ROBERT J. STULL Department of Geology, California State University, Los Angeles, Los Angeles, California 90032 KENT McMILLAN Department of Geological Sciences, Stanford University, Stanford, California 94305 Origin of Lherzolite Inclusions in the Malapai Hill Basalt, Joshua Tree National Monument, California ABSTRACT dence that the nodules could be the end product of more than one partial melting. Alkali olivine basalt at Malapai Hill, Cali- fornia, occurs as a late Cenozoic stock that has INTRODUCTION intruded the Cretaceous White Tank Monzon- Basalts and ultramafic nodules from the ite. The basalt is chemically and mineralogi- Mojave Desert have been described in several cally similar to other alkaline basalts in the papers. Ross and others (1954) reported min- Mojave Desert, although it is devoid of zeolites. eral compositions of peridotite inclusions from The strontium isotopic composition of the Dish Hill and noted the world-wide similarity basalt (Sr87/Sr86 = 0.7030 ± 0.0006) suggests of nodules. Hess (1955) published one analysis that it is derived from a mantle that has of a nodule from Dish Hill (Fig. 1). Wise already experienced one period of partial (1966, 1969) described the characteristics of ba- melting. OUvine-rich lherzolite nodules in the salts in the Mojave Desert and provided a basalt are high in Mg and low in Si, Al, Ca, viable theory for the sequence of lava at Na, and K. The nodules are xenomorphic Pisgah Crater. Most petrologists agree that granular with a tectonite fabric and forsteritic basalt forms in the upper mantle (for example, olivine (F094-88). The Sr87/Sr86 ratio of the Green and Ringwood, 1967; Jackson and nodules is 0.7043 + 0.0008. Olivine xenocrysts Wright, 1970), and that nodules probably form and corroded xenolith margins suggest that as a result of a variety of upper mantle processes reaction between the nodules and basalt has (Wilshire and Binns, 1961; White, 1966). This occurred. paper examines the chemical character of the Similar Sr87/Sr86 ratios of lherzolite nodules Malapai Hill basalt and ultramafic nodules, and alkali olivine basalt hosts at Malapai Hill and the relation of the xenoliths and host rock. are consistent with a related origin for the two rocks. The tectonite fabric and olivine com- position of the nodules rule out a cognate origin by fractional crystallization of the basalt. Instead, the nodules are mantle ma- terials that have been depleted in the most easily fusible elements by one or more periods of partial melting. The nodules may represent refractory residue from the area of partial melting or depleted mantle material, unrelated to the basalt but accidentally incorporated into it. Interpretation of the nodules as re- fractory residue from the site of melting is favored due to the similarity of the nodules and host rocks in strontium isotopic composi- tion, the extreme chemical and petrologic uni- formity of the inclusions, and chemical evi- Pisgah, Dish Hill, and Amboy volcanoes. Geological Society of America Bulletin, v. 84, p. 2343-2350, 1 fig., July 1973 2343 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2343/3433296/i0016-7606-84-7-2343.pdf by guest on 23 September 2021 2344 STULL AND McMILLAN ANALYTICAL PROCEDURES TABLE 1. MODES AND PETROGRAPHIC SUMMARY OF MALAPAI HILL BASALT AND ULTRAMAFIC NODULES Basalt number 1 (Table 3) and ultramafic nodule number 1 (Table 4) were analyzed with Modal Mode wet chemical techniques by Dr. Ken-Ichiro minerals (%) Pétrographie summary Aoki. These samples served as standards when determining Si by x-ray fluorescence methods Basali' and the remaining elements by atomic absorp- Olivine 10 Anhedral phenocrysts and tion spectrophotometry. groundmass; 0.1 to 4 mm diameter; 2V = -80° to -85° The strontium isotopic compositions were phenocrysts have uniform or determined with a 9-in.-60° mass spectrometer undulatory extinction and with a single rhenium filament. Data were re- kink bands Tan, euhedral to subhedral corded digitally and reduced with a computer Augite 25 86 88 crystals; 0.1 to 0.2 mm long; and normalized to a Sr /Sr ratio of 0.1194. replaces bronzite and quartz The Sr87/Sr86 ratio of the Eimer and Amend xenocrysts standard SrCC>3 was determined to be 0.7080, Plagioclase 48 Euhedral; 0.1 to 0.2 mm long; consequently no normalization of the data normal zoning An56_3o Alkali feldspar Anhedral; interstitial to was necessary, Ultramafic sample number 1 plagiochse laths was exactly repeated as a test of analytical Oxides and 16 Magnetite calcite, chlorite, precision. secondary biotite, ¡ipatite Olivine compositions were determined by minerals x-ray diffraction. The position of the (130) Lherzolitet and (241) olivine peaks were accurately mea- Olivine 75 Equigranular, anhedral, 1 to 4 sured with the aid of a Cominco ultrapure mm diameter; 2V = 90°, undulatory extinction and silver standard. deformation lamellae sub- Optic axial angles were measured with a parallel :o (100) 4-axis universal stage. Bronzite 13 Nonpleochroic, subhedral, 1 to 4 mm long; bent clinopyroxene LOCATION, GEOLOGY, exsoluticin lamellae; 2V = AND PETROGRAPHY -85° Diopside Anhedral, 1 to 2 mm long; bent Malapai Hill is a small basalt stock in Joshua orthopy::oxene exsolution Tree National Monument, California (Fig. 1). lamellae ;2V = +77° Rogers (1954) mapped the stock and observed Serpentine 6 Along fractures Spinel 1 Brown; anhedral no associated lava flows. The stock intruded Oxides and tr. the late Mesozoic White Tank Monzonite and secondary is probably late Cenozoic in age. minerals The lherzolite nodules are moderately abun- dant and strikingly uniform throughout the 'Basalt texture: Holocrystalline, aphanitic; porphy- ritic with olivine phenocrysts. stock. They are rounded, 1 to 5 cm in diameter, t Nodule texture: Tectonite fabric; medium-grained, medium-grained, equigranular, allotriomor- xenomorphic granular. phic, and not layered. The mineral composition of the inclusions (Table 1) and the forsterite composition of the olivine (Table 2) are similar groundmass and commonly replaces olivine, to lherzolite in alkaline basalt from many parts bronzite, or rarely, quartz xenocrysts. Simi- of the world (Ross and others, 1954; White, larly, subhedral augite crystals occur in reaction 1966). A tectonite fabric is expressed as bent rims where bronzite is at the exterior of a pyroxene exsolution lamellae, undulatory ex- lherzolite nodule. tinction, and kink bands. Xenoliths of White Tank. Monzonite, up to The basalt is aphanitic alkali olivine basalt 20 cm in diameter, are rare and are corroded, with phenocrysts of olivine and a groundmass suggesting partial assimilation into the basalt. of olivine, augite, plagioclase, alkali feldspar, Sodic andesine xenocrysts are invariably par- and magnetite. Olivine phenocrysts with kink tially resorbed. bands and undulatory extinction are xenocrysts derived from the nodules. Optic axial angles CHEMISTRY OF THE BASALT indicate that olivine xenocrysts are more The Malapai Hill basalt is typical alkali magnesian (Fogo) than the cognate olivine olivine basalt approaching basanite in composi- phenocrysts (F075). Augite is ubiquitous in the tion (Table 3). Although the normative nephe- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2343/3433296/i0016-7606-84-7-2343.pdf by guest on 23 September 2021 LHERZOLITE INCLUSIONS: MALAPAI HILL BASALT, CALIFORNIA 2345 TABLE 2. FORSTERITE CONTENTS OF OLIVINE IN raised the SiC>2 content of the basalt and this LHERZOLITE NODULES FROM MALAPAI HILL AS is not apparent. DETERMINED FROM 20ISO AND 20JU The strontium isotopic composition of the Malapai Hill basalt is within the range of Sample no. 2 0Î41 20130 oceanic island basalt reported by Peterman 1 90 90.6 and Hedge (1971), and at the lower extreme 2 91 94 of Basin and Range basalt analyzed by Leeman 3 92 90 (1970) and Peterman and others (1970). The 4 88 90 Malapai Hill basalt was derived from a source 5 92 91 region beneath the Mojave Desert which is 6 93.5 91 isotopically similar to oceanic mantle with respect to strontium. lene is less than 5 percent, these rocks are Wise (1969) found that the Pisgah lava field chemically similar to basanite of the Pisgah represents a sequence of eruptions that ranged lava field described by Wise (1969). The from basanite through alkali olivine basalt to Malapai Hill basalt is higher in Rb and Sr and subalkaline, low-alumina basalt. The sequence lower in CaO than the basalts from both Dish was interpreted as an upward-migrating zone Hill and Pisgah Crater (Wise, 1966, 1969). of melting that was successively affected by Over-all, the chemical differences are minor. lower pressures. The sequence from basanite to The chemical data indicate no reaction be- subalkaline basalt was marked by progressive tween the basalt and the White Tank Monzo- decreases of alkalis and increases of alumina nite unless it is reflected in the slightly raised and silica. The chemical similarity of the Rb values. The low Sr87/Sr86 ratios (Table 3) Malapai Hill basalt to the early members of are not indicative of significant reaction be- this sequence suggests a deep level origin fol- tween the basalt and the more radiogenic lowed by rapid intrusion into the upper crust quartz monzonite. Contamination should have with a minimum of sialic contamination. TABLE 3. CHEMICAL ANALYSES OF MALAPAI HILL BASALT Sample number 1 2 3 4 5 6 7 Oxides Si02 45.31 45.20 46.10 45.90 45.20 45.00 45.00 Ti02 2.77 2.50 2.74 2.73 2.70 2.79 2.83 AI2O3 14.76 14.94 14.81 14.95 14.70 14.81 14.72 Fe2Oa 4.03 FeO 8.63 12.73 12.32 12.21 12.40 12.37 12.79 MnO 0.19 0.21 0.22 0.21 0.21 0.21 0.21 MgO 7.70 8.28 7.85 8.02 7.90 7.69 7.46 CaO 7.85 7.97 8.45 8.34 8.26 8.34 8.25 Na20 3.64 3.33 3.58 3.11 3.19 4.01 3.86 K2O 1.70 1.77 1.91 1.71 1.75 1.20 1.26 H2O+ 1.49 H2O- 0.87 2.50 2.09 2.56 2.97 2.34 2.85 P2O6 0.58 Total 99.52 99.43 100.07 99.74 99.28 98.66 99.23 Elements Co 74 85 81 95 67 81 Cr 200 170 125 195 95 130 Ni 197 226 219 210 201 201 Cu 42 40 42 41 44 45 Rb 37 41 29 30 29 41 Sr 830 834 798 921 760 750 K/Rb 396 382 490 477 345 255 Rb/Sr 0.044 0.050 0.036 0.033 0.038 0.055 87 88 Sr /Sr 0.7036 0.7028 0.7028 2 a 0.0006 0.0006 0.0006 Trace elements are in ppm.