Geochemical Reconnaissance of Late Proterozoic 

Volcanic and Mafic Plutonic Rocks 

of the AI 'Awshaziyah Quadrangle 

and Related Rocks in the Qufar 

Quadrangle, Northern Arabian Shield 

U.S. GEOLOGICAL SURVEY BULLETIN 1680 
r 

.. 

.. 

"' 
Geochemical Reconnaissance of late Proterozoic 

Volcanic and Mafic Plutonic Rocks 

of the AI 'Awshaziyah Quadrangle 

and Related Rocks in the Qufar 

Quadrangle, Northern Arabian Shield 

By GERHARD W. lEO 

U.S. GEOLOGICAL SURVEY BULLETIN 1680 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1986 

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Leo, Gerhard W. 

Geochemical reconnaissance of late Proterozoic volcanic and 

mafic plutonic rocks of the AI 'Awshaziyah quadrangle 

and related rocks in the Qufar quadrangle, Northern Arabian Shield. 

(U.S. Geological Survey bulletin; 1680) 

Supt. of Docs. no.: I 19.3:1680 

1. Geochemistry-Saudi Arabia-'Awsajiyah Region. 

2. Geochemistry-Saudi Arabia-Qufar Region. 

3. Geology, Stratigraphic-Pre-Cambrian. 4. Volcanic 

ash, tuff, etc.-Saudi Arabia-'Awshaziyah Region. 

5. Volcanic ash, tuff, etc.-Saudi Arabia-Qufar Region. 

I. Title. II. Series: Geological Survey bulletin ; 1680. 

QE75.B9 no. 1680 [QE515] 557.3 s 86-600114 

[551.9'0953'8] 

The use of brand names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. 
CONTENTS 

Abstract 1 

Introduction 1 

Geologic setting 3 

Aqab and Nuf formations 3 

Gabbro and pyroxenite 3 

Hadn formation 5 

Chemistry 5 

Analytical methods 5 

Analytical results 6 

Major elements 6 

Trace elements other than rare earths 6 

Rare-earth elements 8 

Discussion 9 

Appendix: Petrographic descriptions of analyzed samples 12 

Acknowledgments 15 

References cited 15 

FIGURES 

1. Index map of the Arabian Shield showing the location of the AI 'A wshaziyah 

quadrangle 2 

2. Generalized geologic map of the AI 'Awshaziyah quadrangle 4 

3. Harker diagrams of late Proterozoic volcanics and mafic intrusives in the AI 

'A wshaziyah and Qufar quadrangles 8 

4. AFM diagram of Nuf, Aqab, and Hadn formations and mafic intrusives 9 

5. Fe0*/Mg0-Si02 plot of mafic rocks only 10 

6. Plot of Th, Zr, and Hf against Si02 10 

7. Plot of Rb against Sr 1 0 

8. Rare-earth element plots for Aqab and Hadn formations and mafic plutons in the 

AI 'A wshaziyah quadrangle 11 

9. Th-Ta-Hf/3 paleotectonic discrimination diagram of volcanic rocks and mafic 

intrusives from the AI 'Awshaziyah quadrangle 13 

10. La!Th diagram for mafic volcanics and intrusives 14 

11. La/Nb diagram for mafic Aqab volcanics and intrusives 15 

TABLES 

1. Major- and trace-element chemistry of 14 samples from the Hadn and Aqab 

formations and mafic intrusive rocks in the AI 'Awshaziyah quadrangle 7 

2. Summary of trace-element abundances in the Hadn, Aqab, and Nufformations and 

in mafic intrusives of the AI 'A wshaziyah quadrangle compared to island-arc 

abundances 12 

Contents Ill 

Geochemical Reconnaissance of Late Proterozoic 

Volcanic and Mafic P'lutonic Rocks of the . 

AI 'Awshaziyah Quadrangle and Related Rocks 

in the Qufar Quadrangle, Northern Arabian Shield 

By Gerhard W. Leo 

Abstract 

The AI 'Awshaziyah quadrangle and adjacent Qufar 

quadrangle in the northern part of the Arabian Shield are underlain by two distinct volcanic sequences and a variety of 

intrusive rocks ranging from gabbro to peralkaline granite. The 

older, layered sequence (Nuf and Aqab formations) is tentatively correlated with the Halaban formation (now Hulayfah 

Group) in the north-central and southeastern part of the shield; 

its age is estimated at 738 Ma. 

The Nuf and Aqab formations consist of a basal sequence of andesite, basalt, and graywacke intruded by plugs of 

gabbro locaiJy grading to pyroxenite and are overlain by intermediate to rhyolitic flows and tuffs (upper Nuf and upper Aqab 

formations). The basal Nuf contains pillow basalts and interbedded marbles, whereas the Aqab apparently lacks these but 

contains sandstone and locally thick conglomerate, especially 

in its upper part. The entire sequence is deformed and is metamorphosed at greenschist to lower amphibolite facies grade. A 

mostly felsic, relatively undeformed and unmetamorphosed 

volcanic-volcaniclastic sequence (Hadn formation, approximately 650-600 Ma old) unconformably overlies the Nuf and 

Aqab formations. The Hadn has been correlated with the 

Shammar group, and it could also be equivalent to the upper 

rhyolite of the Arfan formation (upper Halaban). Two source 

calderas for the Hadn have been identified. 

The mafic basal part of the Nuf and Aqab shows some 

tholeiitic characteristics, but overall trace-element abundances, and plots based on Th, Ta, Hf, and La suggest a calcalkaline trend. No clearcut oceanic (mid-ocean ridge basalt) 

affinity is apparent for these rocks, an observation somewhat at 

odds with lead-isotope data in the northern shield and geochemical interpretation on the Halaban. The mafic to intermediate volcanics are compatible with an origin in a maturing 

island arc. The Hadn, and possibly also the felsic upper part of 

the Aqab, may have been derived from a continental crustal 

source. According to current plate-tectonic models, the Hadn, 

in particular, has been related to magmatic activity accompanying Pan-African cratonization of the Arabian Shield. 

INTRODUCTION 

A growing body of published data on the petrology, 

geochemistry, and geochronology of the Arabian Shield has 

been accumulating over the past decade (Jackaman, 1972; 

Dodge and others, 1979; Fleck and others, 1980; Duyverman and others, 1982; Roobol and others, 1983; Darbyshire 

and others, 1983; Stacey and Stoeser, 1983; Stoeser and 

others, 1984; Stuckless and others, 1982 and in press). Most 

of the data and related interpretations are from the southern 

and central part of the shield, whereas relatively little detailed petrologic and geochemical work has been done thus 

far on Proterozoic volcanic rocks of the northeastern shield. 

As a result, the stratigraphy and correlation of the latter 

rocks with better-studied rocks to the south remain conjectural and their chemistry largely unknown. The present report growing out of reconnaissance mapping of the AI 

'Awshaziyah quadrangle in 1982 and 1983 (Leo, 1984, see 

fig. 1) presents and interprets major- and trace-element data 

on two late Proterozoic volcanic suites widely exposed in 

the AI 'A wshaziyah quadrangle and the Qufar quadrangle 

to the north (Kellogg, 1983; Leo and Kellogg, 1984). To 

some extent the chemical data serve to support but also to 

constrain the increasingly elaborate and detailed platetectonic models for the shield as a whole (see Stoeser and 

Camp, 1985, and references cited therein). Nevertheless, it 

is recognized that, owing to the relatively small data base 

of this report, conclusions and interpretations herein must 

be recognized as being preliminary and reconnaissance in 

nature. 

The Al 'Awshaziyah quadrangle is located in the 

northern part of the Arabian Shield (fig. 1). The quadrangle 

was mapped at 1:100,000 scale as part of the cooperative 

program between the U.S. Geological Survey and the Ministry of Petroleum and Mineral Resources, Kingdom of 

Saudi Arabia. Nearby quadrangles mapped at 1:100,000 

include the Qufar (Kellogg, 1983), Harrat Hutaymah (Pallister, unpub. data, 1983), Ghazzalah (Quick, 1983), Rak 

(Kellogg, 1984), and Hf il (Kellogg and Stoeser, 1983) 

quadrangles (fig. 1). The area between 26° and 27°N. and 

40°30' and 42°E., which includes the AI 'Awshaziyah quadrangle, has recently been compiled at 1:250,000 as the Wadi 

Ash Shu'bah quadrangle (Quick and Doebrich, 1986; see 

fig. 1). The entire region was earlier mapped in reconnaissance at 1:500,000 scale (Brown and others, 1963). Chevremont (1982) made a reconnaissance study of the mafic and 

ultramafic plutonic rocks of the Hf il region, with observations on volcanic and volcaniclastic rocks. 

