Fig. 11

Photomicrographs of clastic parts of OPS of the Franciscan Complex with emphasis on the clastic/olistostromal horizons. Mineral abbreviations are chl chlorite, gln glaucophane, hb hornblende, jd jadeitic clinopyroxene, lws lawsonite, phen phengite, qtz quartz, trem tremolite/actinolite. a Metaultramafic schist clast in the pebbly sandstone from Marin Headlands shown in Fig. 10d. The metamorphic assemblage in this clast is higher grade than the prehnite-pumpellyite facies host rock. Note that this clast exhibits an internal fabric that predates deposition, and this fabric appears to be the result of progressive deformation of serpentinized peridotite, rather than a deformational overprint on metaultramafic sandstone of the sort shown in “e” and (“i”). Plane light. b Cross-polarized light view of (“a”). c Imbricate and block-in-matrix zone in metaultramafic schist from the block in Fig. 10r, showing alternating blueschist layers and lenses with abundant sodic amphibole (blue in this plane light view) and pale tremolite/actinolite (trem)-phengite (phen)-chlorite (chl) schist. Note the contrast in textures between the metaultramafic schist and the serpentinite/metaultramafic sandstones shown in “d, e, g, and h,” as well as the clast in (“q”). d Serpentinite sandstone matrix from the outcrop in Fig. 10o at Ring Mountain. The clasts in this view are primarily antigorite schist (lighter colored) or tremolite schist (darker). Although some of the rounding of the grains is a result of the folding of these grains (two examples shown with arrows) and development of locally curved shears, there are many rounded grain boundaries that abruptly truncate the internal grain fabric without drag; these are original detrital grain boundaries (examples shown by arrows). Plane-polarized light. e Cross-polarized light view of (“d”). f Rutile-bearing hornblendite clast (long dimension about 5 cm) in the same serpentinite matrix sampled about 1 m away from the sample in (“d”). The internal fabric in this clast is truncated along the rounded grain boundary with the enclosing serpentinite sandstone/conglomerate (dark) without drag features (most apparent in the upper left hand part of the view). Hornblende (hb) locally has small glaucophane (gln) rims, and there is some late growth of chlorite (chl). This is a cm-scale analog of the larger high-grade blocks found in this mélange. Plane-polarized light. g Serpentinite matrix from the same locality showing hornblende (hb) grains, some of which are rounded and rimmed with glaucophane (gln). Note the truncation of the hornblende and rim against the rounded grain boundary, best seen at the left-rounded termination of the hb grain in the upper left corner of the photo. Other grains in this view include tremolite/actinolite, and chlorite-tremolite-antiorite schist. Plane-polarized light. h Serpentinite matrix from serpentinite mélange, Sunol Regional Wilderness (sample location on Fig. 7) showing abrupt truncation of internal grain fabric along grain boundaries, similar to that seen at the Ring Mountain locality. Clasts in this view are primarily antigorite-chlorite schist with some grains of tremolite/actinolite, hornblende, and chlorite. Plane-polarized light. i Chlorite-rich sandstone from Sunol Regional Wilderness (Fig. 7). This sandstone is medium green in outcrop and resembles clastic serpentinite in outcrop appearance. Much of the chlorite rich zones (chl) have fine greenish phengite, and polycrystalline growths of chlorite form textures similar to antigorite schist. Chrome spinel is locally present in the chlorite-rich zones. The chlorite-rich zones probably originated as serpentinite clasts. Other clasts or minerals include jadeitic clinopyroxene (jd), lawsonite (lws), quartz (qtz), and glaucophane. Some of the metamorphic mineral grains are part of polymineralic clots (jd, lws or lws, qtz or jd, lws, qtz) and the polymetamorphic clots, and many of the metamorphic mineral grains have rounded grain boundaries that truncate their internal fabric, suggesting a detrital origin. Many of these grains also exhibit late neoblastic overgrowths with sharp terminations. Plane-polarized light. j Cross-polarized light view of (“i”). k Serpentinite clast with chrome spinel in mostly siliciclastic conglomerate, Sunol Regional Wilderness. Long dimension of the clast is about 10 mm. Such serpentinite clasts make up 5–10% of the clasts in some horizons in this conglomerate. Plane-polarized light. l Cross-polarized view of (“k”). m Sandstone that makes up most of non-mélange siliciclastic outcrops at Sunol Regional Wilderness. This sandstone has much less chlorite than “i,” but some chlorite (chl) clasts have the appearance of serpentinite (best viewed in the cross-polarized light view of “n”) such as the clast labeled that is directly below the shale chip in the center of the photo. This sample has neoblastic lawsonite (lws), jadeitic clinopyroxene (jd), and glaucophen (gln), but relict albite is still present as seen by the albite twinning seen in the cross-polarized light view of “n” (good example along the upper left edge of photo). o–u El Cerrito quarry area, eastern San Francisco Bay area (general location on Fig. 2). Two serpentinite clasts from prehnite-pumpellyite siliciclastic conglomerate, matrix of the Hillside mélange unit. This conglomerate/pebbly sandstone contains up to 15% serpentinite clasts. This photo shows a lizardite-rich and an antigorite-rich clast, both of which appear to have been derived from intact peridotite (they are not themselves clastic). Plane-polarized light. p Cross-polarized light view of (“o”). q Serpentinite sandstone clast from the same thin section showing the internal rounded grains that truncate fabric within them. Compare to the serpentinite matrix samples shown in “d” and (“h”) The grains are primarily antigorite schist. Plane-polarized light. r Cross-polarized light view of (“q”). s Clast with jadeite-chlorite ± lawsonite, along with another serpentinite clast in the same thin section. t View of blueschist facies metasandstone unit of the upper El Cerrito quarry. This particular sample consists of more than 50% detrital blueschist clasts, most of which have hornblende relics. Much of the blue amphibole growth is truncated on the detrital grain boundaries indicating that these were blueschist-overprinted amphibolite clasts (equivalent of the high-grade blocks) at the time of deposition. There is also some late neoblastic glaucophane growth along one of the margins of this clast. u Same unit as “t” collected about 10 m away showing serpentinite clast with chrome spinel. For a higher resolution image, please see Additional file 2