Geochemistry and origin of mafic rocks from the Pelona, Orocopia, and Rand Schists, southern California

Earth and Planetary Science Letters, 92 (1989) 371-385 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

371

[31

Geochemistry and origin of mafic rocks from the Pelona, Orocopia, and Rand Schists, southern California M. Robert Dawson and Carl E. Jacobson Department of Earth Sciences, Iowa State University, Ames, Iowa 50011 (U.S.A.) Received August 27, 1988; revised version received January 24, 1989 Metabasites from the Pelona-Orocopia-Rand (POR) schist, a presumed subduction complex in southern California, have been analyzed for major, minor, and trace element concentrations to investigate the nature, origin, and tectonic implications of these rock bodies. Element abundances indicate that these rocks are tholeiitic and alkalic basalts generated from several sources. Most samples (group 1) resemble normal to transitional mid-ocean ridge basalts (N-type to T-type MORB). Variation of MORB-like samples can be accounted for by low-pressure crystal fractionation. A second group (group 2), which includes five samples from the Rand Mountains and one from the San Gabriel Mountains, has elemental abundances similar to those of tholeiitic ocean-island basalts or E-type MORB, although phosphorus is unusually low. Several samples of group 2 have high MgO, Ni, and Cr, characteristics which are commonly associated with crystal accumulation. A final group (group 3) consists of two samples from the Orocopia Mountains that resemble alkalic ocean-island basalts. The analyzed samples were collected from six different bodies of POR schist. There seem to be no major differences in basalt type which would imply a unique and separate source for any one area. Various sites of accumulation have been suggested for the protoliths of the POR schist: open ocean, Gulf of California-type basin, trapped back-arc basin, and rifted back-arc basin. The first three settings are entirely consistent with the basalt geochemistry. The last option is less likely, but not impossible.

1. Introduction

The Pelona, Orocopia, and Rand (POR) Schists are similar metamorphic complexes, of Late Cretaceous age, located in southern California and southwesternmost Arizona (Fig. 1). These units lie structurally beneath continental basement rocks of North American affinity along low-angle faults assigned to the Vincent-Chocolate Mountains thrust [2]. The POR complexes are composed predominantly of quartzofeldspathic (90%) and mafic (10%) schist believed to be derived from graywacke and basalt, respectively. Minor amounts of ferromanganiferous quartzite (probably metachert), marble, serpentmite, and talc-actinolite rock also are present. Metamorphism is pervasive and is mostly of greenschist to lower amphibolite facies. The local occurrence of rocks transitional to the blueschist facies, and other features, indicates that the metamorphism occurred at relatively highP/low-T conditions [3,4]. Because of the presumed oceanic nature of the protoliths and the 0012-821X/89/$03.50

© 1989 Elsevier Science Publishers B.V.

high-P style of metamorphism, these rocks are generally thought to represent a relict subduction complex [1,5-9]. The exact details of the proposed subduction event are poorly understood. Some workers believe that it was of Andean style [5-8]. Others consider that it involved the closing of a marginal basin, either trapped, Japanese type or Gulf of California type [2,9-11]. In some cases, basalts erupted in marginal basins are compositionally distinct from normal oceanic ridge basalts [12,13]. Thus, the major goal of this study was to see if the geochemical characteristics of metabasalts from the POR schist could be used to determine their environment of eruption. A second problem involves the degree of certainty in correlating the various bodies of POR schist. Although all contain similar rock types and have undergone generally similar metamorphism, certain evidence suggests that some of the individual bodies (e.g., the Rand Schist) may have formed at a slightly different time a n d / o r in a different

372

basin than the majority of the schists (see reviews in [1,14]). It was thought that a geochemical comparison of mafic rocks from the various areas might help resolve this question. 2. Description of the marie schist The mafic schist is interlayered with metagraywacke and ranges in thickness from millimeters to several hundred meters. At some locations the schist is laterally discontinuous, whereas at others, it can be traced for several km along strike. It is typically associated with ferromanganiferous quartzite. The mafic schist is composed predominantly of albite, chlorite, amphibole, and epidote. Albite commonly forms porphyroblasts, which typically are about one to several m m in diameter. The schists range from compositionally uniform to finely layered due to variations in mineral abundance and grain size. The rocks are thoroughly recrystallized and schistosity is generally well developed. Isoclinally refolded folds and as m a n y as two additional sets of more open folds are present,

as well as brittle deformation structures [1,15]. Few primary structures were seen in the mafic schist, although relict pillow structures are preserved at one location in the Sierra Pelona ([9]; SP25, this study). Some field evidence m a y indicate that the protolith of the mafic schist underwent local seafloor alteration (metamorphism). At one location in the Rand Mountains there is a deformed breccia that contains centimeter- to decimeter-scale mafic clasts in a matrix rich in muscovite and epidote [16]. Associated with the breccia is a laterally extensive, meter-thick quartz-epidote vein. These features are reminiscent of the seafloor hydrothermal systems described by Harper et al. ([17], and personal communication). The breccia has been observed at only one locality, however quartz-epidote veins and pods are present throughout the P O R schist.

