GEOCHEMICAL ANALYSIS OF THE RICA

The Miocene Ricardo Group of the southern El Paso Mountains consists of lucustrine sedimentary rocks interlayered with felsic tuffs and basalt flows. The basalts were sampled and analyzed geochemically and petrographically. The results of those analyses were compared to data from other Mojave/Owens Valley basalt fields. Major elements show dramatic differences. Ricardo volcanics have significantly lower alkali (Na₂O + K₂O) content, plotting as tholeiites on a basalt tetrahedron. This is in marked contrast to the distinctly alkaline character of other fields. CIPW analyses support the geochemical data. Ricardo basalts are quartz normative, while basalts from other fields are typically olivine normative. In a seeming paradox, hand samples of Ricardo basalt appear to contain phenocrysts of olivine. Thin sections reveal, however, that only remnants of olivine remain; most grains having been replaced and pseudomorphed by iron oxides, siderite and iddingsite. Similar remnant olivines from the Big Pine field have been characterized as partially digested xenocrysts. Perhaps the altered olivines of the Ricardo basalts represent a reaction between an earlier alkaline magma that mixed with a pulse of later, more tholeiitic magma.

Trace elements were plotted on a spider diagram and compared to other fields. The dramatic variations seen for major elements are less apparent. In general, all Mojave fields display a characteristic barium spike and incompatible element enrichment relative to the MORB standard. Ricardo basalts are slightly depleted in some incompatible elements, but the trends are less well defined than those for major elements. Trace element data suggests a common parentage and/or tectonic setting for all Mojave basalts.

Any model that relates Mojave basalts must explain the major element variations. Wang (2002) proposed that Cenozoic basalt composition varies from east to west across the Mojave as a function of depth of melting. To the east, magmas tapped deeper fertile mantle having higher alkali and incompatible trace element concentrations. To the west, shallower melting resulted in more siliceous, alkali-poor magmas derived from a depleted mantle. Our data appears to support this hypothesis.

The basalts of the Ricardo Formation are located in, and to the east of Red Rock Canyon State Park. The basalt field lies at the south end of the El Paso Mountains; the southern boundary marked by the Garlock fault (Figure 1). Outcrops occur along California Highway 14, and to the east within Last Chance Canyon. Since the Ricardo basalts are situated north of the Garlock fault they are technically within the Sierra Nevada geomorphic province. However, geologically they are many similarities to Mojave basalts and most researchers consider the Ricardo to represent the westernmost bimodal basalt field within the Mojave Desert province.

The sinistral-slip Garlock fault zone is one of the most conspicuous geomorphic features in southern California, marking the northern boundary of the Mojave block, as well as the southern margin of the Sierra Nevada and Basin and Range provinces. Although the Garlock has not experienced surface rupture in historic time, there have been a few moderate earthquakes recorded. The most recent was a magnitude 5.7 near the town of Mojave in July, 1992; an apparent aftershock of the Landers earthquake, a few weeks earlier. At least one section of the fault has shown movement by creep in recent years. Research by McGill (1993) suggests Holocene slip rates of 5mm/yr.

The El Paso fault is a normal fault a few kilometers north of the main trace of the Garlock fault near Red Rock Canyon. It forms the southern scarp of the El Paso Mountains. The El Paso fault is thought to be inactive, with some uncertainty about the timing of its last movement. The western boundary of the study area is marked by Dibblee's Sierra Nevada fault. This was thought to be an extension of the Owens Valley fault system to the north. However, unlike the Owens Valley fault, the Sierra Nevada fault shows no evidence of Holocene movement.

Dibblee (1952) (Figure 2) mapped a dark gray micaeous, schist exposed within Mesquite Canyon as the oldest unit in the El Paso Mountains. He termed the unit informally the Mesquite Schist and assigned a provisional Precambrian age. Dibblee indicated the Mesquite Schist was unconformably overlain by a series of metastratified rocks of Paleozoic age, the Garlock Series. More recent mapping throughout the central and southern Sierra Nevada suggests the Precambrian (?) Mesquite Schist may be as young as Triassic and the unconformable contact an east-dipping thrust fault.

