Petrological parameters of sands and sandstones from DSDP and ODP holes in arc-related areas

Detrital modes for 524 deep-marine sand and sandstone samples recovered on circum-Pacific, Caribbean, and Mediterranean legs of the Deep Sea Drilling Project and the Ocean Drilling Program form the basis for an actualistic model for arc-related provenance. This model refines the Dickinson and Suczek (1979) and Dickinson and others (1983) models and can be used to interpret the provenance/tectonic history of ancient arc-related sedimentary sequences. Four provenance groups are defined using QFL, QmKP, LmLvLs, and LvfLvmiLvl ternary plots of site means: (1) intraoceanic arc and remnant arc, (2) continental arc, (3) triple junction, and (4) strike-slip-continental arc. Intraoceanic- and remnant-arc sands are poor in quartz (mean QFL%Q < 5) and rich in lithics (QFL%L > 75); they are predominantly composed of plagioclase feldspar and volcanic lithic fragments. Continental-arc sand can be more quartzofeldspathic than the intraoceanic- and remnant-arc sand (mean QFL%Q values as much as 10, mean QFL%F values as much as 65, and mean QmKP%Qm as much as 20) and has more variable lithic populations, with minor metamorphic and sedimentary components. The triple-junction and strike-slip-continental groups compositionally overlap; both are more quartzofeldspathic than the other groups and show highly variable lithic proportions, but the strike-slip-continental group is more quartzose. Modal compositions of the triple junction group roughly correlate with the QFL transitional-arc field of Dickinson and others (1983), whereas the strike-slip-continental group approximately correlates with their dissected-arc field.

Geochemistry and structural formulas of heavy minerals from sands and sandstones of ODP Leg 126 samples

Lower Oligocene to Pleistocene volcaniclastic sands and sandstones recovered around the Izu-Bonin Arc during Ocean Drilling Program Leg 126 were derived entirely from Izu-Bonin Arc volcanism. Individual grains consist of volcanic glass, pumice, scoria, basaltic or andesitic fragments, plagioclase, pyroxene, and minor olivine and hornblende. In Pliocene-Pleistocene samples plagioclase and heavy minerals in the volcaniclastic sands and sandstones are present in the following abundances: plagioclase > orthopyroxene > clinopyroxene > pigeonite > olivine. In contrast, plagioclase and heavy minerals found in Oligocene-Miocene samples occur in the following order: plagioclase > clinopyroxene > orthopyroxene > hornblende. The low concentration of Al, Ti, and Cr in calcium-rich clinopyroxenes in Oligocene to Holocene sediments suggests that the sources of the volcaniclastic detritus were nonalkalic igneous rocks. There are, however, some distinctive differences in the chemical composition of pyroxene between the Pliocene-Pleistocene and Oligocene-Miocene volcaniclastic sands and sandstones. Orthopyroxene belongs to the hypersthene-ferrohypersthene series (Fe-rich) in Pliocene-Pleistocene sediments, and the bronzitehypersthene series (Mg-rich) in Oligocene-Miocene sediments. Clinopyroxene is characterized by augite and pigeonite in Pliocene-Pleistocene sediments, and by the diopside-augite series in Oligocene-Miocene sediments. Mineral assemblages and mineral chemistry of the volcaniclastic sands and sandstones reflect those of the volcanic source rocks. Therefore, the observed changes in mineralogy record the historical change in volcanism of the Izu-Bonin Arc. The mineralogy is consistent with the geochemistry of the volcaniclastic sands and sandstones and the geochemistry of forearc volcanic rocks of the Izu-Bonin Arc since the Oligocene.

Geochemistry of Pliocene-Pleistocene volcanic sands of ODP Site 136-842

Several distinct, thin (2-7 cm), volcanic sand layers ("ashes") were recovered in the upper portions of Holes 842A and 842B. These holes were drilled 320 km west of the island of Hawaii on the outer side of the arch that surrounds the southern end of the Hawaiian chain. These layers are Pliocene to Pleistocene in age, graded, and contain fresh glass and mineral fragments (mainly olivine, plagioclase, and clinopyroxene) and tests of Pleistocene to Eocene radiolarians. The glass fragments are weakly vesicular and blocky to platy in shape. The glass and olivine fragments from individual layers have large ranges in composition (i.e, larger than expected for a single eruption). These features are inconsistent with an explosive eruption origin for the sands. The only other viable mechanism for transporting these sands hundreds of kilometers from their probable source, the Hawaiian Islands, is turbidity currents. These currents were probably related to several of the giant debris slides that were identified from Gloria sidescan images around the islands. These currents would have run over the ~500-m-high Hawaiian Arch on their way to Site 842. This indicates that the turbidity currents were at least 325 m thick. Paleomagnetic and biostratigraphic data allow the ages of the sands to be constrained and, thus, related to particular Hawaiian debris flows. These correlations were checked by comparing the compositions of the glasses from the sands with those of glasses and rocks from islands with debris flows directed toward Site 842. Good correlations were found for the 110-ka slide from Mauna Loa and the ~1.4-Ma slide from Lanai. The correlation with Kauai is poor, probably because the data base for that volcano is small. The low to moderate sulfur content of the sand glasses indicates that they were derived from moderately to strongly degassed lavas (shallow marine or subaerially erupted), which correlates well with the location of the landslide scars on the flanks of the Hawaiian volcanoes. The glass sands may have been formed by brecciation during the landslide events or spallation and granulation as lava erupted into shallow water.

