Chemical composition of igneous rocks and origin of the sill and pillow-basalt complex at DSDP Site 61-462

The sill and pillow complex cored on Deep Sea Drilling Project Leg 61 (Site 462) is divided into two groups, A and B types, on the basis of chemical composition and volcanostratigraphy. The A-type basalt is characterized by a higher FeO*/MgO ratio and abundant TiO2, whereas the B-type basalt is characterized by a lower FeO*/MgO ratio and scarcity of TiO2. The A type is composed of sills interbedded with hyaloclastic sediments, and the B type consists of basalt sills and pillow basalt with minor amounts of sediment. However, the structure of pillow basalts in the B type is atypical; they might be eruptive. From paleontological study of the interbedded sediments and radiometric age determination of the basalt, the volcanic event of A type is assumed to be Cenomanian to Aptian, and that of B type somewhat older. The oceanic crust in the Nauru Basin was assumed to be Oxfordian, based on the Mesozoic magnetic anomaly. Consequently, two events of intraplate volcanism are recognized. It is thus assumed that the sill-pillow complex did not come from a normal oceanic ridge, and that normal oceanic basement could therefore underlie the complex. The Site 462 basalts are quartz-normative, and strongly hypersthene-normative, and have a higher FeO*/MgO ratio and lower TiO2 content. Olivine from the Nauru Basin basalts has a lower Mg/(Mg + Fe**2+) ratio (0.83-0.84) and coexists with spinel of lower Mg/(Mg + Fe**2+) ratio when compared to olivine-spinel pairs from mid-ocean ridge (MAR) basalt. The glass of spinel-bearing basalts has a higher FeO*/(FeO* + MgO) ratio (0.58-0.60) than that of MAR (<0.575). Therefore, the Nauru Basin basalts are chemically and mineralogically distinct from ocean-ridge tholeiite. That the Nauru Basin basalts are quartz-normative and strongly hypersthene-normative and have a lower TiO2 content suggests that the basaltic liquids of Site 462 were generated at shallower depths (<5 kbar) than ocean-ridge tholeiite: Site 462 basalts are similar to basalts from the Manihiki Plateau and the Ontong-Java Plateau, but different from Hawaiian tholeiite of hot-spot type, with lower K2O and TiO2 content. We propose a new type of basalt, ocean-plateau tholeiite, a product of intraplate volcanism.

Composition of sedimentary rocks from the Burun-Tas Formation, Bol'shoi Lyakhov Island (New Siberian Islands)

Graywackes and shales of the Bol'shoi Lyakhov Island originally attributed to Mesozoic were subsequently considered based on microfossils as Late Proterozoic in age. At present, these sediments in the greater part of the island are dated back to Permian based on palynological assemblages. In the examined area of the island, this siliciclastic complex is intensely deformed and tectonically juxtaposed with blocks of oceanic and island-arc rocks exhumed along the South Anyui suture. The complex is largely composed of turbidites with members displaying hummocky cross-stratification. Studied mineral and geochemical charac¬teristics of the rocks defined three provenances of clastic material: volcanic island arc, sedimentary cover and/or basement of an ancient platform, and exotic blocks of oceanic and island-arc rocks such as serpentinites and amphibolites. All rock associations represent elements of an orogenic structure that originated by collision of the New Siberian continental block with the Anyui-Svyatoi Nos island arc. Flyschoid sediments accumu¬lated in a foredeep in front of the latter structure in the course of collision. Late Jurassic volcanics belonging to the Anyui-Svyatoi Nos island arc determine the lower age limit of syncollision siliciclastic rocks. Presence of Late Jurassic zircons in sandstones of the flyschoid sequence in the Bol'shoi Lyakhov Island is confirmed by fission-track dating. The upper age limit is determined by Aptian-Albian postcollision granites and diorites intruding the siliciclastic complex. Consequently, the flyschoid sequence is within stratigraphic range from the terminal Late Jurassic to Neocomian. It appears that Permian age of sediments suggested earlier is based on redeposited organic remains. The same Late Jurassic-Neocomian age and lithology are characteristic of fossiliferous siliciclastic sequences of the Stolbovoi and Malyi Lyakhov islands, the New Siberian Archipelago, and of graywackes in the South Anyui area in the Chukchi Peninsula. All these sediments accumulated in a spacious foredeep that formed in the course the late Cimmerian orogeny along the southern margin of the Arctic conti¬nental block.

Geochemistry of sheeted dike complex minerals of ODP Hole 140-504B

Diabases were recovered during Legs 137 and 140 at Hole 504B from depths between 1621.5 and 2000.4 meters below seafloor in the lower sheeted dike complex. The samples contain multiple generations of millimetric to centimetric veins. The orientation of the measured veins suggests that two main vein sets exist: one characterized by shallow dipping and the other by random trend. Thermal contraction during rock cooling is considered the main mechanism responsible for fracture formation. Vein infill is related to the circulation of hydrothermal fluids near the spreading axis. Some veins are surrounded by millimeter-sized alteration halos due to fluid percolation from the fractures through the host rock. Vein-filling minerals are essentially amphibole, chlorite, and zeolites. Amphibole composition is controlled by the microstructural site of the rock. Actinolite is the main amphibole occurring in the veins and also in the groundmass away from the halos. In the alteration halos, amphibole shows composition of actinolitic hornblende and Mg-hornblende. Late-stage tension gashes and interstitial spaces in some amphibole-bearing veins are filled with zeolites, suggesting that the veins likely suffered multiple opening stages that record the cooling history of the circulating fluids. Evidence of deformation recorded by the recovered samples seems to be restricted to veins that clearly represent elements of weakness of the rock. On the basis of vein geometry and microstructure we infer structural interpretations for the formation mechanism and for deformation of veins.