3. Sampling and analytical methods Twenty-eight metabasites were analyzed for major, minor, and trace elements by X-ray flu-

PELONA-OROCOPIA-RAND SCHISTS VINCENT-CHOCOLATE MOUNTAIN-RAND THRUST

Fig. 1. Map of southern California showing sample localities of the Pelona, Orocopia, and Rand Schists (after [1]). B R = Blue Ridge area of San Gabriel Mountains; C M = Chocolate Mountains; E F = East Fork area of San Gabriel Mountains; O R = Orocopia Mountains; P R = Portal and Ritter Ridges; RA = Rand Mountains; T M = Tehachapi Mountains; S P = Sierra Pelona. Mesozoic phitonic rocks are indicated by unpatterned outlined areas.

373 orescence (XRF). Of these samples, 20 were analyzed for additional trace elements by instrumental neutron activation analysis (INAA). The samples are Pelona Schist from Portal Ridge, the Sierra Pelona, and the Blue Ridge and East Fork areas of the San Gabriel Mountains (terminology of [18]); Orocopia Schist from the Orocopia Mountains; and Rand Schist from the Rand Mountains (Fig. 1). The analyzed samples were selected from a much larger sample suite collected from the six different areas and were chosen to encompass the range of metamorphic facies exhibited by the schist and to show a relatively wide range in M g / F e (presumed degree of igneous fractionation), as inferred from probe data and modal mineralogy. In general, samples were chosen to avoid veins, inhomogeneous bulk composition, and highly unusual bulk composition. One exception from the Orocopia Mountains (OR00) was of interest because it contained particularly large (2-3 cm) albite porphyroblasts. Two other samples from the Orocopia Mountains (280A and 280B) were collected because they were directly adjacent to each other, but of obviously different composition. Sample 280A has much less albite than 280B. A complication is that both contain thin calcite veins and show late-metamorphic partial replacement of epidote by calcite. This should be kept in mind, as these two samples turned out to have very different trace element compositions than the rest of the samples. Approximately 150 grams of each sample was ground in a corundum shatter mill to produce a single aliquot of powder. All analyses were performed on splits from these powders. Whole-rock concentrations of major and minor elements were determined by X-ray fluorescence (XRF) using a Siemens X R F spectrometer at Iowa State University. Analyses for Si, A1, Fe, Ca, Mg, K, and Ti were performed on glass discs prepared by fusing the sample with a lanthanum-bearing lithium borate mixture [19]. Samples for fusion were first heated to 1000 ° C for four hours to drive off all volatiles and convert all iron to Fe20 3. The elements N a and P were determined from pressed powder pellets. The elements Mn, Ba, Ni, Rb, Sr, Y, Zr, and N b were determined on a Kevex energy dispersive X R F at Iowa State University, using loose powders. The rare earth elements (La, Ce,

TABLE 1 Determined and established values of standards run as unknowns

SiO2 (wt.%) A1203 TFe203 MgO CaO Na 2° K20 TiO2 MnO P205 Cr (ppm) Ni Ba Rb Sr Y Zr Nb La Ce Sm Eu Yb Lu Hf Ta Th

Determined values

Established values

Standard

50.21 13.89 4.24 7.03 1.99 2.70 5.44 0.66 0.15 0.49

49.90 13.85 4.31 7.31 2.04 2.77 5.51 0.66 0.14 0.60

BHVO-1 BHVO-1 GSP-1 BHVO-1 GSP-1 OB-1 GSP-1 GSP-1 BCR-1 OB-1

350 13 682 48 323 37 176 11 15.5 38.3 6.3 2.1 2.1 0.29 4.4 1.4 1.2

3130 13 678 47 330 39 191 14 16.9 39.2 6.1 2.0 1.9 4.3 1.1 1.0

BHVO-1 BCR-1 BCR-1 BCR-1 BCR-1 BCR-1 BCR-1 BCR-1 BHVO-1 BHVO-1 BHVO-1 BHVO-1 BHVO-1 BHVO-1 BHVO-1 BHVO-1 BHVO-1

Notes: Established values for BHVO-1, BCR-1, and GSP-1 from [1]. Values for OB-1 from S.E. Thieben and K.E. Seifert (unpublished data).

Sm, Eu, Yb, Lu) and Cr, Hf, Ta, and Th were determined by instrumental neutron activation analysis (INAA) at the University of Missouri Research Reactor, Columbia, Missouri. Analytical techniques and sample preparation for I N A A are similar to those of [20]. Accuracy for all techniques was monitored by standards run as unknowns. Analyzed values are compared to established values in Table 1.

4. Results W h o l e - r o c k c o m p o s i t i o n s and n o r m a t i v e analyses for all 28 samples are listed in Table 2. It will be shown below that all samples can be divided into three groups (termed "groups 1 - 3 " )

374

based on their rare earth clement (REE) patterns and the concentrations of certain other trace elements. It will be argued that the three groups correspond to the following types of oceanic basalt: (1) N-type to T-type MORB, (2) tholeiitic ocean-island basalt or E-type MORB, and (3) alkalic ocean-island basalt. Most samples, including some from each of the areas sampled, belong to group 1. Group 2 contains one sample from the Blue Ridge area of the San Gabriel Mountains

(BR224) and five samples from the Rand Mountains (RA38, RA66, RA69A, RA73, and RA74). Group 3 consists of the two somewhat altered samples from the Orocopia Mountains (280A and 280B).

