IGNEOUS PETROLOGY

General Procedures

Procedures used during Leg 176 for describing igneous rocks in general follow the outline presented in the "Explanatory Notes" chapter of Leg 153 (Shipboard Scientific Party, 1995), and much of the discussion presented here comes from that source. Observations on hard-rock petrology and petrography were stored in ODP written and electronic media and in Excel spreadsheet files according to the definitions given below and the terminology and outlines described in "Linked Spreadsheets". For a detailed discussion of the hard-rock core "barrel sheets," the reader is referred to "Introduction". Hard-rock macroscopic observations of igneous rocks were recorded on HRVCD forms by the igneous petrologists.

Measurements were not made in traditional ODP shift mode but were performed by the entire igneous petrology group working in tandem. For consistency in the measurements, qualitative measurements (especially the selection of igneous contacts) were made by the entire group. Each quantitative measurement (e.g., grain size, mode, etc.) was measured by a single team member through the entire core. This, we believe, resulted in a better and more consistent measurement of macroscopic features than is possible by teams working in shifts.

Igneous units were defined on the basis of primary igneous lithologies and textures. Mineral modes were estimated (each mineral by an independent observer), totaled, and rechecked if the total was not 100% 10%. Plagioclase and augite were estimated to 5%. Because the abundance of each mineral was determined independently, there is no constraint that would require a total of 100% for the mode, as would be the case if a single observer estimated the abundance of all the minerals. Inasmuch as the total derived by this method provides an indication of the accuracy of the method, the modes were not arbitrarily renormalized to 100%. Mineral habits, igneous structures, and igneous fabrics were also recorded, as well as the nature of igneous contacts. Observations were recorded in Excel spreadsheets for each lithologic unit in the core. Details of the individual measurements are given below.

Differences between Leg 176 and Leg 118 VCDs

Differences in the procedures used during Legs 118 and 176, and the opportunity to incorporate insights into the geology of Hole 735B gained on the earlier leg, have resulted in significant differences in the presentation of the macroscopic visual core descriptions (VCDs) of Leg 176. The Leg 118 report recognized only six major lithotectonic units in the Hole 735B core. A subsequent redescription of the core divided these units into 495 primary igneous lithologic intervals (Dick et al., 1991b). The description of the core during Leg 176 extended the philosophy of that later study, but the larger group involved in the description enabled a more extensive set of measurements to be made.

To provide some continuity between these three studies, the lower 50 m of the Leg 118 core from Hole 735B was redescribed using the new system. This is illustrated in Figure F6, where the interval sequences derived by the two data sets are shown side by side. As much as possible the old interval numbers were retained in the new description. Where old intervals were subdivided by the new description, they were labeled A, B, C, and so forth. Where old units were not recognized in the new description, the interval numbers of the old description were skipped in the new sequence.

Rock Classification

The classification of mafic phaneritic rocks recovered during Leg 176 closely follows the International Union of Geological Sciences (IUGS) system (Streckeisen, 1974; Le Maitre, 1989; Figs. F7, F8). Minor modifications were made to subdivide the rock types more accurately in the Hole 735B core on the basis of significant differences rather than arbitrary cutoffs based on the abundance of a single mineral. For gabbroic rocks, the modifier "disseminated oxide" is used when the abundance of Fe-Ti oxide is 1%-2%, and the modifier "oxide" is used when the abundance is >2%. The terms "orthopyroxene-bearing gabbro" and "gabbronorite" are used to denote the presence of discrete orthopyroxene grains; orthopyroxene present as reaction rims is ignored for the purpose of assigning a rock name. Rocks with between 1% and 5% orthopyroxene are called orthopyroxene-bearing gabbros and samples with >5% orthopyroxene are called gabbronorite. The term "troctolitic" is used to describe olivine gabbros with 5%-15% clinopyroxene and the rock name "troctolite" is used for rocks with <5% clinopyroxene. "Anorthositic" is used for gabbros with >80% plagioclase. High proportions (>65%) of mafic minerals are noted by the prefix mela-, and high proportions (>65%) of plagioclase are noted by the prefix leuco-. The modifier "micro" is used to distinguish gabbroic rocks with a dominant grain size of <1 mm (e.g., microgabbro, microtroctolite, and microgabbronorite). Leucocratic rocks with >20% quartz and <1% K-feldspar (a restricted part of the "tonalite" field of the IUGS system) are called "trondhjemites" in keeping with previous usage in the ocean crust literature. On the HRVCDs, the rock names as described above are given at the top of each interval description, the official IUGS names calculated from the mode are given in the text.