A diverse assemblage of weakly metamorphosed (lower greenschist facies) rock units of Paleozoic age, the Garlock Series, lie stratigraphically above of the Mesquite Schist. Fossil evidence suggests the rocks range in age from Cambrian/Ordovician through Permian. Similar Paleozoic rook pendants sequences have been mapped along both the east and west flanks of the central and northern Sierra Nevada. The Mesquite Schist and Garlock Series have been intruded by a granitic pluton of the Sierra Nevada batholith. Age of the intrusion is uncertain, but similar granitic plutons have yielded Cretaceous ages.

The Paleozoic and Mesozoic rocks are deeply eroded and unconformably overlain by the northeast dipping Paleocene Goler Formation. The Goler is comprised of a thick sequence of terrestrial conglomerates and sandstones. Finer grained lithologies in the Goler Formation have produced diagnostic mammal vertebrates. The Goler Formation is best exposed in Scenic Canyon.

The Middle to Late Miocene volcanics and clastic sediments overlying the Goler Formation were termed the Ricardo Formation by Dibblee (1952), after a long-abandoned gas stop on the old Highway 6 (now 14). Loomis (1984) subdivided the volcanic rocks and the younger, overlying, fluviatile succession into two formations; the Cudahay Camp and overlying Dove Springs. The volcanic rocks at the base of the Cudahay Camp Formation are coarse pyroclastics and andesites best exposed in Last Chance Canyon. They also form most of the rocks of Black Mountain. Age dates from interbedded basalt and andesite flows have yielded a range of dates from 15 to 19 million years (Cox, 1987). The volcanics are overlain by 1800 m of primarily fluviatile and lacustrine sediments (Dove Springs Formation). These form the multicolored badlands of Red Rock Canyon. The lower 800 m contains numerous volcanic ash layers, two lapilli tuff breccias and two basalt flows. A volcanic ash layer yielded an age slightly in excess of 10 million years. The Ricardo Formation is overlain by two episodes of Quaternary alluvium derived primarily from the Sierra Nevada. These deposits illustrate a complex history of downcutting and terrace development related to periodic uplift of the Sierra Nevada.

Sampling for this research focused on basalts within both the Dove Springs and Cudahay Camp Formations exposed in Red Rock and Last Chance Canyons. The satellite image to the left (Source - Google Maps) shows the area sampled. Note, California Highway 14 is the dark line on the left side of the image. Red Rock Canyon Sate Park lies in the southwest one-quarter of the image. The basalt flows form dramatic northwest-dipping ridge caps (darker shades) within and to the east of Red Rock State Park. The image to the right is a detailed geologic map by Loomis (Figure 3 4MB Image). Loomis mapped 20 separate flows, five within the Dove Springs Formation and as many as 15 thinner flows within the Black Mountain Basalt Member of the Cudahay Camp Formation. All of these basalt flows were sampled. In addition, a smaller number of samples were collected from tuffs and flows both above and below the basalts to better constrain the compositional variation of all Ricardo Group volcanics.

Table 1. CIPW Normative Analysis Ricardo basalt
Mineral	Normative wt %
Q	11.8
or	7.6
ab	19.8
an	26.9
ne	0.0
di	13.8
hy	14.8
ol	0.0
mt	3.0
il	1.8
ap	0.5

Figure 4 summarizes the compositional variation within the Ricardo volcanics. Notice that nearly all Ricardo volcanics lie within the typical basalt-andesite-dacite-rhyolite lineage. Table 1 is a CIPW normative analysis calculated for all rocks containing less than 57% SiO₂ (basalt and basaltic andesite). The CIPW analysis indicates that the Ricardo basalts are quartz normative; containing no normative olivine making the Ricardo basalts tholeiites. This presents a paradox when compared to thin section petrology (see discussion below). It is important to note that all other Mojave basalt fields (Darrow, 1972, Groves, 1996 and Waits, 1995) have been characterized as olivine normative.

Figure 5 is a series of Spider diagrams calculated from the average trace element content of 20 Ricardo basalt samples. Trace element concentrations were determined with XRF utilizing an in-house software routine optimized for the determination of trace elements in basalt. Spider diagrams were constructed with Ig-Pet 2001 software. Rock standards are from Sun and McDonough (1989).