4.1. Major elements The major elements indicate the rocks are basaltic, with SiO2 between 45 and 52 wt.%. An AFM diagram (Fig. 2; fields of [22]) shows that

TABLE 2 Major-element (wtX), trace-element (ppm) abundances and CIl~ norms of Petona, Orocopia and Rand mafic schists Orocopia Mts

Sample: Group:

0R96 1

SiO2 A[203 TFe203 MgO

P205 LOl

46,56 14,08 14,95 6.93 9.40 3.50 0.12 1.86 0.23 0.17 2.31

"OTAL

100.11

CaO Na20 KO T202 MnO

Hg-number 50 Cr Ni Ba Rb Sr Y Zr Nb la Ce Sm Eu Yb Lu Hf Ta Th Q C Or Ab An Ne Di Hy Ot Mt It Ap

167 69 28 6 85 42 122 5 6.3 18.5 5.3 1.7 5.5 0.74 4.0 0.46 0.51

0.74 29.33 23.17 0.74 19.88 19.40 2.67 3.66 0.42

0R133 1

San Gabriel Mts

0R171 1

0R180 1

ORO0 I

260A 3

280B 3

EF5 1

EFIO 1

EF20 1

EF25 1

47.28 44.07 13.29 12.48 1 2 . 9 4 13.18 7.40 5.95 11.54 10.90 2.86 3.00 0.11 0.24 1.50 2.67 0.18 0.23 0.12 0.24 1.87 5.29