If a mafic 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 are used. The textural terms "mylonitic," "schistose," and "gneissic" are added to metamorphic rock names, such as "greenstone," "amphibolite," or "metagabbro" to indicate that the rocks exhibit the effects of dynamic metamorphism. The methods for describing the metamorphic and structural petrology of the core are outlined in subsequent sections of this chapter.

Primary Silicate Minerals

The principal rock-forming silicate minerals in the core are plagioclase, olivine, clinopyroxene, and orthopyroxene. For each of these minerals, the following data were recorded in the HRVCDs and in Excel spreadsheets: (1) estimated modal percent of the primary minerals; (2) smallest and largest size of mineral grains (measured along the longest axis in millimeters); (3) average crystal size for each mineral phase using fine-grained (<1 mm), medium-grained (1-5 mm), coarse-grained (5-30 mm), and pegmatitic (>30 mm); (4) mineral shapes using terms such as equidimensional, tabular, prismatic, platy, elongate, acicular, and amoeboidal; (5) mineral habit using the terms euhedral, subhedral, anhedral, rounded, deformed, and fractured; and (6) mineral occurrence if minerals occurred as chadacrysts or oikocrysts.

Primary Fe-Ti Oxide and Sulfide Mineralogy

The abundance of oxides in the core was visually estimated using a binocular microscope with 10 magnification. A continuous log of oxide abundance was compiled along with average abundances in each interval. Oxide habits in hand sample are divided into the following categories: disseminated, interstitial network, concordant seams, discordant seams, and matrix. There is a strong correlation between the textures and abundance. "Disseminated" is used to describe scattered grains or grain clusters of oxides with no pronounced fabric, and is most commonly observed in samples with low oxide abundances. "Interstitial network texture" is used to describe oxides that occur interstitial to the silicates, surrounding or partially surrounding groups of silicate grains, and was most commonly observed in samples with low to moderate oxide abundances. "Concordant seams" is used to describe thin, elongate lenses or patches of oxides parallel to the silicate fabric, and "discordant seams" is used to describe those that crosscut the silicate fabric. Both types of seams are most commonly observed in samples with low to moderate oxide abundances. "Matrix" is used to describe continuous networks of oxides that completely surround and separate the silicate phases and is most commonly observed in samples with high oxide abundances. Oxide shapes in hand sample are divided into the following categories: euhedral, anhedral, angular aggregates, amoeboidal aggregates, and interstitial lenses. "Euhedral" and "anhedral" are used when it appeared that isolated individual grains were present; "aggregates" are used to describe what appeared to be contiguous grain clusters.

Relative sulfide abundance in hand specimen was estimated using a 10 binocular microscope and a scale of 0 to 5, where 0 = not observed, 1 = rare, 2 = average, 3 = above average, 4 = abundant, and 5 = very abundant. Many samples contain sulfide grains too small to be seen on the saw-cut surface at 10 magnification, and for these samples the estimated abundances are too low. Sulfide habits in hand specimen are divided into the following categories: disseminated, disseminated with oxides, disseminated in silicates, concordant seams, discordant seams, clusters with oxides, clusters in silicates, and massive. "Disseminated in silicates" is used to describe low concentrations of scattered sulfide grains enclosed in silicates, and "disseminated with oxides" is used to describe low concentrations of scattered sulfide grains enclosed in either silicates or oxides but associated with oxide concentrations. "Concordant seams" is used to describe thin, elongate lenses or patches of sulfides parallel to the silicate fabric, and "discordant seams" is used to describe thin, elongate lenses or patches of sulfides that crosscut the silicate fabric. "Clusters in silicates" is used to describe sulfide grains that are present near other sulfide grains, and are included in silicates, and "clusters with oxides" is used to describe sulfide grains that are present near other sulfide grains included in either silicates or oxides but associated with oxide concentrations. Massive is used to describe large patches or lenses of sulfides with an average dimension >1 cm. Sulfide shapes in hand sample were divided into the following categories: euhedral, anhedral, globular inclusions, angular inclusions, globular interstitial, and angular interstitial. "Euhedral" and "anhedral" are used when isolated individual grains are present; "globular" and "angular" are used to describe what appear to be contiguous grain clusters that are nearly spherical masses or are not nearly spherical, respectively.

