The procedures for describing igneous rocks during Leg 179, in general, follow the outline presented in the "Explanatory Notes" chapters for Leg 147 (Shipboard Scientific Party, 1993a), Leg 153 (Shipboard Scientific Party, 1995) and Leg 176 (Shipboard Scientific Party, 1999). All igneous petrography and petrology observations are stored in electronic form in project-designed spreadsheets called the igneous-metamorphic core log and are based on the following definitions given below. For a detailed discussion of core curation and handling, the reader is referred to the "Introduction" section of this chapter. Standard ODP visual core description (VCD) sheets were used to document the igneous rock cores and include both a graphical representation and summaries of the dominant macroscopic features. Descriptions of microscopic features were archived on thin-section description forms. For both the macroscopic and microscopic descriptions, see the "Core Descriptions" contents list.
Igneous intervals were defined on the basis of primary igneous lithologies, textures, and contacts and do not necessarily imply intrusive relationships. The nature of the igneous contacts was systematically recorded together with igneous structures, mineralogy, and textures. Mineral modes and grain sizes were visually estimated and textures and fabric were evaluated based on macroscopic observations. The observations were recorded for each lithologic interval. A summary of the individual measurements is given below.
The classification of mafic and ultramafic plutonic rocks follows International Union of Geological Sciences (IUGS) recommendations (Le Maitre, 1989; Fig. F6) with minor modifications. We use the term oxide gabbro if the mode exceeds 5% oxide minerals and the modifier oxide-bearing if oxide minerals are present in abundances <5%. The modifier troctolitic was used to describe olivine gabbro when pyroxene represents 5% to 15% of the ferromagnesian assemblage. Lithologies with high proportions (>65%) of mafic minerals were noted as melanocratic, and those with high proportions (>65%) of plagioclase were noted as leucocratic. The modifier micro- was used to distinguish gabbroic rocks with a dominant grain size <1 mm (e.g., microgabbro, microtroctolite, and microgabbronorite). The modifier pegmatitic was used when the average grain size of an interval exceeds 30 mm. Leucocratic rocks with >20% quartz and <1% K-feldspar (a restricted part of the tonalite field of the IUGS classification) were called trondhjemites in keeping with previous usage in the ocean-crust literature.
If a mafic plutonic rock exhibits the effects of dynamic metamorphism such that the assemblage consists of secondary hydrous minerals that completely obliterate the protolith mineralogy and texture, or if the rock is made up of recrystallized primary minerals such that the original igneous protolith cannot be recognized, the appropriate metamorphic rock names were used. The textural terms mylonitic, schistose, and gneissic were added to metamorphic rock names, such as greenstone, amphibolite, or metagabbro, to indicate that the rocks exhibit the effects of dynamic metamorphism.
The principal rock-forming minerals in the cores are plagioclase, olivine, clinopyroxene, orthopyroxene, Fe-Ti oxides, and sulfides. For each of these minerals, the following data were recorded on the VCDs and in electronic form on spreadsheets (igneous lithology and contacts log; see Fig. F7):
From these data, a lithology name based on IUGS definitions was generated by a "Visual Basic for Applications" macro in the spreadsheet. An overall grain size was assigned using the terms fine grained (<1 mm), medium grained (1-5 mm), coarse grained (5-30 mm), and pegmatitic (>30 mm). Also recorded were descriptions of mineral shapes using terms such as equidimensional, tabular, prismatic, platy, elongate, acicular, skeletal, and amoeboidal, and descriptions of mineral habit using the terms like euhedral, subhedral, anhedral, rounded, deformed, and fractured.
Textures of the plutonic rocks were characterized on the basis of grain shape, mutual contacts, and preferred mineral orientation. Rock textures such as equigranular, inequigranular, intergranular, and granular were used to describe the overall texture of each lithologic interval. Poikilitic, ophitic, subophitic, and interstitial textures were distinguished according to the predominant grain shapes in each interval. No attempt was made to apply classic cumulus terminology (Wager et al., 1960) during macroscopic rock descriptions, but these terms were used in the thin-section descriptions where appropriate (see "Thin-Section Description"). Igneous fabrics that were distinguished include lamination and lineation. These terms were reserved for rocks exhibiting a clear preferred dimensional orientation of mineral grains that was likely derived from magmatic processes.