Mojave basalt fields of Cenozoic age are generally thought to be derived from subducted oceanic mantle and a byproduct of residual heat from the East Pacific Rise. Spider diagrams can test this hypothesis. The lower right diagram compares Ricardo basalts to the "primitive mantle" of Sun and McDonough. Note that all trace elements are in enriched in Ricardo basalts with the most incompatible elements showing the greatest enrichment. This is consistent with a primitive mantle source. NMORB (top left), EMORB (top right) and OIB (bottom left) are unlikely "source" rocks, but similarities to the Ricardo basalts would suggest a common source. Of these, the most dissimilar is the NMORB. This implies that the Ricardo basalts were not derived from the typical depleted mantle reservoir that acts as a source for NMORBs. However, Ricardo basalts appear to be only slightly enriched relative to an EMORB and slightly depleted relative to an OIB. OIBs are a product of enriched or fertile mantle while EMORBs, although somewhat enigmatic, probably represent some type of middle ground between enriched and depleted mantle. The similarity of Ricardo basalts to both the model OIB and NMORB thus suggests that fertile mantle acted as the "source" for Ricardo magmas.

Thin sections were also examined (see image to the right), They revealed that Ricardo basalts are generally comprised of 1-1.5 mm, euhedral, plagioclase crystals set in a matrix of skeletal pyroxene and glass. Occasional, large (5-10 mm) phenocrysts of augite and highly altered (1-3 mm) phenocrysts of olivine are also present. In many cases only the cores of olivine grains remain unaltered, the rims consisting of iddingsite and hydrated iron oxides. Locally, Ca-siderite completely replaces and pseudomorphs olivine as is shown in the image to the right.. The alteration of olivine suggests that it may represent a xenocrystic phase. Subsequent reaction with the magma altered the olivine to iddingsite/siderite. Perhaps the modal olivine xenocrysts were engulfed by a more siliceous pulse of magma which was then extruded before digestion of the olivine grains was complete. The result would be an olivine-bearing rock that chemically appears to have no olivine. Darwin (1972) studied the basalts of the Big Pine field and found similar olivine xenocrysts. He suggested these were inherited from an earlier magma. Winter (2001) states that resorbed olivine with orthopyroxene reaction rims can occur in tholeiitic basalts. Another possibility that must be considered is some form of subsequent alteration to the Ricardo basalts perhaps due to hydrothermal or meteoric fluids. The latter is unlikely since many of the samples showed little or no visible alteration, even the characteristic iron oxidation was not present in the fresher outcrops adjacent to California Highway 14. The former possibility, however, cannot be discounted. Certainly, hydrothermal fluids played a role in borate mineralization in many localities throughout the Mojave. These same fluids could have altered nearby basalt fields.

Major element chemistry for Ricardo basalts differs substantially from that of other Mojave basalt fields. Ricardo basalts have lower total alkalis (Na₂O + K₂O) and higher silica (SiO₂) (Figure 6). Compositional differences are more apparent when plotting on a basalt tetrahedron (Figure 7).Ricardo basalts are, for the most part, tholeiitic; those form the Darwin field olivine tholeiites, and all others generally alkali basalts. Chemical differences may be related to a variety of factors including differing source rocks, differences in melting depth, percentage of partial melt or even age. In the latter case, Ricardo basalts are 8-10 Ma, those of the Darwin Plateau 6-8 Ma, and all others 4 Ma or younger. An argument can be made for a trend from older, silica-oversaturated, tholeiitic basalts (Ricardo), to younger, silica-undersaturated, alkaline basalts (Coso, Cima).

Spider diagrams are considered to be the best discriminators of subtle differences in source rocks. Figure 8 is a MORB-based spider diagram for selected Mojave basalt fields. The diagram is based upon the MORB standard of Pearce (1983) and was chosen simply because it had the most complete range of elements for the available data. Note that the Ricardo basalts show no significant differences from other Mojave basalts, suggesting a commonality of source. Spider diagrams can also serve as indicators of crustal contamination. While all Mojave basalts are enriched in incompatible elements relative to the MORB standard, the geographic distances between the fields makes it highly unlikely they would have experienced similar degrees of contamination as necessitated by the similarity of data plots. Spider diagrams, therefore rule out major differences in source rocks or significant crustal contamination. Both are consistent with tectonic models. The tectonic setting of the Mojave has changed little over the past ten million years, so source rocks would not be expected to differ. Furthermore, extension rates have been rapid enough throughout that period to allow basaltic magmas unimpeded access to the upper crust with little opportunity for contamination. Perhaps the only point of debate involves changes in the rate of extension from the Miocene through the Holocene and the degree to which certain areas of the Mojave have been extended.