Igneous Textures

Textures of igneous rocks are characterized on the basis of grain size, grain shape, preferred mineral orientation, and mineral proportions. The dominant grain size for each unit is recorded as fine grained (<1 mm), medium grained (1-5 mm), coarse grained (5-30 mm), or pegmatitic (>30 mm). Rock textures such as "equigranular," "inequigranular," "intergranular," and "granular" are used to describe the overall texture of each lithologic unit. "Poikilitic," "ophitic," and "subophitic" textures are distinguished according to the predominant grain shapes in each unit. Igneous fabrics that are distinguished include "lamination" for rocks exhibiting a preferred orientation of mineral grains that was likely derived from magmatic processes, "clusters" for mineral aggregates, and "schlieren" for lenses of igneous minerals.

Igneous Structures

Igneous structures noted in the core description include layering, gradational grain-size variations, gradational modal variations, gradational textural variations, and breccias. "Layering" is used to describe planar changes in grain-size, mode, or texture within a unit. Grain-size variations are described as normal if the coarser part was at the bottom and as reversed if the coarser part was at the top. Modal variations are described as normal if mafic minerals are more abundant at the bottom and as reversed if mafic minerals are more abundant at the top. "Igneous breccia" is noted where the breccia matrix appears to be of magmatic origin.

Contacts between Lithologic Intervals

The most common types of contacts are those without chilled margins. These are either planar, curved, irregular, interpenetrative, sutured, or gradational. Sutured refers to contacts where individual mineral grains are interlocking across the contact. In many cases, contacts are obscured by subsolidus or subrigidus deformation and metamorphism and are 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.

Thin-Section Description

Thin sections of igneous rocks were examined to complement and refine the hand-specimen observations. In general, the same type of data were collected from thin sections as from hand-specimen descriptions, and a similar terminology is used. Modal data were collected using standard point-counting techniques. All data are recorded in the thin-section spreadsheet (see "Appendix," in the "Leg 176 Summary" chapter) and 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 is noted, and the apparent order of crystallization is suggested in the comment section for samples with appropriate textural relationships. The presence and relative abundance of accessory minerals such as oxides, sulfides, apatite, and zircon are noted. The percentage of alteration is also reported (see "Metamorphic Petrology").

Linked Spreadsheets

Because the hard-rock application of JANUS was not yet complete, and the old database (HARVI) had been discontinued, igneous data collected during this leg are recorded in a set of linked Microsoft Excel spreadsheets. There are five interlinked spreadsheets (I_LITH.XLS, I_TEX.XLS, I_MIN.XLS, I_COMM.XLS, and DEPTHS.XLS) and three independent spreadsheets (I_OPAQUE.XLS, I_VEIN.XLS, and I_XRF_TS.XLS) in this collection (For detailed information on the spreadsheets, see "Appendix," in the "Leg 176 Summary" chapter). The interlinked spreadsheets cover lithologies, mineralogy, textures, and comments. Most data in these spreadsheets are recorded as numerical values, and the numeric codes are translated to English and back via a pair of "Visual Basic for Applications" (VBA) look-up macros. Information about igneous contacts is recorded in the lithology spreadsheet (I_LITH.XLS). This includes information about the position of the contact, its depth, the thickness and type of rock above the contact, the type of contact (e.g., igneous or tectonic), and the form of the contact (e.g., planar or curved).

The other three databases link to the I_LITH.XLS spreadsheet columns defining the position of the lower contact of each interval. The I_MIN.XLS spreadsheet additionally records the mode of the igneous minerals (plagioclase, clinopyroxene, olivine, orthopyroxene, oxides, and sulfides), as well as the grain size of the silicate minerals (plagioclase, clinopyroxene, olivine, and orthopyroxene). From these data, a rock name is automatically generated by a VBA macro. The I_TEX.XLS spreadsheet records mineral shapes (e.g., acicular, platy, etc.) and habits (e.g., chadacrystic, ophitic, etc.). The I_COMM.XLS spreadsheet contains number codes for igneous textures and fabrics, as well as text comments on each interval. An independent (unlinked) spreadsheet, I_OPAQUE.XLS, records the approximate oxide abundance in the recovered core on a centimeter by centimeter basis.