Introduction 1 
36• 

Ill .. -· .... 

0 50 100 

16. - I I I I I I I I 

I 

200 

I 

38. 39. 40. 44. 46. 48. 

\ 

300 KILOMETERS ! y· I .11 .h : ~· \ 

46. 

Figure 1. Index map of the Arabian Shield showing the location of the 1 :100,000-scale AI'Awshaziyah quadrangle (shaded) and 

other quadrangles cited in this report: A, He} il (Kellogg and Stoeser, 1983); B, Qufar, (Kellogg, 1983); C, Rak (Kellogg, 1984); 

D, Ghazzalah (Quick, 1983); E, Harrat Hutaymah (Pallister, unpub. data, 1983); heavy outline, Wadi Ash Shu'bah 1 :250,000 

quadrangle (Quick and Doebrich, 1986). 

2 Geochemistry of the AI' Awshaziyah Quadrangle 
GEOLOGIC SETTING 

A generalized geologic map of the AI 'Awshaziyah 

quadrangle is shown in figure 2. The lithology is dominated 

by ( 1) large and abundant syntectonic and posttectonic plutons with radiometric ages ranging from about 740 to 580 

Ma and (2) a variety of late Proterozoic volcanic rocks with 

associated volcanogenic sediments and mafic plutons. The 

granites were not included in this study for several reasons, 

including their great extent and local compositional variations; difficulty of obtaining fresh material; and already 

available data on several large posttectonic plutons in the 

region (Stuckless and others, 1982 and in press). 

Brown and others (1963) defined three layered sequences in the northeastern shield. From oldest to youngest, 

these are (1) the dominantly andesitic Halaban group, 1 

(2) the overlying, shallow-marine Murdama group, and 

(3) the posttectonic Shammar group comprising felsic 

flows, ashflow tuffs, and associated sediments. Owing to 

the very broad and largely unconfirmed shieldwide correlations inherent in this terminology (e.g., rocks correlated 

with the Halaban group are widely distributed in the northeast, east-central, and southeast parts of the shield; see 

Schmidt and Brown, 1984, fig. 1), these names have not 

been used in the course of recent mapping of the northern 

shield. Nevertheless the general volcanic lithology of units 

correlated with the Halaban resembles that of Brown and 

others ( 1963). The Halaban has also been correlated with the 

Hulayfah group in the north-central shield (see Delfour, 

1977), a usage recently extended to the newly named Wadi 

Ash Shu'bah 1:250,000 quadrangle (Quick and Doebrich, 

1986). 

AQAB AND NUF FORMATIONS 

The older, more mafic volcanic sequence (currently 

Hulayfah group) formerly correlated with the Halaban by 

Brown and others (1963) was named Aqab formation in the 

AI 'Awshaziyah quadrangle (Leo, 1984) and Nuf formation 

to the north (Chevremont, 1982; Kellogg, 1983; Nuf not 

shown in fig. 2). The names Nuf and Aqab are retained in 

this report for purposes of comparison with their original 

descriptions. The Nuf, and by extension the Aqab, are indirectly dated by the K-Ar method at approximately 740 Ma 

(Delfour, 1977). A more direct U-Pb zircon date of 738 Ma 

has recently been obtained on a granodiorite that cuts Hulayfah rocks but is overlain by the Hadn (C.E. Hedge, pers. 

commun., cited by Quick and Doebrich, 1986). This must 

be regarded as a minimum age for the Hulayfah group. 

1 The Halaban and Murdama were referred to as formations and the 

Shamrnar as "Shamrnar rhyolite" by Brown and others (1963), but owing 

to their subsequent elevation to group status (see Schmidt and others, 

1973), the term group is used throughout this paper to designate these units. 

The Aqab formation is moderately to strongly deformed and is metamorphosed at greenschist facies grade, 

whereas the Nuf locally reflects amphibolite facies metamorphism (Kellogg, 1983). The Aqab and Nuf show lithologic similarities, especially in the lower parts of their section, where both consist dominantly of mafic metavolcanic 

rocks; however they show some consistent distinctions suggesting somewhat different depositional environments (Leo, 

1984, table 1). In particular, the upper part of the Aqab 

contains extensive conglomerate consisting of cobbles in a 

matrix of highly indurated graywacke. This association suggests a significant erosional hiatus within the Aqab volcanic 

episode and further suggests a depositional environment 

ranging from subaerial to nearshore (compare Greenwood 

and others, 1976, p. 16). The Nuf, by contrast, contains no 

conglomerate but has local marble lenses as well as local 

pillow basalts (Kellogg, 1983). Thus the Nuf appears to 

reflect a marine environment of deposition. This paper is 

concerned in part with geochemical differences in the two 

formations that would reflect their distinct depositional 

environments. 

The Aqab formation has a maximum thickness of 

6,000 min the AI 'Awshaziyah quadrangle and is divided 

into three members (Leo, 1984). From bottom to top they 

are the basalt-andesite member, the dacite-rhyolite member, 

and the conglomerate member, which is not considered 

further here. The basalt-andesite member consists of darkgray to greenish-black, aphanitic to fine-grained, locally 

amygdaloidal, typically nonporphyritic lava (see appendix 

for more detailed descriptions of analyzed samples). These 

rocks show a well-preserved primary texture generally consisting of felted plagioclase laths in a largely amorphous 

dark matrix. Plagioclase phenocrysts are faintly cloudy to 

moderately saussuritized; extensive breakdown of plagioclase is uncommon. Mafic constituents are represented by 

patches of chlorite+epidote, ±amphibole (actinolite?). 

Amygdules are filled by crystalline epidote and (or) quartz. 

Disseminated magnetite constitutes 3 to 5 percent. 

The dacite-rhyolite member is much more abundant 

and widely distributed than the basalt-andesite member. It 

includes purple to brown dacitic ash-flow tuffs that commonly show eutaxitic texture and have small white to pink 

phenocrysts, and greenish-gray, poorly sorted lithic tuffs 

and tuffaceous sandstones. 

The dacitic tuffs have silicic, de vitrified matrices that 

are slightly to moderately altered. Crystal tuffs contain 

subidiomorphic, cloudy to saussuritized plagioclase phenocrysts, subordinate quartz, and volcanic rock fragments. 

More mafic rocks transitional to andesite contain green 

hornblende, which apparently replaces primary pyroxene. 

GABBRO AND PYROXENITE 

Numerous masses of gabbro are distributed through 

the region and constitute three general types. Black, distincGabbro and Pyroxenite 3 
+ + 

+ + + 

+ + 

+ + + + + 

+ 

+ 

+ 

0 

I 

0 

5 

I I 

5 

10 

I 

+ + + 

+ + + + 

+ + + + 

+ + + + + 

+ + + + + 

+ + + + + 

+ ~+ + + 

+ + + + 

+ + + + + + 

+ + + + + + 

+ + + + + + 

+ + + .+gr+ + + 

+ + + -t- + + 

+ + + + + + 

+ + + + + + + + + + + + + + + + + + + 

+ + + + + + 

+ + + + + + 

+ 

+ + + + + + .+ + + + /+ + + + + + + + + + + ( + + + + + + + + + + + ~ + + + + + + + + + + + ~ + + + + + + + + + + 

I 

10 MILES 

20 KILOMETERS 

I 

Figure 2. Generalized geologic map of the AI 'Awshaziyah quadrangle, Kingdom of Saudi Arabia (modified from Leo, 1984). 

