ROCKS OF THE BERKELEY HILLS: LEONA & NORTHBRAE RHYOLITES (Indian Rock, Cragmont Rock, Alvarado Parks)
Scattered throughout the Berkeley Hills are
silica-rich rocks, which have been mapped as Tertiary (Dibblee, 1980, 1981;
Radbruch and Case, 1967; Robinson, 1956), but which are now considered to be
Upper Jurassic (Jones and Curtis, 1991).
The Geologic Map of the San Francisco-San Jose Quadrangle (California
Division of Mines and Geology, 1991) identified these rocks as “rhyolite of
uncertain age” and tentatively grouped them as part of the Coast Range
Ophiolite. Graymer (1996) characterized
them as volcanic rocks (keratophyre and quartz keratophyre) overlying the Coast
Range Ophiolite.
Lawson (1914) named the rhyolites in
the Berkeley Hills “Northbrae” and “Leona,” after the residential areas where
they crop out. Lawson apparently
thought of the Northbrae as a flow-banded subset of the generally featureless
Leona (Robinson 1956) and considered them to be Plio-Pleistocene. In outcrop, the Leona (as seen at Tunnel
Road and at Alvarado Park) is fractured, weathered, crumbling. It has defied definitive analysis because it
lacks distinctive textures and is very altered. In contrast, the Northbrae, which crops out at Indian Rock Park
and in adjacent neighborhoods in North Berkeley and Kensington, forms coherent
prominences. In contrast with the
generally featureless and massive Leona, the Northbrae Rhyolite is flow-banded and contains autobrecciated
clasts, suggesting its origin as a lava
flow or dome. (Autobrecciation occurs
when a silica-rich lava rapidly congeals, becomes too viscous to flow and
breaks into pieces.) We interpret the
brecciated rock at Cragmont Park (within a mile of Indian Rock) as part of this
flow. We have distinguished the
flow-banded Northbrae Rhyolite chemically from the nonflow-banded Leona
Rhyolite-type rocks at Alvarado Park and Tunnel Road by the former’s greater K2O
content, as well as by distinctive trace-element values.
Rock
sites:
Indian
Rock Park
Cragmont
Rock Park
Leona
Rhyolite
Northbrae
Rhyolite
Although they differ significantly in
outcrop and hand sample, all rocks described on this site exhibit relict
volcanic textures in thin section. In hand sample and outcrop, Indian Rock
(Northbrae Rhyolite) and Cragmont Rock differ significantly from the rocks at Tunnel
Road and Alvarado Park (Leona Rhyolite) and from the quartz keratophyre in Del
Puerto Canyon. Indian Rock’s
flow-bands, spherulites, and autobrecciation textures set it apart from the
outcrops of Leona Rhyolite. (A
spherulite is a devritification texture of volcanic glass: the disordered glass
crystallizes into spheres of radiating crystals which go in and out of
extinction as the polarizer of a petrographic microscope is rotated.) The brecciated rock at Cragmont Park is
texturally distinct from all other rocks in the Berkeley Hills.
Devitrification textures in Indian Rock suggest that
it originated as a glass flow. The
clasts of Cragmont Rock may represent silicified autobrecciation of that
flow. At Rock Lane in Berkeley, located
equidistant between Indian Rock and Cragmont Rock, there is an exposure that
exhibits textures that are characteristic of both Indian Rock and Cragmont
Parks. Relatively few Cragmont clasts
retain their original flow-banding; the
majority of the clasts do not, possibly the result of intense
silicification. (An alternative
explanation for the origin of the nonbanded clasts is that they may represent
sicilified spherulites.)
We did not observe any autobrecciation, flow-banding, or spherulitic textures in the Tunnel and Alvarado (Leona Rhyolite) rocks. Nor did we observe in either Indian Rock or Cragmont Park the microlite-intense groundmass that we observed in Tunnel Rd, Alvarado Park, and Del Puerto Canyon keratophyre. Moreover, silica replacement textures are common in Indian and Cragmont Rocks and virtually absent in Leona Rhyolite-type rocks and in the sample from the Del Puerto Canyon keratophyre. The foregoing observations lead to an interpretation that the Northbrae (including Indian Rock and Cragmont Parks) and Leona Rhyolites in the Berkeley Hills have different petrogeneses.
GEOCHEMISTRY
We obtained chemical analyses of six samples: Indian
Rock (Northbrae Rhyolite), rocks from Tunnel Road and Alvarado Park, Richmond
(Leona Rhyolite), Del Puerto Canyon (Coast Range Ophiolite quartz keratophyre)
and two samples from Cragmont Rock Park: clast and a mixture of clast and
matrix. We plotted the geochemical data
to attempt to determine tectonic setting and petrogenetic relationship between
our samples.
Na2O/K2O: Leona Rhyolite-type rocks (Tunnel and Alvarado) are Na2O-rich
and K2O-poor, whereas Northbrae Rhyolite has intermediate values for
both oxides. Cragmont Rock samples plot
near zero values for both oxides, possibly reflecting the dilution of Na and K
during the silicification of Cragmont Rock (98-99% silica).
