The procedures and methods for igneous and metamorphic descriptions of the core during Leg 180 generally follow those adopted during Leg 147 (Gillis, Mével, Allan, et al., 1993), Leg 153 (Cannat, Karson, Miller, et al., 1995a), Leg 176 (Dick, Natland, Miller, et al., 1999b), and Leg 179 (Pettigrew, Casey, Miller, et al., 1999c). To ensure accurate core descriptions, thin-section petrography of representative samples was integrated with the VCDs (Fig. F8). When possible, identification of mineral phases was confirmed by XRD analyses according to standard ODP procedures outlined in previous Initial Reports volumes (e.g., Volume 118; Shipboard Scientific Party, 1989). The XRF analyses were also conducted according to ODP standards. After coarse crushing, samples were ground in a tungsten carbide shatterbox. Ignited rock powder in 600-mg aliquots was 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 (LOI) for each sample was determined by the standard practice of heating an oven-dried (110ºC) 20-mg sample to 1100ºC for 12-24 hr. A list of the elements analyzed and the operating conditions for Leg 180 XRF analyses are presented in Table T3.
Alteration intensity was classified as follows: negligible (<2%), slight (2%-10%), moderate (10%-40%), high (40%-80%), and pervasive (80%-100%). A column on the VCDs shows the variation in the degree of alteration (see "Core Descriptions" contents list).
All igneous petrography and petrology observations are stored in paper and electronic form and in project-designed spreadsheets following the definitions given below. Details of core curation and handling are discussed in "Core Handling". Standard ODP VCD sheets were used to document the igneous rock cores. The VCD sheets include graphical representations and summaries of the dominating macroscopic features (see "Core Descriptions" contents list). Descriptions of microscopic features were archived in text files (see "Core Descriptions" contents list).
Igneous intervals were defined on the basis of primary igneous lithologies, textures, and contacts. The intervals do not necessarily imply intrusive relationships. The natures of the igneous contacts were systematically recorded together with igneous structures, mineralogies, and textures. Mineral modes and grain sizes were visually estimated, and textures and fabric evaluated. The observations were recorded for each lithologic interval. A summary of the individual measurements is given below.
The classification of plutonic rocks followed International Union of Geological Sciences (IUGS) recommendations (Le Maitre, 1989; Figs. F9A, F9B) with minor modifications.
The appropriate metamorphic rock names were used for mafic plutonic rock exhibiting the effects of dynamic metamorphism such that the assemblage consists of secondary hydrous minerals that obliterate the protolith mineralogy and texture, and for rock made up of recrystallized primary minerals such that the original igneous protolith cannot be recognized. The textural terms mylonitic, cataclastic, 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.
Volcanic rocks were classified according to the nature and abundance of phenocryst assemblages as aphyric (1%), sparsely phyric (1%-2%), moderately phyric (2%-10%), or highly phyric (>10%). Volcanic rocks were further classified by phenocryst type using mineral name modifiers given in the order of decreasing abundance. Therefore, a moderately plagioclase-olivine phyric basalt contains 2%-10% phenocrysts with plagioclase more abundant than olivine.
Volcanic rock names are provisional even after thin-section description, unless the rock has been analyzed by XRF in which case it can be assigned a name in the IUGS system (Le Maitre, 1989) on the basis of normative mineralogy.
The principal rock-forming minerals in the cores are plagioclase, olivine, clinopyroxene, hornblende (or other amphiboles), biotite, Fe-Ti oxides, and sulfides. For each of these minerals, data were recorded on the VCDs and in electronic form on spreadsheets: (1) estimated modal percent of the primary minerals, (2) largest size of mineral grains (measured along the longest axis in millimeters), (3) in the case of alteration, the estimated original percentage of primary minerals replaced by secondary phases, and (4) the percentages of secondary minerals such as chlorite, epidote, serpentine, and calcite. A lithology name based on IUGS definitions was generated from these data 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). Descriptions of mineral shapes were recorded using terms such as equidimensional, tabular, prismatic, platy, elongate, acicular, skeletal, and amoeboidal. Descriptions of general appearance were recorded using terms such as 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. Igneous fabrics that were distinguished include lamination and lineation for rocks exhibiting a preferred dimensional orientation of mineral grains that was likely derived from magmatic processes, clusters for mineral aggregates, and schlieren for lenses of igneous minerals.