4 Geochemistry of the AI' Awshaziyah Quadrangle 
,a:l 1.1.1< a::o Q.< 

EXPLANATION 

Alluvium 

Post-orogenic granites 

Hadn formation 

Dacite-rhyolite member and undivided 

Aqab; cgi-Conglomerate member 

// :::1, Basalt-andesite member. Pattern follows 

approximate trend 

gd--gabbro-diabase, g--gabbro 

Pretectonic and syntectonic granitic rocks 

Gabbro/pyroxenite 

----- Contact 

----- Fault. Dashed where inferred; dotted where 

concealed 

.5 Sample locality (see table 1) 

u 

a 

N 

0 

0::: 

~ 0 

0::: 

0.. 

tive gabbro-pyroxenite with rhythmic banding on the scale 

of a few millimeters to several meters forms two bodies in 

the northern part of the AI 'A wshaziyah quadrangle (gp on 

fig. 2). This rock consists of diopsidic clinopyroxene partly 

replaced by tremolitic amphibole, 0 to 50 percent plagioclase (An37_65), and rare grains of serpentinized olivine. The 

gabbro-pyroxenite is intruded by the oldest granite in the 

region (gr on fig. 2) and possibly can be correlated with 

Hf il mafic-ultramafic suite (Chevremont, 1982). 

A more widely distributed gabbro type (gd on fig. 2) 

consists of green-brown hornblende and prismatic, strongly 

zoned plagioclase (An32_ 75). Texturally this gabbro is gradational to diabase and is locally associated with Aqab basalt. 

Hence it is tentatively correlated with the Aqab formation. 

A third type of gabbro, forming relatively small and 

steep-sided plugs (g on fig. 2), is cut by synorogenic granite 

and therefore may also be of Aqab age. This gabbro is 

medium grained, dark gray, and consists dominantly of 

calcic plagioclase (An70-82) and diopsidic pyroxene, locally 

accompanied by serpentinized olivine, deep red-brown amphibole (kaersutite?), red biotite, and accessory apatite. 

HADN FORMATION 

The Hadn formation (the term is used here to include 

the AI 'Awshaziyah formation of Leo, 1984) overlies both 

the Aqab and Nufformations. The age of the Hadn has been 

indirectly determined at approximately 650 to 600 Ma 

(C.E. Hedge, pers. commun., 1985). The contact with the 

Aqab is locally exposed along an angular unconformity in 

the Al 'Awshaziyah quadrangle. In the Qufar quadrangle 

(Kellogg, 1983) no contacts between Hadn and Nuf have 

been found. The upper parts of both the Aqab and Nuf 

contain dacitic and rhyodacitic flows and tuffs. Thus, these 

two older volcanic sequences overlap the compositional 

range of the Hadn. This compositional overlap, which is 

particularly well exposed in the upper part of the Aqab 

formation (Leo, 1984, fig. 2 and pl. 1), makes the contact 

with lithologically similar Hadn hard to locate. The similarity between the upper Aqab and the Hadn notwithstanding, 

eruption of the two units is separated by a hiatus of more 

than 100 Ma. 

The Hadn formation comprises a sequence of rhyolitic 

to dacitic flows, ash-flow tuffs, tuffaceous sandstones, and 

local conglomerate layers (much less massive than the Aqab 

conglomerate). The unit is regionally correlated with the 

more widely distributed Shammar group (Kellogg, 1983). A 

major source area for the Hadn is a caldera in the westcentral part of the AI 'Awshaziyah quadrangle (AI 'Awshaziyah eruptive center, Leo, 1984). Broad exposures of 

Hadn formation in the western part of the AI 'A wshaziyah 

quadrangle are flat-lying to gently dipping. The Hadn formation in the southeastern and northeastern parts of the 

Qufar quadrangle is highly folded and locally overturned 

(Kellogg, 1983). This indicates that the crust in the eastern 

and northeastern part of the study area was tectonically more 

active throughout the late Proterozoic than in the west and 

southwest, where tectonism has been virtually negligible at 

least since the Hadn volcanism. 

The welded tuffs typically have a devitrified groundmass and 5 to 25 percent phenocrysts including elongated, 

well-twinned plagioclase prisms, quartz, and sanidine (the 

latter in a minority of samples). Accessory constituents include magnetite± biotite. Alteration in most samples ranges 

from very slight to moderate. Fragmental rocks are generally more altered than welded tuffs and lava flows, and 

rocks of more mafic composition are more altered than the 

siliceous rocks. The overall extent of alteration of these 

rocks is less than that of the Aqab and Nuf formations. 

Although the investigated area is relatively small, correlation of the major underlying units with more broadly 

distributed volcanic suites makes it likely that the assemblage of the AI 'Awshaziyah quadrangle is representative of 

a significantly larger area. 

CHEMISTRY 

Analytical Methods 

Eleven samples of valcanic rocks and three samples 

of mafic plutonic rocks from the AI 'Awshaziyah quadrangle were analyzed for major-element chemistry by standard 

Chemistry 5 
rapid-rock procedures in the U.S. Geological Survey laboratories at Reston, Va. The samples selected for analysis 

ranged from virtually unaltered to moderately altered, consistent with partial recrystallization at greenschist facies 

grade (especially Aqab samples, see appendix), but showed 

a minimum of fractures and (or) obviously introduced material. Rb, Sr, Ba, Nb, Zr, andY were determined by X-ray 

fluorescence, and 26 trace elements including rare-earth 

elements (REE) were determined by instrumental neutron 

activation (INA). Results are tabulated in table 1. Norms 

were calculated by the GNAP program (Stuckless and 

VanTrump, 1979; T.L. Wright, unpub. data) on a waterfree basis (adjusted oxides not shown in table 1). Fe20 3 was 

adjusted where necessary according to the principle of 

Irvine and Baragar ( 1971) that limits the amount of Fe20 3 to 

the weight percent Ti02 + 1. 5, the remaining Fe20 3 being 

converted to FeO and combined with the FeO value. Analytical data obtained by Karl S. Kellogg, U.S. Geological 

Survey, from X-Ray Assay Laboratories, Ltd., Don Mills, 

Ontario, on 17 samples of related rocks that include Nuf, 

intermediate to felsic Aqab, and Hadn from the Qufar quadrangle (Kellogg, 1984 and unpub. data) are used here by 

permission. These data are shown in several plots and are 

also utilized in table 2. 

Precision for the various analytical procedures on AI 

'Awshaziyah samples is as follows. Rapid-rock analysis 

(concentration range and relative precision, both in percent): 

>10, 1-2; 1-10, 2-10; 0.1-1, 10-20; and 0.1, 50-100. 

X-ray fluorescence spectrometry: >20 ppm, 5 percent relative; <20 ppm, ±2 ppm absolute. Instrumental neutron 

activation analysis: variable, but nearly all values accurate 

to < 10 percent, most <5 percent. Among rare-earth elements (REE) Gd has an error limit of <10 percent and Tm 

<20 percent, but these elements are mostly omitted from 

REE plots (fig. 8). Precision reported by X-Ray Assay Laboratories, Ltd. is ±2 percent for major oxides and 

± 10 percent for trace elements including REE (oral 

commun., 1986). 

Analytical Results 

Major Elements 

Major-element Harker plots of the volcanic-rock data 

(fig. 3) reflect the continuum of compositions evident from 

megascopic and microscopic characteristics. Nuf and mafic 

Aqab samples generally span the compositional range from 

basalt (47-52 percent Si02) through basaltic andesite and 

andesite (52-62 percent Si02). Hadn samples, combined 

with felsic Aqab, define the general range dacite through 

rhyolite. A slight compositional gap apparent in the range of 

Si02=59-64 percent (fig. 3) could be an artifact of sampling. 

Data points for Nuf, mafic Aqab, and gabbro show 

considerable scatter, which in some cases can be resolved 

6 Geochemistry of the AI' Awshaziyah Quadrangle 

into mutually distinct, more or less linear trends (e.g. , 

Fe0*, 2 MgO, CaO, Na20, Ti02, and P20 5). The following 

observations can be made: 

1. The Aqab points show a tholeiitic trend over the Si02 

range of 48 to 54 percent, notably in the enrichment of 

FeO*, Na20, and P20 5, with accompanying depletion in 

MgO and CaO. Continuation of this trend is not particularly 

apparent in the felsic Aqab. 