Composition of bioclastic sands, carbonates and pyroclastic rocks of the Great Meteor and Josephine Seamounts, eastern North Atlantic

1. Great Meteor Seamount (GMS) is a very large (24,000 km**3) guyot with a flat summit plateau at 330-275 m; it has a volcanic core, capped by 150-600 m of post-Middle-Miocene carbonate and pyroclastic rocks, and is covered by bioclastic sands. The much smaller Josephine Seamount (JS, summit 170- 500 m w. d.) consists mainly of basalt which is only locally covered by limestones and bioclastic sands. 2. The bioclastic sands are almost free of terrigenous components, and are well sorted, unimodal medium sands. (1) "Recent pelagic sands" are typical of water depths > 600 m (JS) or > 1000 m (GMS). (2) "Sands of mixed relict-recent origin" (10-40% relict) and (3) "relict sands" (> 40% relict) are highly reworked, coarse lag deposits from the upper flanks and summit tops in which recent constituents are mixed with Pleistocene or older relict material. 3. From the carbonate rocks of both seamounts, 12 "microfacies" (MF-)types were distinguished. The 4 major types are: (1) Bio(pel)sparites (MF 1) occur on the summit plateaus and consist of magnesian calcite cementing small pellets and either redeposited planktonic bioclasts or mixed benthonic-planktonic skeletal debris ; (2) Porous biomicrites (MF 2) are typical of the marginal parts of the summit plateaus and contain mostly planktonic foraminifera (and pteropods), sometimes with redeposited bioclasts and/or coated grains; (3) Dense, ferruginous coralline-algal biomicrudites with Amphistegina sp. (MF 3.1), or with tuffaceous components (MF 3.2); (4) Dense, pelagic foraminiferal nannomicrite (MF 4) with scattered siderite rhombs. Corresponding to the proportion and mineralogical composition of the bioclasts and of the (Mgcalcitic) peloids, micrite, and cement, magnesian calcite (13-17 mol-% MgCO3) is much more abundant than low-Mg calcite and aragonite in rock types (1) and (2). Type (3) contains an "intermediate" Mg-calcite (7-9 mol-X), possibly due to an original Mg deficiency or to partial exsolution of Mg during diagenesis. The nannomicrite (4) consists of low-Mg calcite only. 4. Three textural types of volcanic and associated gyroclastic rocks were distinguished: (1) holohyaline, rapidly chilled and granulated lava flows and tuffs (palagonite tuff breccia and hyaloclastic top breccia); (2) tachylitic basalts (less rapidly chilled; with opaque glass); and (3) "slowly" crystallized, holocrystalline alkali olivine basalts. The carbonate in most mixed pyroclastic-carbonate sediments at the basalt contact is of "post-eruptive" origin (micritic crusts etc.); "pre-eruptive" limestone is recrystallized or altered at the basalt contact. A deuteric (?hydrothermal) "mineralX", filling vesicles in basalt and cementing pyroclastic breccias is described for the first time. 5. Origin and development of GMS andJS: From its origin, some 85 m. y. ago, the volcano of GMS remained active until about 10 m. y. B. P. with an average lava discharge of 320 km**3/m. y. The volcanic origin of JS is much younger (?Middle Tertiary), but the volcanic activity ended also about 9 m. y. ago. During L a t e Miocene to Pliocene times both volcanoes were eroded (wave-rounded cobbles). The oldest pyroclastics and carbonates (MF 3.1, 3.2) were originally deposited in shallow-water (?algal reef hardground). The Plio (-Pleisto) cene foraminiferal nannomicrites (MF 4) suggest a meso- to bathypelagic environment along the flanks of GMS. During the Quaternary (?Pleistocene) bioclastic sands were deposited in water depths beyond wave base on the summit tops, repeatedly reworked, and lithified into loosely consolidated biopelsparites and biomicrites (MF 1 and 2; Fig. 15). Intermediate steps were a first intragranular filling by micrite, reworking, oncoidal coating, weak consolidation with Mg-calcite cemented "peloids" in intergranular voids and local compaction of the peloids into cryptocrystalline micrite with interlocking Mg-calcite crystals up to 4p. The submarine lithification process was frequently interrupted by long intervals of nondeposition, dissolution, boring, and later infilling. The limestones were probably never subaerially exposed. Presently, the carbonate rocks undergo biogenic incrustation and partial dissolution into bioclastic sands. The irregular distribution pattern of the sands reflects (a) the patchy distribution of living benthonic organisms, (b) the steady rain of planktonic organism onto the seamount top, (c) the composition of disintegrating subrecent limestones, and (d) the intensity of winnowing and reworking bottom current