An independent (unlinked) spreadsheet, I_VEIN.XLS, records the data on "trondhjemite veins." Here, "trondhjemite" represents any of the various quartzo-feldspathic igneous rocks that were recovered (including tonalite and granodiorite), it being the most common. Trondhjemite of apparent igneous origin that crosscuts the core, and that is more than 5 cm in thickness along the length of the core, is recorded as a separate lithologic interval. Trondhjemite of igneous or hydrothermal origin that is less than 5 cm in thickness along the length of the core is recorded as a vein. Although this is an arbitrary cutoff, it provides a workable solution for the description of the hundreds of thin trondhjemite dikelets, fracture fillings, and/or veins that cut the core. The I_VEIN.XLS spreadsheet records the position and igneous interval of each trondhjemite vein, the apparent thickness relative to the vertical axis of the core, the true thickness perpendicular to the plane of the vein, the thickness of the zone in which the vein or vein network occurs, the percentage by area of trondhjemite within that zone, and comments about the vein. Although many of the thicker "veins" appear to be of igneous origin, for many of the veins it is not possible in hand specimen to determine whether they are of igneous or hydrothermal origin. Although the log excludes veins of clearly hydrothermal origin, it includes any veins of unclear origin.

A macro in the I_MIN.XLS file extracts a condensed data report on each igneous interval. This macro is accessed from the "Report" worksheet of the I_MIN.XLS spreadsheet. All other spreadsheets must first be loaded and translated using their respective "Translate" buttons; then the Up and Down buttons on the report page scroll through the entire database one line at a time. It is possible to jump to any part of the database by typing an interval number in the appropriate cell and pressing the "Go" button. This loads the values from that interval into the report and copies the entire report onto the (Windows or Mac) clipboard.

Multi-interval reports are generated by switching to the "Mineralogy" worksheet and highlighting a group of interval numbers, switching back to the report sheet and pressing the "Multi" button. The corresponding unit reports are formatted and transferred via OLE to Microsoft Word. Word is then brought to the foreground, where further editing can take place.

Depths below seafloor for the igneous spreadsheets and for other purposes are calculated using the EDepth and CDepth functions of the DEPTHS.XLS spreadsheet. This spreadsheet by default hides itself after loading. Its functions can be used by selecting the Tools|Add-ins menu item in Excel. From the Add-in manager dialog box select "Browse," and go to the DPTHSMTH\EXCOM directory where DEPTHS.XLS resides. Double click on DEPTHS.XLA to install this add-in to Excel. If a dialogue box asks you to copy the add-in to the Microsoft Default Library, it is important to select "NO." The item "Hole 735B Depths" should now be listed as an item in the Add-in manager. It is now permanently installed in Excel until the checkbox next to its name is deselected, which will uninstall it. The depth calculator will now reload automatically every time Excel is started. To use the depth calculator, type in a destination cell

=EDepth(A1,B1,C1/100).

Make sure to replace A1 in this equation with the cell where the core number of interest resides, making sure that the core number is an integer rather than the alphanumeric combination (i.e., 90 instead of 90R). Replace B1 with the cell where the section number of interest resides and C1 with the cell where curated depth in the section resides (in cm). Expanded/compressed depth should appear in the destination cell, which can be copied and pasted to subsequent cells.

An additional add-in menu item was also written that calculates a spatial moving average of irregularly spaced data points. This add-in item resides in the DPTHSMTH directory in a subdirectory named SMOOTH. The "Smooth" add-in is installed the same way as the depths calculation add-in (see above). Once the add-in is installed, it adds the item "Smooth" to the Tools menu. To use this function, make sure there is a free column in a destination spreadsheet for the output. Depths and values for those depths must reside in two other columns. Highlight a few cells of the output destination column and select "Smooth" from the tools menu. A "Set-up" dialog box will appear; ensure the destination range is correct. Enter into the cells below the destination range the number of columns that the depth and value range are offset from the output column and select a smoothing interval. Selecting the "Fast" checkbox speeds up the smoothing process by hiding the individual cell updates. The "Keep Going" checkbox instructs the algorithm to keep working its way downward as long as there is data to process. Once everything is entered, selecting the "Go" icon should output smoothed data to the destination column.

Igneous Lithology, Interval Definitions, and Summary

For a complex sequence of plutonic rocks, interpretations of the vertical 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 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 (cm), and piece number. If the contact was not recovered, but a significant change in lithology or texture is observed, the contact is placed at the bottom of 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 were entered into the standard ODP-format program 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.

Notes: To open the Excel spreadsheets, either click on the file-names in the text or open the files through Microsoft Excel. To use the in-text links, your computer must be configured to automatically launch Microsoft Excel when files with .XLS extensions are opened. The spreadsheet files can be found in the \176_IR\ SUPP_MAT\APPENDIX\ directory on the CD-ROM.

Add-in functions may not operate in all versions of Microsoft Excel.