47.51 13.11 12.60 4.40 11.69 3.26 0.23 1.94 0.17 0.25 4.55

46.35 12.87 15.57 5.80 10.02 2.80 0.15 3.22 0.22 0.26 2.16

43.96 18.45 17.76 4.78 2.55 5.75 0.70 1.90 0.25 0.12 3.40

49.67 16.79 10.22 5.62 4.63 1.55 1.49 2.87 0.16 1.03 5.85

49.73 16.86 8.95 5,39 5.06 4.75 0.17 2.71 0.14 1.12 5.07

47.56 11.64 12.77' 10.67 9.37 1.97 0.25 1.86 0.20 0.12 3.38

49.37 13.65 11.83 7.64 12.09 1.46 0.14 1.34 0.17 0.09 3.36

48.15 13.27 11.44 7.43 8.07 2.10 0.21 1.25 0.29 0.08 6.38

99.09

98.25

99.71

99.42

99.62

99.88

99.95

99.79

101.14

58

52

45

48

38

54

57

65

177 96 12 3 111 23 83 4 4.5 12.8 3.3 1.4 3.1 0.36 2.6 0.38 0.23

55 48 28 9 113 46 175 10 10.2 29.4 6.4 2.1 5.1 0.71 5.3 0.89 0.70

127 61 89 11 71 43 118 4 4.1 14.0 4.1 1.6 5.3 0.67 3.7 0.32 0.15

0.68 25.19 24.04 28.56 1.59 14.37 2.32 2.97 0.30

0R135 1

1.55 25.13 21.65 1.36 29.35 12.35 2.48 5.52 0.62

NO 41 23 4 172 42 145 8 NO NO NO ND ND ND ND ND NO

ND 72 28 10 127 63 180 6 ND NO ND ND NO ND ND ND ND

1.45 29.33 21.75

0.92 24.71 23.05

31.86 2.80 5.97 2.31 3.92 0.63

22.44 14.09 4.97 2.81 6.38 0.64

ND 169 69 15 76 53 102 4 NO ND ND ND ND ND ND ND ND

4.10 4.37 38.18 12.54 7.16

26.30 3.25 3.81 0.30

3 7 380 38 171 59 351 81 59.0 124.3 12.5 3.8 6.5 0.84 9.1 6.22 6.07

3 44 49 3 131 63 360 74 59.8 134.2 13.5 4.0 6.9 0.93 9.3 6.40 6.69

16.39 7.17 9.46 14.08 17.44

2.24 2.49 1.07 42.72 18.91

25.15

22.72

1.90 5.85 2.62

1.63 5.47 2.82

BR224 BR258 BR311 2 1 1

BR318 1

47.44 13.39 16.57 5.48 8.26 2.84 0.29 2.67 0.23 0.20 2.81

49.26 13.64 14.40 7.80 8.55 2.58 0.35 2.02 0.12 0.09 0.84

46,97 13.79 14,56 7.51 8.21 3.87 0.64 2.23 0.20 0.20 2.09

48.32 13.65 14.41 7.52 8.76 2.73 0.19 2.12 0.20 0.16 2.10

98.67

100.18

99.65

100.25

100.27

100.16

56

59

44

55

43

55

56

67 57 15 4 236 24 80 4 4.6 12.4 3.1 1.2 3.2 0.43 2.2 0.43 0.28

112 80 191 8 209 25 72 6 6.0 11.7 3.2 1.1 2.8 0.45 2.0 0.30 0.32

66 56 91 5 77 60 155 7 6.5 20.8 6.6 2.2 7.7 1.16 5.2 0.32 0.29

1200 579 17 4 77 21 104 9 7,8 24.4 4,2 1.2 1,7 0,24 2.8 0,60 0,57

3.24

3.82

1.55 17.49 23.28

0.86 12.77 31.30

1.36 19.47 28.66

1.79 25.06 23.91

2.12 22.38 25.22

20.04 25.52 5.79 2.33 3.71 0.30

24.66 22.22

11.99 29.74

2.11 2.63 0.22

2.16 2.60 0.21

14.59 24.96 0.95 2.98 5.29 0.49

1 4 . 5 3 15.87 27.49 25.41 1.56 2.54 2.93 3.93 3.64 0.22 0.37

52.01 11.76 16.41 5.22 6.32 4.13 0.52 1.86 0.10 0.15 1.77

ND 35 73 14 85 34 124 2 NO ND NO ND ND ND ND ND ND

ND 71 76 16 69 50 146 8 ND ND ND ND ND ND ND ND ND

ND 74 30 9 100 36 109 5 ND ND ND ND ND ND ND NO ND

0.24

Notes: ND = not determined, Loss on ignition (LOl) at I000°C. Norms calculated on Fe20~/FeO = 0.15, deletion of LO! and normalizing. Bracketed points cocstd n~t be separated from background, actual values are [ess than those reported.

3.17 36.01 12.38

3.90 29.09 18.95 2.55 17.90 20.15 2.60 4.37 0.49

1.16 23.87 25.24 15.31 23.28 4.03 2.59 4.38 0.49

375

line (NE) normative. Most of the samples that are nepheline normative are only slightly undersaturated, and probably can be attributed to the effects of seafloor alteration (see below). The samples show a wide range in M g # (Mgnumber), where M g # = 100Mg/(Mg + Fe 2+) in atomic proportions, and Fe 2÷ is obtained by assuming Fe3+/Fe 2+= 0.15. Most of the samples have M g # < 63, which implies that the basalts have been modified by fractionation (cf. [23]).

most samples follow a tholeiitic iron-enrichment trend. Several samples, in particular one of the group 3 samples (280B), lie outside the tholeiitic field. A tholeiitic nature for most of the samples is also indicated by the relationship of alkalies (Na + K) to SiO2 (not shown). Normative compositions confirm the general tholeiitic character (Table 2) and suggest that most rocks were olivine tholeiites. A few samples are quartz (Q) or nephe-

Rand Mts

RA16 I

RA38 2

RA66 2

RA69A 2

RATE I

RA73 2

RA74 2

RA76 I

Sierra Pelona

Porta[ Ridge

SP18 I

PR20A I

PR42 I 49.05 13.93 13.98 6.32 9.00 4.61 0.24 2.62 0.16 0.38 0.60

SP25 I

49.81 13.79 12.14 6.32 8.70 2.78 0.11 1.72 0.14 0.16 2.70

48.50 46.96 12.53 9.16 1 5 . 8 2 12.06 5.78 16.16 6.76 7.01 3.51 0.70 1.22 0.00 1.95 1.97 0.15 0.17 0.08 0,02 2.51 4,44

48.45 49.82 48.27 49.12 44.66 1 3 . 5 0 1 2 . 4 7 12.87 11.14 12.11 1 2 . 1 8 1 3 . 0 0 1 1 . 7 6 1 0 . 3 2 18.43 4.60 7.81 12.88 1%03 6.40 10.18 9.06 4.68 10.33 7.50 3.25 4.81 1.71 1.38 2.22 0.55 0.12 0.04 0.28 0.57 2.14 1.02 1.84 1.92 3.30 0.16 0.20 0.18 0.14 0.23 0.20 0.08 0.03 0.08 0.26 3.78 1.82 4.29 2.95 2.69

46.26 49.05 1 7 . 6 3 13.02 9.39 15.13 6.23 5.03 10.82 8.67 1.98 3.67 1.32 0.09 1.16 2.76 0.13 0,14 0.10 0,20 4.64 1,96

49.42 15.31 10.03 8.86 11.96 2.80 0.49 0.79 0.15 0.06 1.41

98.30

98.81

98.65

98.99

99.66

101.28 100.89

46

74

55 243 84 27 6 114 36 98 3 4.8 14.8 3.8 1.4 4.4 0.58 2.9 0.21 0.25

201 1240 203 777 59 6 27 3 152 34 12 19 113 88 9 9 4.7 3.8 18.8 16.2 2.9 3.4 1.1 1.3 1.3 1.5 0.21 0.19 3.1 2.6 0.61 0.66 0.64 0.27

3.08

0.51

0.69 24.88 26.26

7.60 31.29 15.63

15.47 23.71

15.99 10.53 11.98 2.89 3.90 0.20

2.22 3.46 0.23

6.36 23.46 11.01 52.36 2.24 4.02 0.05

48 83 78 43 11 217 28 124 10 11.7 30.1 5.9 2.0 2.5 0.32 3.6 0.77 0.84

3.45 29.21 21.91 25.24 11.47 1.68 2,23 4,32 0,50

100.21

98.55

60

71

195 869 102 424 48 13 I 5 97 38 21 20 49 95 2 6 1.3 5.2 4.8 16.9 2.0 3.6 0.8 1.3 3.2 1.9 0.40 0.24 1.7 2.8 (0.19) 0.53 (0.17) 0.24