REE Diagram (Nakamura 1974): Leona Rhyolite-type rocks, Tunnel and
Alvarado, plot like the keratophyre of Del Puerto Canyon: a flat pattern with
no Europium (Eu) anomaly. This pattern
is characteristic of keratophyres in the Coast Range Ophiolite (Blake and Jones
1981). Northbrae Rhyolite at Indian
Rock Park exhibits a negative Eu anomaly and light rare earth element (REE)
enrichment. Cragmont clast and Cragmont
clast/matrix, however, show extreme light REE depletion. Cragmont Rock is the most silicified of our
samples ( 98-99% SiO2).
Light REE depletion is possibly due to silica replacement (dilution);
greater depletion of light REE may be due to trace minerals fixing the heavy
REE. Both Indian Rock and Cragmont
Rock exhibit negative Eu
anomalies. One possible interpretation
of this diagram, taking into account the extreme silicification at Cragmont
Rock, is a common origin for Indian and Cragmont Rocks, an origin that is
different from that of Leona Rhyolite-type rocks and the keratphyre of Del
Puerto Canyon.
Wood (1980): The plot (Th-Hf/3-Ta) can be applied to intermediate and silicic
lavas, as well as to basalts and is
good at identifying volcanic-arc basalts (Rollinson 1993). The Alvarado and Tunnel rocks (Leona
Rhyolite) plot as a volcanic arc, consistent with the widely held view that the
Coast Range Ophiolite formed as a supra-subduction zone ophiolite (Shervais
1990). The major question posed by this
plot is why Del Puerto Canyon does not also plot as a volcanic arc. (Caveat: We believe that our Del Puerto
sample may not be entirely representative of Coast Range Ophiolite
keratophyres. Its low Zr values are not
consistent with published values [Jones 1981].)
Pearce et al
(1984): These plots are based on a study of granites
(defined as any plutonic rock with >5% modal quartz) from known tectonic
settings: ocean ridge (ORG); volcanic arc (VAG); within plate (WPG); and
collisional (SYN-COLG). Where there is
evidence of ocean setting, as in our samples, supra-subduction zone granites
can be identified on the Nb-Y plot by their lower Nb content. Leona Rhyolite-type rocks (Alvarado and
Tunnel) and Del Puerto Canyon keratophyre have low Nb values, consistent with the
interpretation that they are part of the Coast Range Ophiolite, which has a
supra-subduction zone geochemical signature.
By contrast, the tectonic setting of the Northbrae Rhyolite at Indian
Rock Park plots as an anomalous ridge.
SiO2-Zr: In the rocks studied, Zr generally increases with SiO2,
however, this plot illustrates the significant difference between
Northbrae-type rocks (including Cragmont) and Leona Rhyolite-type rocks and the
Del Puerto Canyon keratophyre. Even
when SiO2 of a Leona-type rock approaches that of a Northbrae
Rhyolite (Indian Rock), Zr values are significantly lower for the Leona
Rhyolite.
Our Na2O/K2O plot indicates
that flow-banded rocks at Indian Rock Park and nearby locations contain
significantly higher amounts of K2O and less Na2O than do
the massive Leona Rhyolite rocks (Tunnel Rd and Alvarado Park) and the quartz
keratophyre of Del Puerto Canyon. Some
workers who believe that both the Leona and Northbrae Rhyolites are both Coast
Range Ophiolite keratophyres classify the silica-rich rocks south of the
University of California, Berkeley campus as the Leona Rhyolite and the
silica-rich rocks north of the campus as Northbrae. Our Na2O/K2O
plot indicates that Leona-type rocks are located both north and south of the
campus, “sandwiching” the chemically distinctive Northbrae rocks. Therefore, we question the validity of this
north/south of campus distinction.
Our REE diagram suggests that neither Indian Rock
Northbrae Rhyolite nor Cragmont Rock share the same petrogenetic environment as
do the Leona Rhyolite (Tunnel and Alavardo rocks) and the Coast Range Ophiolite
keratophyre at Del Puerto Canyon. Our
Pearce (1984) plots further support this observation in that the Leona rocks and
the keratophyre plot as volcanic arc/collision granites, whereas Indian Rock
plots as an anomalous ridge.
The data and observations discussed above support
the interpretation that:
·
Indian
Rock Northbrae Rhyolite and Cragmont
Rock share the same petrogenetic environment;
·
Northbrae
and Leona Rhyolites do not have the same petrogenesis;
·
The
tectonic and accretionary history of the Berkeley Hills area is more complex
than is indicated by current mapping of these rock units.
Possible explanations of our
findings regarding the Northbrae include:
1. The Northbrae is a
heretofore unrecognized distinctive member of the Coast Range Ophiolite;
2. The Northbrae has a
provenance similar to rocks of the
Franciscan Complex, which are found to the west and east of it in the Berkeley
Hills; and
3. The Northbrae has
affinities not yet studied.
Comparative radiometric
dating of the Northbrae Rhyolite is not feasible as we have found no datable
zircons in any of our Indian Rock or Cragmont Park samples. We intend to collect more Northbrae
Rhyolite samples for geochemical analysis and comparision with other volcanics
of the Berkeley Hills. We recognize
that the Berkeley Hills are a complex area whose geology is poorly exposed due
to intense development and whose rocks have been juxtaposed and rearranged from
their original geometries due to faulting and massive landsliding.
The research set forth on
this site was presented at the meeting of the Cordilleran Section of the
Geological Society of America in Berkeley, June 1999. The geochemical analysis was funded, in part, by a grant from the
Graduate School of California State University, Hayward.
We invite comment,
criticism, and input. Email Lin Murphy
at: Lmarphy@haywire.csuhayward.edu.