2. The greater scatter of Nuf points relative to mafic 

Aqab for Al20 3 , FeO*, and CaO, together with the distinctly different fields of Nuf and mafic Aqab for nearly all 

the oxides, suggests one or more of several possibilities: 

(a) the Nuf and Aqab may not be comagmatic; (b) some of 

the Nuf samples, e.g., the basalt containing 49 percent 

Si02, 8.2 percent Al20 3, 10 percent MgO, and 19.2 percent 

CaO, are not primary liquids but rather represent cumulates 

(the aforementioned sample is not far from an augite composition with excess Al20 3 that could be contained in 25 percent plagioclase); (c) some of the differences between the AI 

'Awshaziyah and Qufar samples could be related to analytical procedures in two different laboratories (see section on 

trace elements). In this connection it may be noted, however, that the data points for felsic rocks, representing analyses by both laboratories, show less scatter (figs. 3, 6). Because of the writer's lack of familiarity with the analyzed 

Qufar samples and lack of knowledge concerning point 2c 

(above), the above possibilities cannot be fully evaluated. It 

seems safe to assume that 2b is a factor, especially inasmuch 

as gabbro-pyroxenite (anal. 12-13, table 1) show somewhat 

similar compositions as some of the Nuf "basalt." Finally, 

the alteration of most of the analyzed samples probably 

contributes to the observed scatter. 

On an AFM plot (fig. 4) most of the mafic samples 

fall into the tholeiite field as defined by Irvine and Baragar 

(1971), whereas most of the felsic samples, both Hadn and 

Aqab, fall in the calc-alkaline field. Most, but not all of the 

same mafic samples also plot as tholeiites in figure 5. In 

paleotectonic diagrams (figs. 9-11) the tholeiitic and calcalkaline categories are somewhat mixed. Aqab samples plot 

well in the calc-alkaline field in figure 9, plot (along with 

Nuf samples) in the orogenic andesite field (fig. 10), and 

skirt the mid-ocean ridge basalt-orogenic andesite boundary 

in figure 11 (data for Nuf not available). Thus, none of the 

mafic volcanics for which data exist has clear-cut oceanic 

affinities. The three gabbro samples, by contrast, partially 

plot in mid-ocean ridge basalt fields (see esp. figs. 9, 11). 

Trace Elements Other Than Rare Earths 

Trace elements are listed in table 1, and several plots 

of elements obtained for both sets of samples are shown in 

2Total iron as FeO. 
Table 1. Major- and trace-element chemistry of 14 samples from the Hadn (AI 'Awshaziyah formation of Leo, 1984) and Aqab 

formations and mafic intrusive rocks in the AI 'Awshaziyah quadrangle 

[NO, not determined;--, nil or unreliable value; sample locations shown in figure 2; descriptions are in the appendix] 

Hadn formation Aqab formation Gabbro and pyroxenite 

Si02------- -------- 70.7 

Al203 ------------- 13.8 

Fe203 ------------- 1.8 

FeO--------------- 1.2 

MgO -------------- 0.3 

CaO--------------- 1.3 

Na20-------------- 5.2 

K20--------------- 3.3 

H20--------------- 0.94 

Ti02 -------------- 0.31 

P205 -------------- 0.02 

MnO -------------- 0.09 

C02--------------- 0.16 

Total------------ 99. 

Q ----------------- 25.54 

c -----------------

Or---------------- 19.86 

Ab ---------------- 44.82 

An---------------- 4.65 

Ac ----------------

Wo---------------- 0.32 

En ---------------- 0.76 

Fs------ ----------- 0.38 

Fo ----------------

Fa-----------------

Mt---------------- 2.66 

Hm----------------

11----------------- 0.60 

Ap ---------------- 0.05 

Cc ---------------- 0.37 

Total------- -----100.00 

Salic----------- 94.87 

Fernie---------- 5.13 

Di----------------- 0.61 

Hy ---------------- 0.84 

01-----------------

01 1---------------- 90.22 

Rb ---------------- 62 

Sr----------------- 70 

Ba ----------------717 

Th ---------------- 5.4 

Zr--- --------------267 

Hf ---------------- 6.8 

Nb---------------- 10 

Ta ---------------- 0.51 

y ----------------- 49 

Co---------------- l. 

Cu---------------- ND 

Ni-----------------NO 

Zn ---------------- 73 

Cr----------------- 4. 

Sc----------------- 3.09 

u ----------------- 2.2 

La---------------- 27.5 

Ce ---------------- 54.4 

Nd ---------------- 28. 

Sm--- ------------- 6.85 

Eu ---------------- 1.01 

Gd ---------------- 6.40 

Tb ---------------- 1.13 

Tm------- --------- 0.72 

Yb---------------- 5.23 

Lu ---------------- 0.799 

K/Rb --------------450 

Rb/Sr- ------------- 0.89 

I Differentiation index. 

2 

76.1 

13.2 

0.92 

0.62 

0.25 

1.0 

3.5 

4.5 

1.1 

0.08 

0.03 

0.04 

0.02 

101. 

36.22 

0.87 

26.65 

29.68 

4.65 

0.62 

0.42 

0.64 

0.15 

0.07 

0.05 

100.00 

98.06 

1.94 

0.62 

92.54 

3 

69.9 

15.4 

1.7 

1.9 

1.6 

3.0 

2.7 

2.2 

1.63 

0.39 

0.12 

0.05 

0.27 

101. 

37.63 

4.07 

13.10 

23.02 

12.49 

4.02 

1.55 

2.48 

0.75 

0.29 

0.62 

100.01 

90.31 

9.70 

5.56 

73.76 

4 

70.7 

14.7 

1.8 

1.2 

0.22 

0.62 

6.7 

2.3 

0.74 

0.25 

0.05 

0.12 

0.04 

99. 

21.56 

0.28 

13.77 

57.44 

2.53 

0.56 

0.53 

2.64 

0.48 

0.12 

0.09 

100.00 

95.58 

4.43 

1.09 

92.77 

131 77 34 

106 640 84 

842 1505 723 

11.2 8.23 4.85 

130 242 387 

3.82 5.3 9.12 

<5 10 

0.79 0.5 0.86 

24 24 43 

l. 5.55 0.4 

ND ND ND 

ND ND ND 

30 56 108 

4. 19.4 I. 

1.87 6.7 8.97 

3.5 2.6 2.02 

26.6 27.9 38.3 

47.6 48.1 79.8 

17. 22. 43. 

3.39 4.39 9.29 

0.438 1.01 I. 79 

3.50 4. 8. 

0.405 0.43 1.21 

0.22 0.3 0.6 

1.8 2.08 4.33 

0.277 0.330 0.615 

286 347 570 

1.24 0.12 0.4 

5 6 7 8 9 

Major Oxides (in percent) 

68.2 

14.4 

2.14 

2.78 

1.3 

2.5 

4.9 

2.2 

1.3 

0.64 

0.07 

0.16 

0.03 

101. 

73.3 

13.8 

1.8 

1.9 

0.22 

0.28 

5.2 

3.1 

0.58 

0.25 

0.08 

0.08 

0.05 

101. 

70.9 

13.6 

1.8 

2.0 

0.5 

1.3 

5.3 

3.3 

1.0 

0.31 

0.07 

0.16 

0.26 

101. 

69.0 

14.4 

1.9 

1.2 

0.98 

1.9 

5.3 

2.0 

1.2 

0.39 

0.16 

0.06 

0.91 

99. 

50.9 

18.9 

2.7 

6.5 

4.5 

8.9 

3.4 

1.1 

1.26 

1.2 

0.35 

0.15 

0.1 

100. 

Normative Minerals-H20 free (in percent) 

23.46 

13.09 

41.75 

10.87 

0.40 

3.26 

2.60 

3.12 

1.22 

0.17 

0.07 

100.01 

89.17 

10.84 

0.79 

5.47 

78.29 

29.82 

1.69 

18.31 

43.98 

0.55 

0.55 

1.74 

2.61 

0.48 

0.19 

0.11 

100.01 

94.34 

5.67 

2.28 

92.10 

24.21 

19.60 

45.07 

3.59 

0.33 

1.25 

1.98 

2.62 

0.59 

0.17 

0.59 

100.01 

92.47 

7.53 

0.66 

2.90 

88.88 

28.39 

2.60 

12.04 

45.67 

2.68 

2.48 

0.10 

2.81 

0.75 

0.39 

2.11 

100.01 

91.37 

8.64 

2.59 

86.09 

0.52 

6.59 

29.15 

33.50 

3.46 

11.35 

8.11 

3.97 

2.31 

0.84 

0.23 

100.02 

69.75 

30.27 

6.78 

16.14 

36.26 

Trace Elements (in ppm) 

48 45 

397 Ill 

1170 827 

7.74 5.29 

246 404 

6.17 9.51 

7 13 

0.5 0.9 

30 45 

7.1 1.2 

ND ND 

ND ND 

68 108 

38.3 4. 