0.73 33.20 12.42 4.69 27.36 17.11 2.31 1.99 0.19

98.91 69 ND 375 38 9 100 36 109 5 XD ND HD ND XD ND ND ND ND

98.37 45 47 47 212 14 76 66 192 6 9.1 27.2 7.6 2.2 8.4 1.10 6.3 0.54 0.29

62 ND 103 160 26 90 24 91 4 XD ND NO ND ND ND ND ND ND

99.72 44

68

51

65 33 26 5 205 45 164 80 9.1 26.1 5.9 2.1 4.9 0.65 5.3 0.91 0.63

474 117 71 11 245 19 48 2 1.6 5.8 2.0 0.9 2.8 0.33 1.4 (0.17) (0.16)

151 56 46 5 188 57 153 5 7.4 22.7 6.5 2.2 6.4 0.80 4.7 0.41 0.24

2.93 23.26 28.04 0.36 25,60

1.43 34.42 16.76 2.69 21.31

16,39 1,76 1,52 0,14

15.03 2.44 5.02 0.91

3.21 1.69 0.25 15.52 24.69

1.74 12.30 24.61

3.58 19.97 22.75

8.28 17.79 37.50

0.55 32.20 19.48

48.62

23.29 26.82

2.19 3.75 0.08

2.05 3.84 0.20

12.51 26.34 4.16 3.39 6.66 0.65

15.38 6.05 10.70 1.72 2.34 0.25

19.84 17.50 1.80 2.71 5.44 0.49

•

3.16

376 Fe~03 + FeO

No20 * Ka0

these samples are probably not and a direct comparison with fields in Fig. 3 cannot be made, indicate an alteration event prior morphic veining.

MO0

Fig. 2. A F M (A = N a 2 0 + K 2 0 , F = FeO+ Fe203, M = MgO) diagram with POR mafic schist data. Tholc~itJ¢ field is above the solid line [22]. See Table 2 for sample grouping and text for discussion.

Relatively unfractionated samples are present, but uncommon (PR20A M g # = 68). Fig. 3 shows the concentrations of Fe, Ti, A1, Na, K, and Ca versus M g # for all samples. Fields for MORB glasses from the Atlantic, Pacific, and Indian Oceans [24] are shown for comparison. For Fe, Ti, and AI, most of the samples fall within or near the fields for basalt glasses, which is consistent with the observation that these elements tend to be relatively immobile during seafloor metamorphism. For the above three elements, samples RA66, RA73, and RA74 from group 2 consistently plot away from the trends exhibited by the other samples. They all have high M g # (75, 71, and 67, respectively) and, for their M g # , have high Fe and Ti and low AI. For Na, K, and Ca, the majority of samples fall within the fields for oceanic glasses. However, some show depletion of Ca a n d / o r enrichment of Na and/or K, which is characteristic of altered submarine basalts [25,26]. Sample OR00 previously noted as being unusual, is anomalous for many of the elements (Fig. 3) and presumably is highly altered. Not surprisingly, of the pair of group 3 samples, the albite-rich one (280B) is higher in Na (Table 2). Samples 280A and 280B also have very different K contents, but similar abundances for the other major elements. Both samples are low in Ca (Table 2), despite the presence of late-metamorphic calcite. Although

MORB (below), the basalt glass the low Ca may to the late-meta-

4.2. Trace elements It was stated above that groups 1-3 can be distinguished based on REE abundances. As shown in Fig. 4, most group 1 basalts have nearly flat to somewhat LREE depleted patterns, with abundances about 4 × to 30 × chondrite. The more enriched samples typically have a slight Eu anomaly. These patterns are characteristic of Ntype or T-type MORB [28,29]. The nearly parallel nature of most group 1 patterns and correlation of REE abundances with M g # presumably are due to varying extents of crystal fractionation from a common parent magma or similar group of magmas. Samples from group 2 have abundances of the LREE similar to those of the group 1 samples, but are relatively depleted in Yb and Lu. Group 3 samples have HREE abundances like those of group 1, but are very strongly enriched in LREE (note the different vertical scale for group 3 in Fig. 4). On a plot of Ti versus Zr (Fig. 5), samples of group 1 and group 2 define a hnear trend on or near the 'field of mid-ocean ridge basalts. Samples which are depleted in Ti and Zr are, in general, Mg-rich. Variation of Ti-Zr is parallel to vectors representing fractionation by crystallization of olivine, pyroxene, and plagioclase + ilmenite or magnetite [31,32], consistent with the operation of low-pressure fractionation processes in the formation of these rocks. Samples of group 3 plot in the field of within-plate basalts, outside the area of overlap with MORB, and clearly are not related to the fractionation trend of the other samples. The proportions of Th-Ta-Hf (Fig. 6, after [33]) for group 1 samples are similar to those of N-type and T-type MORB. Group 2 samples are similar to E-type MORB or within-plate tholeiites. Group 3 samples he in the field of alkalic within-plate basalts. In Fig. 7, various elements normalized to MORB values [34] are plotted for the 20 samples analyzed by INAA. The large ion lithophile (LIL) elements (Sr-Ba), commonly mobile during metamorphism [35,36], are plotted on the left side of

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Fig. 3. Plots of FeO, TiO2, Al203, N a 2 0 , K 2 0 , and CaO (wt.%) versus M g # for P O R schist. Stippled area represents field for oceanic glasses (data from [24]). For K 2 0 , the outlined area contains 90% of all points.

the diagram. The high field strength (HFS) elements, which tend to be less mobile, are plotted to the right (Ta-Yb). The latter elements are arranged with increasing D to the right, where D is the distribution coefficient between garnet lherzolite and melt [37]. Fig. 7A-D show the elemental

abundances for group 1. The patterns are similar to those of N-type or T-type MORB, being generally fiat to slightly enriched in Th-Ce. The dements thought to be relatively mobile (Sr, K, and Rb) a r e more erratic than the others, but not totally unsystematic. Overall, the trace element

378 I0o

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I0

//At-- -

I0

W J • RAI6 • RA72 • RA76

La Ce

• 0R96 • 0R133 • 0R135

Sm Eu

Yb Lu

~00

La Ce

Sm Eu

Yb Lu

~000 RAND BLUE RIDGE GROUP 2

w

OROCOPZA GROUP 3

E

iO

I

F

Io0

)

• • •

RA38 RA66 RA69A

• RA73 0 BR224 .