In plutonic rocks, igneous structures can be especially important in defining the mechanism of crystallization of the primary mineralogy (Irvine, 1982). Igneous structures noted in the core description include layering or lamination, igneous contacts, gradational grain-size variations, gradational modal variations, gradational textural variations, and breccias. Layering was used to describe vertical changes in grain size, mode, or texture within an interval. Grain-size variations were described as normally graded if the coarser fraction is at the bottom and reversely graded if the coarser fraction is at the top. Modal variations were described as normal if mafic minerals are more abundant at the bottom and reversed if mafic minerals are more abundant at the top. Igneous breccia was noted if the breccia matrix appears to be of magmatic origin.
The nature of igneous and lithologic contacts is an important observation as it provides clues to the origin of the downhole lithologic and structural variation in the core. Attempts were made to distinguish internal layer contacts within a single pluton, igneous intrusive contacts between plutons, and/or structural contacts between intervals. The most common types of contacts observed were those without chilled margins. These were described as planar, curved, irregular, interpenetrative, sutured, or gradational. In many cases, contacts were obscured by subsolidus or subrigidus deformation and metamorphism and were called sheared if an interval with deformation fabric is in contact with an undeformed interval, foliated if both intervals have deformation fabrics, or tectonic if the contact appears to be the result of faulting or localized ductile shearing.
Thin sections of igneous rocks were examined to complement and refine the hand-specimen observations. In general, the same types of data were collected from thin sections as from hand-specimen descriptions. Cumulate terminology (Wager et al., 1960) was applied where appropriate. Modal data were collected using standard point-counting techniques or visual estimation. All data are summarized in ODP format thin-section descriptions (see the "Core Descriptions" contents list). Crystal sizes were measured using a micrometer scale and are generally more precise than hand-specimen estimates. The presence of inclusions, overgrowths, and zonation was noted, and the apparent order of crystallization was suggested in the comment section for samples with appropriate textural relationships. The presence and relative abundance of accessory minerals such as Fe-Ti oxides, sulfides, apatite, and zircon were noted. The percentage of alteration was also reported (see "Metamorphic Petrology").
For a complex sequence of plutonic rocks, interpretations of the lithologic successions in the core are difficult because of overprinting of synmagmatic, metamorphic, and tectonic processes. The lithologic intervals adopted here are defined by vertical sections with consistent internal characteristics and lithology and are separated based on geological contacts defined by significant changes in modal mineralogy, grain size, or primary texture, as encountered downhole. To the extent possible, boundaries were not defined where changes in rock appearance were only the result of changes in the type or degree of metamorphism or the intensity of deformation. Both sharp and gradational contacts occur between intervals. If the contact was recovered, its location is recorded by the core, section, position (in centimeters), and piece number. If the contact was not recovered, but a significant change in lithology or texture was observed, the contact was placed at the lowest piece of the upper interval. The information recorded for each section of the core includes the rock type; igneous, metamorphic, and/or deformational texture; evidence for igneous layering; extent, type, and intensity of deformation; primary and secondary minerals present; grain shape for each primary mineral phase; evidence of preferred orientation; the position of quartzo-feldspathic veins (whether of igneous or hydrothermal origin); and general comments. The description and summary of each interval was entered on the standard ODP form where the general lithological description and top and bottom of the interval are recorded with reference to curated core, section, piece(s), depth, and thickness.
Samples considered by the Shipboard Scientific Party to be representative of the various lithologies cored were analyzed for major oxide and selected trace element compositions with the shipboard ARL 8420 wavelength-dispersive XRF apparatus. Full details of the shipboard analytical facilities and methods are presented in previous ODP Initial Reports volumes (e.g., Leg 118, Shipboard Scientific Party, 1993a; Leg 140, Shipboard Scientific Party, 1992b; Leg 147, Shipboard Scientific Party, 1993a; Leg 153, Shipboard Scientific Party, 1995). A list of the elements analyzed and the operating conditions for Leg 179 XRF analyses are presented in Table T1.
After coarse crushing, samples were ground in a tungsten carbide shatterbox. Then, 600-mg aliquots of ignited rock powder were intimately mixed with a fusion flux consisting of 80 wt% lithiumtetraborate and 20 wt% heavy absorber La2O3. The glass disks for the analysis of the major oxides were prepared by melting the mixture in a platinum mold in an electric induction furnace. Trace elements were determined on pressed powder pellets prepared from 5 g of rock powder (dried at 110ºC) mixed with a small amount of a polyvinyl alcohol binder solution. The calibration of the XRF system was based on the measurement of a set of reference rock powders. A Compton scattering technique was used for matrix absorption correction for trace element analysis. Loss on ignition for each sample was determined by the standard practice of heating an oven-dried (110ºC) 20-mg sample to 1010ºC for several hours.