10.3 7.44 

2.3 2.05 

25.5 38. 

47.4 80. 

25. 43. 

5.5 9.24 

1.16 1.02 

4. 9. 

0.63 1.21 

0.4 0.62 

2.54 4.13 

0.376 0.579 

384 572 

0.12 0.4 

38 

149 

1061 

5.32 

379 

9.35 

11 

0.89 

45 

0.5 

ND 

ND 

117 

<2 

9.63 

2.11 

39.6 

84.3 

46. 

9.8 

2.01 

9. 

1.24 

0.7 

4.26 

0.680 

725.00 

0.25 

54 28 

215 709 

722 523 

3.7 1.87 

147 129 

3.47 2.7 

8 6 

0.3 0.3 

22 24 

5.0 30.3 

ND 32 

ND 37 

46 82 

18.2 140 

7.15 25.5 

1.54 0.65 

15.8 13.8 

30.5 29.3 

17. 18.3 

3.53 4.42 

0.865 1.27 

4. 4. 

0.42 0.62 

0.21 0.2 

1.68 2.2 

0.246 0.325 

314 329 

0.25 0.4 

10 

48.1 

20.3 

2.5 

7.3 

4.9 

10.6 

2.2 

0.58 

1.33 

1.0 

0.22 

0.21 

0.05 

99. 

0.77 

3.50 

19.00 

44.71 

3.00 

12.46 

10.29 

3.70 

1.94 

0.53 

0.12 

100.01 

67.98 

32.03 

5.90 

19.84 

23.27 

11 

53.9 

14.7 

3.2 

8.7 

3.2 

5.3 

4.2 

0.90 

2.44 

1.7 

0.46 

0.2 

0.64 

100. 

8.36 

5.48 

36.60 

19.15 

0.28 

8.21 

11.22 

4.78 

3.33 

1.12 

1.50 

100.03 

69.60 

30.43 

0.55 

19.15 

50.44 

12 

54.7 

9.5 

1.8 

6.5 

10.3 

13.2 

1.7 

0.28 

0.% 

0.29 

0.12 

0.17 

0.15 

100. 

6.24 

1.68 

14.57 

17.69 

19.58 

25.99 

10.42 

2.64 

0.56 

0.29 

0.35 

100.01 

40.18 

59.82 

37.74 

18.24 

22.49 

28 23 13 

664 493 328 

265 573 59 

1.41 3.18 0.4 

76 170 48 

1.2 4.14 0.7 

<5 II 6 

0.2 0.4 <0.3 

18 36 15 

30. 28.1 37. 

22 50 63 

21 22 98 

78 123 61 

36.2 16 726 

30.5 32.6 49.2 

0.34 1.11 <0.5 

9.5 20.9 3.5 

19.1 43.6 9.6 

12. 28. 7. 

3.04 6. 72 1.85 

1.01 1.76 0.51 

3. 6. 2. 

0.42 1.01 0.21 

0.2 0.5 0.1 

1.53 3.55 0.87 

0.250 0.529 0.149 

175 336 185 

.04 0.05 0.04 

13 

53.9 

10.9 

1.9 

7.6 

9.9 

10.9 

1.9 

0.34 

1.44 

0.39 

0.13 

0.18 

0.08 

100. 

5.65 

2.05 

16.39 

20.60 

13.83 

25.13 

12.31 

2.81 

0.70 

0.31 

0.19 

100.01 

44.68 

55.33 

26.81 

24.46 

24.08 

13 

253 

119 

0.7 

56 

0.9 

8 

<0.3 

14 

43.6 

88 

62 

84 

483 

50.4 

<0.5 

5.9 

13.4 

10. 

2.12 

0.58 

0.20 

0.1 

0.79 

0.157 

249 

0.05 

14 

49.9 

20.9 

1.8 

5.3 

7.8 

8.5 

3.3 

0.46 

2.11 

0.31 

0.13 

0.11 

0.02 

101. 

2.76 

28.34 

41.47 

0.14 

9.32 

3.81 

7.28 

3.28 

2.65 

0.60 

0.31 

0.05 

Hl0.01 

72.56 

27.44 

0.28 

13.00 

10.56 

31.10 

40 

1248 

249 

0.4 

64 

0.71 

<5 

0.2 

10 

51.2 

82 

137 

61 

350 

3.5 

<0.3 

5.8 

11.4 

7.4 

1.3 

0.781 

0.12 

0.1 

0.34 

0.05 

97.5 

0.03 

Chemistry 7 
-.......+ I I I I I 

20 - -

' '(. 

' 1"1 

0 

N 15 - + 

"t 

\ 

\ 

~ -

Ci • • 0 

+ 10 - + -

. 

10 - .........-.:!-• - ..,......-¥ + 0 . + 

* + 

0 

u.. 

Q) 

5~ 

~ -

Q1 

0 

10 - -.,. 

+ 

0 

Cl 5- ~...... - -

~ -._..._ --...... 0 

~ 

0 o~o-~ Q) 

20 

+ + 

10 ...... 

' + 0 

0 ......... co 

(.) + '\ 

\ 

5 ~ 

~ 

0 

0 

~~ + ......... ~ ·~ ~ N 

co 

z ,....... . / -++- .. 

0 :r 0~ ~ N 

~ 

.:fll± • ··~ 0 

N } _.,¥ 

J 0 _,. __ _._. 

i= + 

.+ • it- " 0~ Ill 

~0.51 .... _ ...... 

Q.. 0.0 ..t:tij • ~ I o 0~ I Q 

45 50 55 60 65 70 75 80 

Si02 

EXPLANATION 

0 Encloses 13 Hadn analyses o Nuf andesite 

0 Aqab dacite and rhyolite • Nuf basalt 

• Aqab basalt and andesite + Gabbro and pyroxenite 

Figure 3. Harker diagrams of late Proterozoic volcanics and 

mafic intrusives in the AI'Awshaziyah and Qufar quadrangles. 

Data from table 1 and K.S. Kellogg (unpub. data, 1984). 

FeO*, total iron as FeO. Dashed lines show tholeiitic 

trend for part of Aqab (see text). Values are in percent. 

8 Geochemistry of the AI' Awshaziyah Quadrangle 

figures 6 and 7. The following observations can be made: 

1. The relatively stable elements Th and Hf show reasonably linear increases when plotted against Si02 (fig. 6). 

Similar linearity might be expected for Zr, but in fact the 

Zr-Si02 plot shows considerable scatter, and the compositional fields of mafic and felsic rocks are more sharply 

separated than for Th and Hf. The four unusually low Zr 

values (0-20 ppm) for some Nuf samples may reflect analytical error. However, they also have very low Hf contents 

( < 1 ppm) and no detectable Rb (fig. 7). 

2. Rb and Sr both show significant scatter (table 1; 

fig. 7), so that a correlation between Sr content and depth to 

a seismic zone (Hart and others, 1970) cannot be applied 

on the basis of these data. 

3. Th, Hf, and Rb data points for Hadn and felsic Aqab 

show some overlap in figures 6 and 7, adding to the impression that these units are petrologically similar despite their 

apparent structural and temporal separation. 

4. A subtle but consistent distinction, shown by dashed 

lines in figures 6 and 7, is apparent between the mafic Aqab 

and most of the Nuf, in that the latter shows generally lower 

values of the plotted elements than the former; this is true of 

Th and Hf as well as Zr and Rb (Zr values also are consistently lower in felsic Aqab than in Hadn). These data are 

summarized in table 2. Taken together (and assuming the 

low Zr and Rb values of the Nuf samples to be essentially 

accurate) these differences in abundances constitute a basis 

of distinguishing between Nuf and Aqab in accord with the 

more oceanic environment of the Nuf. Trace-element abundances for the Nuf basalts and associated gabbro-pyroxenite 

are within th,e general range of island-arc tholeiites (Jakes 

and White, 1971; Jakes and Gill, 1970). They are consistently lower than abundances of the same elements in the 

mafic Aqab, which has abundances more in the range of 

calc-alkaline rocks . 