La Ce

.

.

.

.

.

.

Sm Eu

.

.

.

.

• •

.

280A 2BOB

~0

Yb Lu

La Ce

Sm Eu

Yb Lu

Fig. 4. Rare earth elem~t patterns no~aliz~ to chondntic ~und~ces of POR mafic sc~sts. No~alizadon values of [27].

abundances are inversely correlated with M g # , and are attributed to varying amounts of crystal fractionation. Group 2 samples differ from those of group 1 in several ways. Both groups show a plateau for the elements Zr to Ti. Relative to that plateau, group 2 shows a pronounced depletion in Y and

Yb, with Yb depleted relative to Y, and a more consistent enrichment in Ta, Nb, and Ce than shown by group 1 (Fig. 7E). Also, most group 2 samples have a negative phosphorus "anomaly" (some also having low Ba and K) and some are particularly high in Cr. The uniqueness of the group 2 samples, including the one from Blue

379 10~

"

.

.

.

.

.

.

.

i

• GROUP 1 o GROUP 2 • GROUP 3

pl0s¢4~oa01o i~ito,os

~10

4

I0~ I0

I00

I05

Zr (ppm) Fig. 5. Ti versus Zr variation of P O R mafic schists (after [30,31]). l A B = island arc basalt, W P B = within plate basalt.

Ridge, is confirmed by neodymium isotope ratios (Bennet, personal communication). The two samples of group 3 (280A and 280B) are distinguished by their moderate enrichment in Zr, Hf, and Sm and their strong enrichment in Th to P (Fig. 7F; note the different vertical scale compared to Fig. 7A-E). 5. Discussion

The mafic rocks of the POR Schist are thoroughly metamorphosed and may also have been exposed to various mechanisms of seafloor alteration. Nonetheless, the trace elements thought to

Hfl3 • GAOUP o GROUP • GROUP

~

ZAT

i 2 3

i

Call Th

To

Fig. 6. Th-Hf-Ta variation of P O R mafic schist (after [33]). Samples (PR20A, RA72) with values below detection limits are not plotted.

be relatively immobile show patterns quite similar to those of pristine igneous rocks. Even many of the major elements and some of the relatively mobile trace elements have basically igneous signatures. We cannot prove that alteration was unimportant, but treating the analyses as generally representative of igneous compositions seems to be a reasonable working hypothesis. Group 1 metabasalts have normative compositions and the iron-enrichment trend typical of tholeiitic basalts. The elemental abundances of group 1, reflected by the shapes of the REE and multi-element (Figs. 6 and 7) patterns, the correlations between these elements and M g # , the Ti-Zr and Th-Ta-Hf diagrams, and all other discrimination diagrams we have plotted are indicative of MORB. Two samples (PR20A and RA72) show LREE-depleted patterns and other features characteristic of N-type MORB. The other group 1 samples, to varying degrees, are more like T-type MORB. The covariation of absolute abundances of trace elements and M g # in the T-MORB sampies, particularly within individual areas, is indicative of varying degrees of crystal fractionation. The six samples of group 2 (BR224, RA38, RA66, RA69A, RA73, and RA74) show extreme variation in Mg # . The Mg-rich samples have very high Cr and Ni and low Na and K. Despite this large intragroup variation, all show certain features that distinguish them from group 1 samples. Specifically, group 2 samples are depleted in the HREE and Y and enriched in Ta, Nb, and Ce. Most also show unusually low P and Ba contents. Other than the low P and Ba, the abundances for the REE and other trace elements are strikingly similar to those of E-type MORB and tholeiitic ocean-island basalt (Fig. 8; [13, fig. 1; 28, figs. 6.2 and 6.8; 29, fig. 1.2.6.12; 37, fig. 6; 39, fig. 15]). The general depletion of HREE and Y implies the presence of garnet in the source region. The high MgO, Cr, and Ni contents of samples RA66, RA73, and RA74 suggest that these samples were rich in olivine. The major element compositions of these rocks are generally similar to known picritic basalts (e.g., [40,41]). Fig. 9 shows the relation of SiO2 to MgO with an olivine control line drawn for reference. The olivine is assumed to have a composition of Foss [42]. A hypersthene control line would plot approximately perpendicular to the olivine line. As can be seen

380 ~0 I 0

~I ~

"

EAST

FORK

:tO.O

a

_

SIERRA PORTAL

PELONA RIDGE

H

[] (Z 0

i.O

1.O

&J J <~ 03

• EF5 • EF~O • EF20 • EF25

• SP25 • PR20A • PR42

O.1

0.~ Sr K RbBaThTeNbCe

P ZrHfSmTi

iO.O

RAND

. . . . .