Textures of volcanic rocks were characterized on the basis of grain size and crystallinity (glassy or crystalline), the shape of the phenocrysts (euhedral, subhedral, anhedral, or skeletal), incipient crystals (crystallites or microlites), phenocrysts (seriate or glomerocrystic), relations between phenocrysts and matrix (intersertal or granular), and vesicularity. Magmatic flow textures include trachytoid (mainly for felsic rocks) and pilotaxitic (mainly for basic rocks).
Glass may be fresh (if basic, it is then called sideromelane) or devitrified. Hydrated basaltic glass is palagonite, which subsequently changes to a variolitic texture that is distinguished by poorly defined spherulite. Devitrification of acid glass (obsidian) leads first to pitchstone and subsequently to the formation of spherulites. Black, optically opaque material found in many of the Leg 180 rocks is known as tachylite (i.e., glass rendered opaque by microcrystalline iron oxides; Fisher and Schmincke, 1984, p. 96).
In plutonic rocks, igneous structures can be especially important in defining the mechanism of crystallization of the primary mineralogy (Irvine, 1982). Igneous structures include layering or lamination, 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. Apart from gradational grain-size variations, these features were not encountered during Leg 180.
Volcanic rock structures were described according to the distribution of phenocrysts, vesicles, chilled margins, alteration, and vesicle fillings.
The nature of igneous contacts is an important observation because it provides clues to the intrusive and/or structural relationships between intervals. The most common type of contact is the chilled contact. Contacts were encountered only on a few occasions during Leg 180.
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. Modal data were collected using visual estimation by reference to standard charts. All data are summarized in ODP format thin-section descriptions (see "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 descriptions of other special features were placed in the comment section. The presence and relative abundance of accessory minerals such as Fe-Ti oxides, sulfides, apatite, and zircon were also noted as well as the amount and nature of alteration.
The lithologic intervals adopted here were defined by vertical sections with consistent internal characteristics, and lithologies were separated based on geological contacts defined by significant changes in modal mineralogy or primary texture as encountered downhole. Generally, 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. If the contact was recovered, its location was 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 (1) rock type; (2) igneous, metamorphic, and/or deformational texture and structures; (3) extent, type, and intensity of deformation; (4) primary and secondary minerals present; (5) grain shape for each primary mineral phase; (6) evidence of preferred orientation; (7) the position of quartzo-feldspathic veins (whether of igneous or hydrothermal origin); and (8) general comments. The description and summary of each interval was entered into the standard ODP format where the general lithologic description and top and bottom of the interval are recorded with reference to curated core, section, piece(s), depth, and thickness.
The VCDs of metamorphic characteristics were compiled together with stratigraphic, igneous, and structural documentation of the core. This information was recorded as completely as possible to provide two types of information: (1) the extent of replacement of primary minerals by metamorphic or secondary minerals, and (2) the extent to which metamorphic or alteration minerals contribute to any subsolidus fabric found in the core. These data, along with any other pertinent observations, were recorded in the VCDs. The following information was recorded for each section: leg, site, hole, core number, core type, section number, piece number (consecutive downhole), and position in the section. Terminologies adopted for metamorphic rock types, metamorphic textures and fabrics, metamorphic facies, and so forth, follow conventional usage (see "Core Descriptions" contents list).
The metamorphic mineral assemblages and alteration intensities obtained from thin-section observations were recorded in the metamorphic VCDs. Primary phases (quartz, plagioclase, pyroxene, amphibole, alkali feldspar, biotite, muscovite, and rutile) and the secondary or retrograde minerals (sericite, chlorite, pyrite, ilmenite, and sphene) that replace them were noted. Where possible, the volume percent of alteration for each phase was estimated in hand specimen and checked by observation of representative thin sections. Portions of pieces where primary textures were ambiguous or obliterated by secondary minerals were termed patches. Where thin-section analysis was available, pertinent observations were used to supplement information in the VCDs.