Rare-Earth Elements 

Rare-earth element (REE) patterns are shown in figure 8. Except for one gabbro pattern (anal. 14, table 1) (fig. 

8A), they are slightly to moderately fractionated with enriched light REE's, depleted heavy REE's, and negligible to 

moderate Eu anomalies, and overall abundances that increase more or less systematically with increasingly felsic 

compositions. 

The weakly fractionated patterns of Nuf samples 

(fig. 8A; K.S. Kellogg, unpub. data) are similar to those of 

calc-alkaline suites (Jakes and Gill, 1970; Arth, 1981). 

Thus, the REE's do not appear to reflect the tholeiitic character suggested in the AFM diagram. The reason for this not 

clear. The Nuf patterns, nevertheless, are less fractionated 

than basaltic Halaban samples of Dodge and others ( 1979). 

Two gabbro-pyroxenite patterns (fig. 8A) show generally 

similar REE abundances to those of the Nuf basalts. The 

negative Eu anomaly suggests a plagioclase residue, 
FeO + 0.9 Fe203 EXPLANATION 

e;. Hadn formation 

Aqab dacite and rhyolite 

Aqab basalt and andesite 

Nut andesite 

Nut basalt 

+ Gabbro and pyroxenite 

--- Halaban crust (Dodge and others, 

1979) 

---- Cascade trend (Carmichael and 

others, 1974) 

MgO 

Figure 4. AFM diagram of Nuf, Aqab, and Hadn formations, and mafic intrusives. TH, tholeiitic field; CA, calc-alkaline field. 

whereas the moderate fractionation of heavy REE' s is compatible with a residue of hornblende (Hanson, 1978, fig. 5). 

The very distinctive remaining pattern (fig. 8A, no. 14), a 

gabbro containing olivine, biotite, augite, and (primary?) 

hornblende as mafic phases, suggests a plagioclase cumulate and a residue containing garnet. 

Aqab patterns of mafic flow rocks (nos. 9-11, 

fig. 8B) are very uniform, moderately fractionated, and 

show negligible to small negative Eu anomalies. The pattern 

for sample no. 8, a dacitic tuff, shows somewhat greater 

fractionation of light REE's and a more conspicuous negative Eu anomaly, both consistent with its relatively more 

silicic and alkalic character, although the overall REE abundances are comparable with those of the mafic samples. 

The Hadn patterns (fig. 8C) show an overall greater 

degree of enrichment and related fractionation than the 

Aqab samples; they are generally comparable with the upper, rhyolitic to dacitic part of the Halaban group (Arfan 

Formation) (Hadley, 1976; Dodge and others, 1979, fig. 5). 

DISCUSSION 

Since the inception of modem studies of the Arabian 

Shield, it has been interpreted in progressively complex and 

detailed models as the product of accretion of a sequence of 

late Proterozoic island arcs, culminating in a collision with 

the African plate during the Pan African orogeny (approx. 

550-650 Ma B.P.) (Greenwood and others, 1976; Schmidt 

and others, 1979; Greenwood and others, 1980; Fleck and 

others, 1980; Greenwood and others, 1982; Duyverman and 

others, 1982; Stoeser and others, 1984; Camp, 1984). The 

Discussion 9 
0 

01 

3 

::!: *"2 0 

(I) 

IL. 

oL_~ __ _L __ ~~L_~ __ _L __ l_~---L--~--L-~ 

48 50 52 54 56 58 

Si02, IN PERCENT 

EXPLANATION 

• Aqab basalt and andesite • Nuf basalt 

o Nuf andesite + Gabbro and pyroxenite 

60 

Figure 5. Fe0*/Mg0-Si02 plot of mafic rocks only (after 

Miyashiro, 1974). FeO*, total iron as FeO; TH, tholeiitic field; 

CA, calc-alkaline field. 

Z400 

0 

:::::; Zr ~ 

~300 

a: 

w 

Q..200 

fJ) 

ta: 

~ 100 

0 

,~F ~ • ·~..t.-II+; o I 

40 50 60 70 80 

Si02, IN PERCENT 

EXPLANATION 

0 Hadn formation (13 analyses) o Nuf andesite 

o Aqab dacite and rhyolite • Nuf basalt 

• Aqab basalt and andesite + Gabbro and pyroxenite 

Figure 6. Plot of Th, Zr, and Hf against Si02 • Dashed lines 

separate most Nuf from Aqab samples (see text). 

10 Geochemistry of the AI'Awshaziyah Quadrangle 

100~--~,--~,--~,--~,--~~--~,--~ 

z 

0 80 f---

:::::i 

...J 

~ 

a: 60 _6. 6. 

w 

Q.. 

~ a: 

<( 

Q.. 

6. 0 

40-

+ + 

6. •• 

-

-

+ -

-

0~~~~---L----~----L---~----~--~ ---~----; I l I I 

0 200 400 600 800 1000 1200 1400 

Sr, IN PARTS PER MILLION 

EXPLANATION 

e:. Hadn formation 

o Aqab dacite and rhyolite 

• Aqab basalt and andesite 

o Nuf andesite 

• Nuf basalt 

+ Gabbro and pyroxenite 

Figure 7. Plot of Rb against Sr. Dashed line separates Nuf and 

Aqab samples. 

most recent formulation (Stoeser and Camp, 1985) postulates the accretion of several microplates, three of ensimatic 

island-arc affinity and two of continental affinity. The region under discussion falls within the so-called Nabitah 

orogenic belt, which separates sialic basement to the east 

from ensimatic island-arc terranes to the west. A different 

interpretation of the evolution of the shield postulates alternating rifting and compression without subduction (Kemp 

and others, 1982). 

Recent lead-isotope studies (Stacey and Stoeser, 

1983) confirm that most of the western shield, correlated 

with island-arc terranes of Halaban and older age and including the present study area, is derived from oceanic crust 

and mantle. An oceanic-crustal source was independently 

postulated by Dodge and others (1979) for rocks of the 

Halaban group (Juqjuq and Arfan formations) in the Bi'r 

Juqjuq quadrangle (Hadley, 1976) in the southeastern part 

of the shield on the basis of geochemical and Sr-isotope 

data. As indicated earlier, the Halaban (now regarded as 

equivalent to Hulayfah) is here provisionally correlated with 

the Aqab and Nuf formations on the basis of lithology and 

stratigraphic position, although a spatial separation of about 

700 km between the Bi'r Juqjuq area and the present study 

area, as well as discrepancies in reported radiometric ages3, 

makes such a correlation rather tentative. Comparison of the 

3Stoeser and others (1984) cite an Rb-Sr whole-rock isochron age 

range of 680-800 Ma for the Halaban, but Darbyshire and others (1983) 

determined an Rb-Sr isochron age of 612 ± 22 Ma for the Juqjuq formation 

and 608 ± 9 Ma for the Arfan formation. These disparities are discussed by 

Darbyshire and others (1983). The younger ages more nearly correspond to 

the Hadn age range than to the heretofore accepted Halaban age. 
200 

A Gabbro and pyroxenite 

100 

50 

w 

1-

a: 

0 20 z 

0 

:I: 

~ 10 (J 

0 

a: 

5 

2 

1 ~~----L---~~--~~--------~--~ 

La Ce Nd Sm Eu Gd Tb Tm Yb Lu 

Figure 8. Rare-earth element (REE) plots for Aqab and Hadn 

formations and mafic plutons in the AI 'Awshaziyah quadrangle. Numbers correspond to analyses in table 1; symbols as in 

figure 7. A, Gabbro and pyroxenite; shaded field is mafic Nuf 

from Qufar quadrangle (K.S. Kellogg, unpub. data). B, Felsic 

and mafic Aqab; shaded. field for mafic Halaban from Dodge 

and others, 1979. C, Hadn formation, with shaded Halaban 

field for tuffaceous rocks (Dodge and others, 1979). Patterns 

for analyses 2 and 6 omitted to reduce clutter. 

geochemical character of the two groups of rocks is nevertheless instructive. 