Y

YbCr

_

Sr K RDBaThTaNbCe

P ZrHfSrnTi

Y YbCr

10.0

OROCOPIA GROUP i

O

[] 0 >-

~.0

1.O uJ 0<~ []

• 0R96 • 0R133 • 0R135

• RA16 • RA72 • RA76

O.i

O.i Sr K RbBaThTaNDCe

P ZrHfSmTi

II,\

Y YbCr

Sr K ROBaThTaNbCe

E

J..0

~.0.0

O.i

Y YbCr

[

o

~ LL~[

P ZrHfSmTi

•

280A

1.0 Sr K RbBaThTaNt]Ce

P ZrHfSmTi

Y YbCr

Sr K RbBaTnTaNbCe

P ZrHfSmTi

Y YbCr

Fig. 7. Large ion lithophile (LIL) and high field strength (HFS) element patterns for selected POR metabasites. Data are normalized to N-MORB values of [34]. Bracketedpoints could not be separated from background, actual values are less than those plotted.

from this figure, these three samples plot along a line of olivine control, suggesting that MgO, Cr, and Ni abundances are indeed due to the accumulation of olivine. Variation of FeO with MgO (not shown) also supports this conclusion. As noted previously, for their M g # ' s , RA66, RA73, and

RA74 are high in Fe and Ti and low in A1 compared to M O R B glasses and the other POR samples. However, this is exactly the way in which high-Mg ocean-island basalts are k n o w n to deviate from M O R B compositions [29, chapter 1.2.6]. It should be noted that the amount of MgO

381 10.0

m nO

1.0 Ld O_ <~ LO

0.~ Sr K RDBaThTaNbCe

P ZrHfSmTi

Y YbCr

Fig. 8. Large ion lithophile (LIL) and high field strength (HFS) dement patterns for representative ocean-island tholeiites (OIT) and E-type MORB. Open circles: tholeiite from Kilanea (HAW-10 [28]); open squares: tholeiite from Manna Loa (HAW-22 [28]); dosed squares: E-type MORB from Reykjanes Ridge (site 408 [37]); shaded area represents the range of group 2 abundances (see Fig. 7C).

also correlates with the degree of P205 depletion in group 2 samples. Sampling of lavas from Kilauea, Hawaii [43, p. 87] has shown that apatite crystallizes near the solidus with approximately 10% melt left. Displacement of melt, presumably by filter pressing from accumulated crystals, before the crystallization-of apatite, may explain this correlation. This mechanism has also been suggested for lavas from Kilauea, Hawaii to explain segregation veins enriched in K and P205 [44]. No 50

"~"~0

'

t

'

I

0 GROUP • GROUP

•

i

49

-0

0

"1 2

-~ 0

oOo ~

oO 48

O,J 47 0

-r-I O3

%

O

4B

8

12

16

20

MgO (wt%) Fig. 9. S i O 2 vs. MgO for group 1 and group 2 samples.Olivine control line assumesolivine~ Foss.

data were found relating Ba to the crystallization sequence of ocean-island tholeiites, but it seems reasonable that Ba might also remain in the melt until late in the sequence. A sample which comes from Blue Ridge and which is virtually identical to our group 2 samples has been described by Haxel et al. ([45], and personal communication). Eight other POR mafic schists analyzed by them are similar to our group 1 samples. They considered the unusual composition of the Blue Ridge sample to be the result of alteration. We believe that our additional samples indicate that the group 2 "pattern" is a consistent one which occurs in several areas and is best interpreted in terms of igneous processes. Group 3 must be treated the most cautiously, because these samples show direct evidence of alteration. If their compositions are primarily of igneous origin, then they are probably alkaline to subalkaline (Y/Nb < 0.6 [46,47]). These samples have low abundances of the relatively compatible trace elements and high abundances of Zr, Y, Hf, Ta, Th, and LREE. Incompatible element abundances exceed those of E-type MORB ([48]; their P-MORB) and resemble abundances of off-axis (i.e., within-plate) plume volcanics (e.g., Bowie seamounts, west coast of British Columbia [49]; Loihi Seamount [50]; and other locations [29]). REE patterns similar to those of group 3 are observed in basalts from the Azores [51], Walvis Ridge DSDP sites, particularly site 528 [52], and dredged basalts from Iceland [53]. None of the POR samples show island-arc affinities. Calc-alkaline island-arc basalts are ruled out by the tholeiitic enrichment trend of groups 1-2 and by the lack of negative Ta and Nb anomalies [12,13]. Tholeiitic island-arc basalts tend to be depleted in high ionic potential elements (Ta-Y) relative to MORB [30,54]. As shown in Fig. 7, the POR mafic schists are generally enriched in these elements. In addition, island-arc basalts are commonly higher in A1203 (16-20 wt.% [29]) than the POR metabasalts. As already noted, there is a controversy as to whether the protolith of the POR schist formed in an open ocean or in a marginal basin. Some marginal basins, in particular rifted back-arc basins, contain basalts different from those occurring at mid-ocean ridges, being transitional between N-type MORB and island-arc basalts [12].