Detailed petrographic descriptions were made aboard ship to aid in identification and characterization of metamorphic and vein mineral assemblages. Stable mineral paragenesis were noted, as were textural features of minerals indicating overprinting events (e.g., coronas, overgrowths, and pseudomorphs). Mineral abundances were visually estimated. These data are recorded in the thin-section tables (see "Core Descriptions" contents list). The modal data allowed accurate characterization of the intensity of metamorphism and helped establish the accuracy of the macroscopic visual estimates of the extent of alteration.
Assignment to metamorphic grade or facies was made on the basis of key index minerals as described in standard texts (e.g., Turner and Verhoogen, 1960; Fry, 1984).
Where metamorphic minerals were included in fabric elements, such as shear zones, cataclastic fabrics or foliations, textures, and associated minerals were recorded. For samples affected by crystal-plastic deformation, textural features noted include identities and abundances (volume percent) of porphyroclasts and their alteration products, neoblasts, and other minerals associated with and defining the fabric.
Breccias were defined as intervals of angular fragments in which clast rotation could be documented. Portions of the core crosscut by dense vein networks may appear to be brecciated; however, if adjacent clasts separated by the veins were not visibly rotated, they were described as net or mesh veined. Characterization of breccias included clast lithology and secondary phase mineralogy, matrix mineralogy, and abundances of clasts and mineral phases.
Fault gouge is defined as soft, uncemented, pulverized, claylike material--a mixture of minerals in finely divided form. The fault gouge was found along the fault or between the walls of a fault commonly associated with breccia.
Metamorphic rocks are classified using a variety of criteria based on the overall aspect of the rock in hand specimen. Examples include slate, greenschist, serpentinite, marble, amphibolite, hornfels, gneiss, mylonite, and metagabbro, depending on factors such as grade, protolith composition, and deformation.
Metamorphic grade category: low grade, medium grade, and high grade, depending on the minerals assemblage.
Composition: for example, pelite, calc-silicate, and ultramafic.
Association: for example, ophiolite, migmatite, and metagabbro.
During Leg 180, we used the rock-type terms in the VCD because further classification is generally not possible unless full mineralogical information is available. In problematic cases the samples have been examined in thin section, which also allows an estimation to be made of the metamorphic grade. For thin-section examination, we have used the guide to compositional categories and their grade indicators given by Fry (1984, pp. 97-102). Our nomenclature for cataclastic and mylonitic rocks follows Table 9.1 (p. 93) in the same book.
Crystalloblastic textures are crystalline textures produced by metamorphic recrystallization. Examples of crystalloblastic textures include
Granoblastic: a nonschistose rock with equidimensional crystals.
Nematoblastic: development during recrystallization of slender parallel prismatic crystals.
Lepidoblastic: foliated or schistose rock with foliation defined by the parallel orientation of minerals with a flaky or scaly habit.
Foliation is the term used to define all planar textures and structures in metamorphic rocks developed during metamorphism. Foliation may be defined by layering of contrasting mineralogies (gneissosity), planar-preferred orientations of individual grains (schistosity), planar-fracture surfaces (cleavage), or any combination of the three.
Porphyroblastic: a crystalloblastic texture with minerals of two or more distinct grain sizes. The large crystals are called porphyroblasts.
Poikiloblastic: a texture in which large porphyroblasts include numerous small mineral grains.
Mosaic: crystals are equigranular and equidimensional and are generally polygonal in shape with simple straight-line or gently curved intergranular boundaries.
Mylonitic: a very fine grained product of mechanical crushing with recrystallization of the primary minerals.
Sutured: crystals have highly irregular boundaries with much interpenetration of each grain into neighboring grains.
Cataclastic textures are produced by mechanical crushing without significant recrystallization. Examples of cataclastic textures include
Mortar: a texture consisting of larger mineral fragments set in a groundmass of crushed material derived from the same crystals.
Porphyroclastic: a cataclastic texture characterized by the presence of large relict mineral grains set in a matrix of smaller crushed grains.
Blastoporphyritic: a relict texture in a cataclastic metamorphic rock in which traces of an original porphyritic texture remain.
Lineation is a general term for the parallel orientation of textural or structural features that are linear.