Geochemical composition of minerals obtained from ODP Sites 180-1109, 180-1117 and 180-1118

During the last 8 m.y. the Papuan Peninsula region of Papua New Guinea has been affected by extension which opened the Woodlark Basin. The present-day spreading tip is located at the foot of the Moresby Seamount, a crustal block whose northern flank is an active low-angle normal fault related to this extension. During Ocean Drilling Program Leg 180 (7 June-11 August 1998), 11 sites (1108-1118) were drilled along a north-south-trending transect across the Woodlark Basin just ahead of the spreading tip. Four of these sites (1118, 1109, 1114, and 1117) reached the crystalline basement, which is composed of diabase and gabbro. Sites 1118 and 1109, located on the Woodlark Rise, belong to the hanging wall block, and Sites 1114 and 1117, located on the crest of the Moresby Seamount, belong to the footwall block and the fault zone itself. Most of the basalt, diabase, and gabbro that were recovered show a well-preserved magmatic texture. The diabase, which is the most abundant rock type, has a coarse-grained ophitic texture composed of poikilitic clinopyroxene including radiating, locally skeletal plagioclase laths with interstitial iron oxide grains. Secondary mineralogy consists of chlorite, zeolite, calcite, albite, and quartz. The gabbro shows a medium-grained granular texture. The magmatic mineralogy consists of euhedral laths of plagioclase and anhedral interstitial clinopyroxene. Secondary mineralogy consists of a magnesio to actinolitic hornblende, chlorite, clinozoisite, zeolite, quartz, and calcite. The retrograde metamorphic evolution of both gabbro and diabase occurred under low amphibolite to subgreenschist facies conditions associated mainly with brittle deformation and the development of a local low-temperature shear zone. This shows no evidence for high thermal gradient in the crust during the continental rifting.

(Table 1) Bulk Fe isotope values and pyrite framboids sizes of Cenomanian-Turonian samples

Organic carbon-rich shales from localities in England, Italy, and Morocco, which formed during the Cenomanian-Turonian oceanic anoxic event (OAE), have been examined for their total organic carbon (TOC) values together with their carbon, nitrogen, and iron isotope ratios. Carbon isotope stratigraphy (d13Corg and d13Ccarb) allows accurate recognition of the strata that record the oceanic anoxic event, in some cases allowing characterization of isotopic species before, during, and after the OAE. Within the black shales formed during the OAE, relatively heavy nitrogen isotope ratios, which correlate positively with TOC, suggest nitrate reduction (leading ultimately to denitrification and/or anaerobic ammonium oxidation). Black shales deposited before the onset of the OAE in Italy have unusually low bulk d57Fe values, unlike those found in the black shale (Livello Bonarelli) deposited during the oceanic anoxic event itself: These latter conform to the Phanerozoic norm for organic-rich sediments. Pyrite formation in the pre-OAE black shales has apparently taken place via dissimilatory iron reduction (DIR), within the sediment, a suboxic process that causes an approximately -2 per mil fractionation between a lithogenic Fe(III)oxide source and Fe(II)aq. In contrast, bacterial sulfate reduction (BSR), at least partly in the water column, characterized the OAE itself and was accompanied by only minor iron isotope fractionation. This change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from ~10 to ~7 µm. The gradual, albeit irregular increase in Fe isotope values during the OAE, as recorded in the Italian section, is taken to demonstrate limited isotopic evolution of the dissolved iron pool, consequent upon ongoing water column precipitation of pyrite under euxinic conditions. Given that evidence exists for both nitrate and sulfate reduction during the OAE, it is evident that redox conditions in the water column were highly variable, in both time and space.

Chemical composition of rocks and minerals from DSDP holes

The book summarizes materials obtained by Soviet scientists during participation in legs of D/S Glomar Challenger and in post-cruise studies. Results on stratigraphy and lithology of the sedimentary cover, petrography and geochemistry of magmatic rocks of the oceanic crust are discussed in the book. A modern analysis of the geophysical structure of the oceanic crust and of the tectonic structure of the ocean floor is given.

Major element compositions, minerals and trace elements at DSDP Leg 67 Holes

Mineralogical (microprobe) and geochemical (X-ray fluorescence, neutron activation analyses) data are given for 18 samples of volcanic rocks from the Guatemala Trench area (Deep Sea Drilling Project Leg 67). Typical fresh oceanic tholeiites occur in the trench itself (Hole 500) and in its immediate vicinity on the Cocos Plate (Site 495). Several samples (often reworked) of "spilitic" oceanic tholeiites are also described from the Trench: their mineralogy (greenschist facies association - actinolite + plagioclase + chlorite) and geochemistry (alteration, sometimes linked to manganese and zinc mineralization) are shown to result from high-temperature (300°-475°C) hydrothermal sea water-basalt interactions. The samples studied are depleted in light rare-earth elements (LREE), with the exception of the slightly LREE-enriched basalts from Hole 500. The occurrence of such different oceanic tholeiites in the same area is problematic. Volcanic rocks from the Guatemala continental slope (Hole 494A) are described as greenschist facies metabasites (actinolite + epidote + chlorite + plagioclase + calcite + quartz), mineralogically different from the spilites exposed on the Costa Rica coastal range (Nicoya Peninsula). Their primary magmatic affinity is uncertain: clinopyroxene and plagioclase compositions, together with titanium and other hygromagmaphile element contents, support an "active margin" affinity. The LREE-depleted patterns encountered in the present case, however, are not frequently found in orogenic samples but are typical of many oceanic tholeiites.