382 These basalts tend to be depleted in high ionic potential dements (e.g., Ta, Nb, Yb, Zr, and Hf) and enriched in the large-ion hthophile elements (e.g., Rb, Ba, and Th), probably the result of mantle source modification due to hydrous fluids or partial melts derived from a subducting slab [55-57]. The analyzed samples show no obvious evidence of these features. However, this is not unequivocal evidence against a back-arc origin of the protolith, because some back-arc basins contain both island-arc-like basalts and basalts which are indistinguishable from N-MORB (e.g., Mariana and Lau Basins [58]). Additionally, incipient or very wide back-arc basins may be predominantly T-type to N-type MORB [13]. Basalts from other types of marginal basins, such as transform (Gulf of California-like) basins and those formed from the entrapment, of older lithosphere (Azores type), do not have unique geochemical characteristics which would distinguish them from open-ocean basalts. The different magma types inferred for the metabasites are entirely consistent with a subduction origin for the POR schist. MORB-like basalts and mafic rocks with geochemical characteristics similar to ocean islands or seamounts have been identified elsewhere in ancient subduction complexes (e.g., [59]) and modem trenches [60]. Some workers (e.g., [61]) believe that the association of metamorphosed basalt, chert, and graywacke in the POR schists is tectonic, due to "shcing" of oceanic crust into an accretionary wedge. Others consider that the interlayering, although modified during subduction, originated through the eruption of basalt flows and sills into a sedimentary pile [44]. Our data indicate a diversity of basalt types in the POR schist. It may be that not all were emplaced in the same fashion. The present distribution of the POR schists is strongly controlled by the San Andreas and other late Tertiary strike-slip faults. When the offset on these faults is removed, the schist bodies define two widely separated dusters [2]. Ong, in the south, includes the schists in the vicinity of the Orocopia and Chocolate Mountains and those that are currently near the latitude of Los Angeles on the southwest side of the San Andreas fault (Sierra Pelona, East Fork and Blue Ridge areas, etc.). The other cluster, in the north, includes the schists of the Rand Mountains, Tehachapi Mountains, and

Portal Ridge. Because of the separation of the two clusters, and the possibihty that the schists of the northern group were metamorphosed prior to those in the south [1,14], it is thought by some that the protoliths of the northern and southern groups formed in separate basins. The other possibility is that the schist extends continuously in the subsurface between the two areas. Although our data can provide only circumstantial evidence bearing on this question, it is interesting to note that all the schist bodies studied contain samples of group 1. For the most part, the group 1 samples are more nearly like T-type than N-type MORB. Because T-type MORB is relatively uncommon, its presence in all the analyzed areas could indicate a common source (compare, for example, to the Shuksan subduction complex in Washington State in which the metabasalts are uniformly LREE depleted [62]). In addition, the very distinctive group 2 samples occur both in the Rand Mountains of the northern cluster and the Blue Ridge area of the southern cluster. On the other hand, the two LREE-depleted samples both occur in the north (PR20A and RA72), and group 3 samples were found only in the south. Overall, however, we are more struck by the similarities between the two clusters than the differences. 6. Conclusions

The POR mafic samples can be divided into three distinct chemical groups. Group 1 metabasalts have tholeiitic major-element compositions and trace-element abundances typical of N-type and T-type MORB. Variation of major and trace elements between samples can be attributed to varying degrees of crystal fractionation. A second group (group 2) is depleted in HREE, Y, and P and enriched in Ta, Nb, and Ce. Several group 2 samples have high MgO, Ni, and Cr, characteristics which are commonly associated with crystal accumulation. Group 2 samples are probably ocean-island tholeiites or E-type MORB. Group 3 samples have exceptionally high contents of LREE and other relatively incompatible trace elements. If not due to alteration, these chemical characteristics imply eruption within an ocean basin as the result of hot spot volcanism. The oceanic nature of the metabasalts is confirmed by the association with metamorphosed carbonates and ferro-

383 m a n g a n i f e r o u s cherts a n d is consistent w i t h the p r e s u m e d s u b d u c t i o n origin of the P O R schist. It does n o t seem likely that the basalts were e r u p t e d in a rifted (Japanese-type) b a c k - a r c basin. H o w ever, d i s c r i m i n a t i o n a m o n g the r e m a i n i n g settings p o s t u l a t e d for the P O R p r o t o l i t h ( o p e n ocean, t r a p p e d b a c k - a r c basin, or t r a n s f o r m basin) c a n n o t be made. Similarity in c o m p o s i t i o n o f the basalts f r o m the d i ff e r e n t areas is consistent w i t h all bodies o f P O R schist b e i n g p a r t o f a single terrane. Acknowledgements F u n d i n g was p r o v i d e d by the I o w a State U n i versity G r a d u a t e College, a g r a n t f r o m the Stand a r d Oil C o m p a n y of California, the U n i v e r s i t y of Missouri R e s e a r c h R e a c t o r , a n d N S F grants EA R- 8 3 1 9 1 2 5 a n d EAR-8617828. W e wish to t h a n k Scott Schlorholtz, Bruce T a n n e r , a n d particularly D z e n g o M z e n g e z a for assistance w i t h the X R F analyses. W e a p p r e c i a t e the discussions a n d suggestions p r o v i d e d by Scott Babcock, G o r d o n Haxel, Bert N o r d l i e , K a r l Seifert, S o r e n a Sorensen, a n d two a n o n y m o u s reviewers.

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