To the extent that data are available, the Halaban 

studied by Dodge and others (1979) is generally similar to 

the Nuf and Aqab except that basaltic rocks of the former are 

somewhat more alkalic than Nuf-Aqab basalts, have greater 

REE concentrations and somewhat more fractionated REE 

trends (fig. 8), and the Halaban suite as a whole shows a 

somewhat more pronounced calc-alkaline trend than the Nuf 

and Aqab (fig. 4). The geochemistry is compatible with that 

of a relatively evolved island-arc assemblage. An oceanic 

crustal source for the Halaban is postulated mainly on the 

basis of relatively low 87Sr initial ratios (0. 7018 for andesite 

and 0. 7028 for alkali basalt) (Dodge and others, 1979). 

Sr-isotope data have not been determined for the Nuf and 

Aqab. 

The upper, rhyolitic part of the Arfan formation 

yielded an 87Sr0 of 0. 7045 to Dodge and others (1979), and 

was assumed by them to reflect a distinct, probably 

continental-crustal source. Such an interpretation also appears most reasonable, albeit speculative, for the Hadn as 

well as for the ash flows in the upper part of the Aqab and 

Nuf. Other considerations that must be taken into account in 

any evaluation of the Nuf, Aqab, and Hadn include the 

differences in depositional environments of the Nuf 

200 

8 Aqab formation 

100 

50 

w 

1-

a: 

c 20 z 

0 

:I: 

~ 10 (J 

0 

a: 

5 

2 

1 

La Ce Nd Sm Eu Gd Tb Tm Yb Lu 

200 

c Hadn formation 

100 

50 

w 

1-

a: 

0 20 z 

0 

:I: 

~ 10 (J 

0 

a: 

5 

2 

1 L_~ ____ L_ __ _L~L-~~--------~--~~ 

La Ce Nd Sm Eu Gd Tb Tm Yb Lu 

(marine) and Aqab (nearshore to subaerial); the lower 

incompatible-element abundances, lower K/Rb, and higher 

Na20/K20 in the Nuf relative to the Aqab and Hadn 

(table 2) that may reflect these distinct environments; dominantly calc-alkaline trends for all the volcanics but particularly for the Aqab and Hadn as indicated by REE plots and 

a discrimination plot utilizing Th, Ta, and Hf (fig. 9); and 

the orogenic character of the mafic volcanic rocks (figs. 

10-11). The latter two plots indicate a possibly significant 

distinction between the volcanic and the gabbroic rocks, 

suggesting enriched type mid-ocean ridge basalt 

(E-MORB) affinity of the latter. In the absence of more 

Discussion 11 
Table 2. Summary of trace-element abundances in the Hadn, Aqab, and Nuf formations and in mafic intrusives of the AI 

'Awshaziyah quadrangle compared to island-arc abundances 

[( ), doubtful value;--, not available; elements except potassium are in parts per million] 

Element/ratio Hadn 

(Average of 13) 

K20 (percent)------- 2.80 

Na20/K20---------- 1.84 

Rb -- -- - - - - - -- - - -- - 52 

Sr----------------- 257 

Ba - - - - - - -- --- -- -- - 978 

K/Rb - - - - - - - - - - - - - - 518 

Rb/Sr- - - - - - - - - - - - - - . 20 

Th ---------------- 6.2 

Zr----------------- 292 

Hf ---------------- 7.2 

Zr/Hf - - - - - - - - - - - - - - 40 

Ta - -- - - --- - - - - --- - . 7 

y - --- - --- -- - - ---- - 37 

Sc----------------- 6.9 

u ----------------- 2.2 

La - -- - --- - - --- -- - - 32 

Yb ---------------- 5.9 

La!Yb ------------- 5.4 

1

Jakes and White, 1971. 

Felsic 

(Average of 4) 

2.72 

2.05 

56 

216 

722 

420 

.26 

6.5 

147 

5.7 

26 

.3 

22 

7.2 

1.9 

28 

3.6 

7.8 

Aqab 

Mafic 

(Average of 4) 

0.92 

3.92 

32 

471 

454 

414 

.07 

2.4 

124 

2.8 

44 

.3 

26 

29.5 

.2 

16 

3.3 

4.8 

data, possible differences in paleotectonic setting cannot be 

evaluated. However, inasmuch as these gabbros very likely 

represent cumulate and not primary magmas, trace-element 

differences such as low La (fig. 11) relative to more 

feldspathic rocks are to be expected. Moreover, the source 

of the gabbro may have been different from that of the mafic 

volcanics. 

Nuf 

Mafic 

(Average of 4) 

0.44 

4.59 

(5) 

158 

(363) 

(.05) 

.7 

(10) 

.8 

(12) 

.3 

4.8 

1.1 

4.4 

Gabbro and 

pyroxenite 

(Average of 4) 

0.33 

6.71 

9 

544 

89 

177 

.02 

.6 

50 

.7 

71 

.3 

13 

(50) 

.5 

6.7 

1.2 

5.6 

Basalt 

Island-arc 

tholeiites 

0.4 

4-6 

5 

200 

75 

1000 

.02 

.5 

70 

1.0 

70 

.15 

1.1 

1.4 

1.0 

Calcalkaline1 

1 

2-3 

10 

340 

115 

340 

.03 

1.1 

100 

2.6 

39 

20 

.2 

9.6 

2.7 

3.5 

presented here do not seem to contradict such a model. In 

summary, field relations and geochemistry of late Proterozoic volcanics in the northernmost Arabian Shield appear to 

reflect on a local scale the sequence of island-arc accretion 

culminating in Pan-African continental collision envisaged 

for the shield as a whole. 

Regarding the volcanic rocks, the following tentative 

conclusions may be drawn from the available data. The Nuf 

and Aqab represent an island-arc assemblage generated 

from oceanic or transitional crust. The Nuf reflects a marine 

environment, and its source was somewhat depleted in 

large-ion lithophile elements while the Aqab was erupted 

from relatively more enriched crust in a nearshore to emergent part of the same arc. The felsic upper parts of the Aqab 

and Nuf formations could reflect a sialic continental source, 

based on their chemistry, although the origin of such crust 

740 Ma B.P. is uncertain. The Hadn formation more probably reflects a continental crustal source. Sr- and Ph-isotope 

data could help to clarify this point. 

APPENDIX: PETROGRAPHIC DESCRIPTIONS 

OF ANALYZED SAMPLES 

Eruption of the Hadn-equivalent Shammar group has 

been generally attributed to renewed igneous activity, beginning about 650 Ma B. P. , and related to collision between 

the Arabian and African cratons (Schmidt and others, 1979; 

Greenwood and others, 1980); or, in the most recent interpretation, collision between the continental Afif terrane with 

western arc terranes along the Nabitah mobile belt after 

about 680 Ma B.P. (Stoeser and Camp, 1985). The data 

12 Geochemistry of the AI 'Awshaziyah Quadrangle 

Numbers keyed to table 1. 

1. Field number 185234; 1 south edge of prominent plateau representing distal part of Hadn formation. Gray-brown porphyritic rhyolite2 consisting of 15 percent tabular, cloudy and 

slightly sericitized 1 to 2 mm plagioclase (An5)

3 phenocrysts, 

locally in glomeroporphyritic aggregates with deep green, 

partly chloritized hornblende. Groundmass consists of 

equigranular aggregate of quartz, slightly cloudy plagioclase, sanidine, and scattered(3 percent) opaque material. 

I All field data for this report, including a sample location map, are 

stored in data file USGS-DF-04-03 in the Jiddah office of the U.S. 

Geological Survey Saudi Arabian mission. A sample location map is also 

available from the author by request. 

2Samples 1 through 7 have compositions generally ranging from 

rhyolite to rhyodacite. 3Based on extinction angles measured on the flat stage, where possible in the zone _1_(010) and (100). 
Convergent 

plate margins 

Primitive 

arc tholeiites 

Hf/3 EXPLANATION 

~:;. Hadn formation 

o Aqab dacite (table 1, analysis 8) 

• Aqab basalt and andesite 

+ Gabbro and pyroxenite 

( + ) Arbitrary value of Ta = 0.2 ppm, 

based on reported values of <0.3 

ppm 

Enriched type midocean ridge basalt; 

tholeiitic withinCalk-alkaline plate basalts 

1:;. • 

(+) 

Th Ta 

Figure 9. Th-Ta-Hf/3 paleotectonic discrimination diagram (Wood, 1980) of volcanic rocks and mafic intrusives from the AI 

Awshaziyah quadrangle. Data for samples from Qufar quadrangle not available. 