Ringwood (1976) demonstrated that variation in percentage of partial melt can have dramatic effect on basalt chemistry. Partial melts of 20% of pyrolite mantle at depths of 40 kilometers (!.3 Gpa) yield alkaline magmas. Increasing the melt percentage to 30% results in a tholeiitic partial melt. Assuming residual heat from the East Pacific Rise (EPR) generated Cenozoic basaltic magmas, older (Miocene) magmas might have a greater heat content, melting more mantle (tholeiites). Younger basaltic magmas (Plio-Pleistocene) would be more alkaline as the heat from the EPR began to dissipate reducing the percentage of melted rock. Residence time could also be a factor in the percentage of partial melt. If the rate of extension in the Mojave/Owens Valley has increased since the Miocene that would reduce the mantle residence time for magmas thus reducing the percentage of partial melt.

Kushira (1968) (Figure 9) was the first investigate the role of pressure in the melting of basaltic magmas. His research suggested that pressure changes significantly effect the position of the eutectic in the system quartz-enststite-forsterite-nepheline. At low pressure (<0.5 Gpa) magmas are dominantly tholeiitic while at higher pressure they become increasingly alkaline. Wang (2002) (Figure 10) utilized an iron index to calculate depth of melting for basaltic magmas. He found that melting depth systematically increased from west to east across the Mojave. The Ricardo volcanics, 20 kilometers to the west of his westernmost volcanic field, would have been generated at a very shallow depth (40 kilometers). Since low pressure partial melts of mantle yield a more tholeiitic magma, it would be expected that Ricardo basalts would to be tholeiites. Thus, the compositional difference of the Ricardo basalts can be a function of melting depth, percentage of partial melt or age of the rocks. It seems less likely that differences in mantle source rocks played a major role in genesis of Mojave basalts.

Darrow, Arthur, 1972, Origin of the basalts of the Big Pine field, California, unpublished Master's Thesis, University of California, Santa Barbara.

Dibblee, Thomas, 1952, Geology of the Saltdale Quadrangle, CA: California Division of Mines Bulletin 160.

Groves, Kristelle, 1996, Geochemical and isotopic analysis of Pleistocene basalts from the southern Coso volcanic field, California, unpublished Master's Thesis, University of North Carolina.

Kushiro, I., 1968, Compositions of magmas formed by partial melting of the earth's upper mantle. Journal of Geophysical Research vol 73.

Loomis, Dana, 1984, Miocene stratigraphic and tectonic evolution of the El Paso Basin, California, unpublished Master's Thesis, University of North Carolina.

McGill, Sally and Sieh, Kerry, 1993, Holocene slip rates of the central Garlock fault in southeastern Searles, Valley, CA: Journal of Geophysical Research vol. 98, no. B8.

Pearce, J.A.., 1983, Role of the sub-continental lithosphere in magma genesis at active continental margins in Continental basalts and mantle xenoliths; papers prepared for a UK Volcanic Studies Group meeting at the University of Leicester. Hawkesworth, C. J. (Open Univ., Dep. Earth Sci., United Kingdom).

Ringwood, A.E., 1976, Composition and Petrology of the Earth's Mantle, McGraw-Hill Book Company Ltd.

Sun, S.S. and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts; implications for mantle composition and processes in Magmatism in the ocean basins. Saunders, A. D. [editor], Geological Society Special Publication, Vol. 42.

Waits, J.R., 1995, Geochemical and isotope study of peridotite-bearing lavas in eastern California, unpublished Master's Thesis, University of North Carolina.

Wang, K., Plank, T., Walker, J.D., and Smith, E.I., 2002, A mantle melting profile across the Basin and Range, SW USA, Journal of Geophysical Research vol. 107, no. B1.

Winter, John D., 2001, An Introduction To Igneous and Metamorphic Petrology, Prentice Hall.

GEOCHEMICAL ANALYSIS OF THE RICARDO VOLCANICS, SOUTHERN EL PASO MOUNTAINS, CA

David R. Jessey and Cami Jo Anderson

Geological Sciences Department

California State Polytechnic University - Pomona