southwestern part of the area, but is absent in the northeast; and the Tacaza Formation above, a volcanic sequence, also present in the southwestern part, shows the thickest sections along the Cordillera Occidental with thinning toward the east. Olivine basalt flows and tuffs of the Sillapaca Formation unconformably overlie the Puno Group. Three orogenies have been identified.