... introduction & background...

... introduction ...

In the 1890's, George Barrow was the first to recognize that assemblages of metamorphic minerals are related to subjection to heat and pressure1. Since his time, many of the relationships have been well defined by field investigation and laboratory experiments. However, many questions remain to be answered. Metamorphic rocks which originated in a subduction zone are relatively rare on the surface of the earth, and geologists have not studied them as extensively as more common rocks. This study will determine whether the sample rocks are in equilibrium, that is, representative of a single temperature-pressure state which controlled crystallization of the rock. If they are not, the rock may represent multiple phases of crystallization under different conditions. These phases may be isolated or snapshots of a continuous gradual process. This study aims to determine whether the samples represent single or multiple metamorphic conditions and compare the compositions found for glaucophane (sodium amphibole).

... regional geology ...

The Franciscan complex of Northern California has been recognized as a unique subduction zone sequence including intact ophiolite sequences, and its study was instrumental in the development of modern understanding of subduction zones in the rock record1. Several rock types characterize the area, all of which originate within or on oceanic crust. Over 80% of the area is underlain by graywacke sandstones, and original turbidite bedding is sometimes preserved (Fig.1: Landscape. Click for larger image.). The structure of the Franciscan has been described as a "plum pudding" or "mega-breccia", wherein coherent blocks of material are floating in sheered, much finer matrix. The matrix is referred to as the "Franciscan Melange" because it includes components of all the common rocks. Although the sheer geologic complexity of the region has made detailed mapping impossible and/or meaningless, geologists have classified the formation into three North-South trending "terrains" (Fig.2: Terrain Map of the Franciscan. Click for larger image.).

The Eastern or Yolla-Bolly Terrain where the samples in this study were collected is characterized by boulders and blocks of high-pressure metamorphic rocks up to tens of meters in diameter embedded in severely sheered matrix material2. As throughout the Franciscan, the matrix material is tectonically deformed mixture ("melange") whose primary component is graywacke sandstones with small amounts of chert, carbonates, and greenstone pillow-lavas. In some areas, serpentinite blocks up to kilometers in length have been thrust into or onto the matrix terrain. These typically contain increased concentrations of the high-pressure metamorphic rocks. The tectonic processes which resulted in the formation of the Franciscan Complex continue to puzzle and challenge geologists, and no mystery is more tantalizing than the question of the origin and exhumation of the high-pressure metamorphic boulders3.

Boulders of greenschist, amphibolite, blueschist, and eclogite facies have been reported throughout the Eastern Terrain. It is likely that the boulders represent the same variety of protoliths as can be observed throughout the terrain, ie. greenstones, graywackes, cherts, and the melange matrix. Although it is uncommon to observe these blocks actually embedded in serpentinite, past contact and/or immersion is evidenced by actinolite + chlorite or talc rinds two to ten centimeters thick found on many boulders.4

Samples were collected from two areas within the Eastern Terrain. The Tiburon Peninsula in Marin County has been extensively studied by previous authors. Shell Beach in Sonoma County has been less extensively studied but nearby Cazadero has been frequented by geologists for decades and is probably geologically indistinguishable.

Field observations and preliminary results of this study indicate consistent observations with previous studies on these areas.

... sample description ...

Samples were collected on the Tiburon Peninsula in Marin County, California and at Shell Beach, Jenner, Sonoma County, California. Blueschist-facies metamorphic rocks were identified by the presence of the sodium-bearing amphibole glaucophane, phengite (white mica), and pale green omphacitic pyroxene. These photomicrographs may have a slight mottled appearance; this is due to the carbon coating and absence of a coverslip and viewing oil on the specimens, to keep them suitable for SEM analysis.

Fig. 3: RM-B4a (PPL)


RM-B4a in plane polarized light (photomicrograph, 100x) showing blue to violet pleichroic glaucophane, colorless material which is primarily feldspar and white mica, and a garnet in the upper left. Small kidney-bean shaped colorless crystal in lower center is lawsonite.
RM-B4a

Sample RM-B4a from Ring Mountain on the Tiburon Peninsula is a schistose, relatively coarse-grained (<.5mm) rock. The schistosity is defined by violet to midnight blue linear amphibole crystals and fine white mica. Albite and lawsonite occur in pockets and planes within the schistosity. Bright red rutile is a minor constituent. This sample was collected on the south-west flank of the Tiburon Peninsula.

Fig. 4: RM-B4a (XPL)


The same view of RM-B4a in cross polarized light. Glaucophane shows second order interference colors (dark blues, purples) The isotropic garnet now appears completely black. White to yellow material in pockets is albite (ok, so the thin section is just a little thick.) Bright "stringy" looking crystals are white mica (phengite).


Fig. 5: SB7 (PPL)


In contrast to the photomicrograph of RM-B4a, SB7 is almost entirely glaucophane in this view. The irregular crystals have ragged boundaries. They strongly display the characteristic glaucophane pleichroism in violet to blue to blue-green. A few crystals of white mica (colorless) are woven in.
SB7

Sample SB7 was collected at Shell Beach, near Jenner, California. It is dense and gneissic in texture. The very fine groundmass is primarily glaucophane, with quartz, omphacite, and phengite in pockets and veins. Sphene and lawsonite are common accessory minerals. Boudins of pyrite reach up to 0.5cm in length. Both photomicrographs were taken at (100x). Note subtle preferred orientation ENE-WSW, with a few crystals cutting across it at high angles.

Fig. 6: SB7 (XPL)


Again, this view is significantly different than RM-B4a under crossed polars. The phengite shows brilliant high 2nd order retardation, while the glaucophane shows anomalous colors such as dark blue and brown.