2. Field number 185376; Hadn formation, top of 265 m (vertical) exposure of nearly continuous ash-flow tuffs. Darkgray, aphanitic felsic ash-flow tuff consisting dominantly of 

a cryptocrystalline, moderately devitrified groundmass 

showing weakly developed eutaxitic texture. Small angular 

quartz and feldspar crystals, mostly filling vesicles, and 

sparse felsic rock fragments constitute 5 percent. Dusty disseminated opaque material is the only mafic constituent. 

3. Field number 185380; Hadn formation, estimated 200 m 

stratigraphically below preceding sample. Red-brown, moderately sorted (fragments <3 mm) ash-flow tuff consisting of 

angular fragments of quartz plagioclase and various felsic 

volcanic rocks in a matrix of more or less devitrified shards 

and nearly unaltered glass that define a well-developed eutaxitic texture. The clastic constituents range from unaltered 

to moderately altered. The deep red color is probably due to 

disseminated hematite. 

4. Field number 185417; Hadn formation, northern part of AI 

'Awshaziyah eruptive center. Red-brown ash-flow tuff, 

strongly eutaxitic texture. Consists of cryptocrystalline, 

barely devitrified groundmass (coarser crystallization along 

flow planes) and 20 percent fresh to slightly altered plagioclase (ADo-10) and subordinate sanidine. Mafic silicates are 

absent. Sparse dusty disseminated hematite and scattered 

clumps of magnetite. Visible alteration is minimal. 

5. Field number 185076-2; top of prominent plateau of no. 1, 

distal part of Hadn formation. Moderately sorted tuffaceous 

sandstone in a sequence of tuffs, ignimbrites, and volcanic 

Appendix 13 
Orogenic andesite\-__ 

$' ---- ......_-..._ ~ /" _..... ---- ~ / . -- .1\~ ~ / / -- I; / 

"<:': / 

~~~i"' 2 20 

0 

:::::; 

...J 

~ 

a: 

w 

£l. 

en 

1-

a:: 

<t 

£l. 

z 

CQ 

-

High-K 

----- ...J 10 Medium-K 

+ 

---

low-K 

EXPLANATION 

• Aqab -basalt and andesite 

o Nuf andesite 

• Nuf basalt 

+ Gabbro and pyroxenite 

2 3 4 5 6 7 7.5 

Th, IN PARTS PER MILLION 

Figure 10. La/Th diagram for mafic volcanics and intrusives (Gill, 1981 ). 

sediments. Consists of angular to rounded volcanic-rock 

fragments, fresh to moderately altered plagioclase, quartz, 

and subordinate sanidine. Estimated 3 percent secondary 

chlorite and epidote, and 1 percent disseminated magnetite. 

6. Field number 185130; Hadn formation, near center of AI 

'A wshaziyah eruptive center. Red-brown, little altered ashflow tuff with well-developed eutaxitic texture. Consists of 

barely devitrified glass, more sparsely devitrified along flow 

planes and cross fractures. Contains 5 percent subrounded 

rock fragments, plagioclase (An3), and quartz. Strong red 

reflection from disseminated hematite(?). 

7. Field number 185391; Hadn formation, ridge east of eruptive 

center. Orange-brown flow or tuff with a holocrystalline 

matrix consisting mostly of plagioclase laths (little alkali 

feldspar identified) and 7 percent moderately sericitized plagioclase (An5) phenocrysts, interstitial chlorite-epidote aggregates, disseminated amorphous and (or) opaque material, 

and < 1 percent secondary carbonate. 

14 Geochemistry of the AI'Awshaziyah Quadrangle 

8. Field number 185055; Aqab formation, dacite-rhyolite member. Well-sorted, tuffaceous sandstone in sequence of tuffs 

and flows. Consists of plagioclase (ADo-5), quartz, and subordinate sanidine, and rock fragments in a cryptocrystalline, 

partly amorphous matrix. Matrix and rock fragments are 

moderately to strongly sericitized and chloritized. The plagioclase clasts are slightly to moderately altered. About 

2 percent disseminated opaques. 

9. Field number 185254; Aqab formation, basalt-andesite member. Dark lava (basalt) with aphanitic groundmass and 20 

percent tabular plagioclase phenocrysts. Groundmass is 

felted intergrowth of plagioclase laths and green metamorphic hornblende, with subordinate ( <3 percent) pale brown 

metamorphic biotite, evenly disseminated granular opaque 

mineral (magnetite?), scattered rounded quartz aggregates 

representing vesicle fillings, and carbonate filling narrow 

fractures. Relict plagioclase phenocrysts are slightly to moderately saussuritized and exhibit albite twinning and relict 

zoning (An13_30). Rock reflects upper greenschist facies. 
2 4 6 8 

Nb, IN PARTS PER MILLION 

EXPLANATION 

• Aqab basalt and andesite 

+ Gabbro and pyroxenite 

( + ) ( • )Arbitrary value of Nb = 4 ppm, 

based on reported values of <5 ppm 

Figure 11. La/Nb diagram for mafic Aqab volcanics and intrusives (Gill, 1981 ). Data for Nuf samples not available. 

10. Field number 185273; Aqab formation, basalt-andesite member. Dark-gray, porphyritic lava (basalt) consisting of intergranular groundmass of felted acicular plagioclase with interstitial magnetite, epidote and chlorite, and 25 percent 

tabular, slightly to moderately altered, calcic plagioclase 

phenocrysts (relict zoning An58_75) and <2 percent aggregates of granular epidote and biotite. Rock reflects low 

greenschist facies. 

11. Field number 18561 0; Aqab formation, basalt -andesite member. Dark-gray aphanitic nonporphyritic flow (basaltic andesite) consisting of intersertal groundmass (intergrown felted 

plagioclase, magnetite, chlorite and epidote) and crosscutting quartz-carbonate-epidote veins. Relict plagioclase is 

normally zoned An22_30. Rock reflects low greenschist 

facies. 

12. Field number 185029; rhythmically layered gabbropyroxenite intrusion at northern boundary of quadrangle. 

Greenish-black rock with aphanitic groundmass and 40 percent euhedral clinopyroxene megacrysts as much as 5 mm in 

diameter, fringed by actinolitic amphibole. Cryptocrystalline 

groundmass consists of amphibole and plagioclase(?). The 

abundance of pyroxene and lack of other megacrysts suggests incipient cumulate development, but clinopyroxene 

crystals are not in mutual contact. 

13. Field number 185154; larger gabbro-pyroxenite intrusion on 

west side of quadrangle, similar to preceding sample. Rock 

is generally similar to preceding sample both texturally and 

compositionally, except that there is more evident plagioclase in the matrix, and replacement of clinopyroxene by 

actinolitic amphibole is somewhat farther advanced. 

14. Field number 185540; isolated gabbro plug. Light-gray, 

medium-grained gabbro consisting of primary plagioclase 

(An50), olivine, red-brown hornblende (kaersutite?), and redbrown biotite. Secondary minerals include reaction rims of 

bowlingite(?) and fibrous actinolitic amphibole around 

olivine, and pale green chlorite and magnetite replacing 

biotite. 

ACKNOWLEDGMENTS 

The report was produced in accordance with a work 

agreement between the Ministry of Petroleum and Mineral 

Resources, Kingdom of Saudi Arabia, and the United States 

Geological Survey. 

I am indebted to Karl S. Kellogg for making available 

his data on the Qufar quadrangle, and for useful discussions. 

The paper has benefited from reviews by K.J. Schutz, M.E. 

Gettings, D.G. Hadley, and comments by J.E. Quick. 

REFERENCES CITED 

Arth, J. G., 1981, Rare-earth element geochemistry of the islandarc rocks of Rabaul and Talasea, New Britain: Geological 

Society of America Bulletin, v. 92, no. 11, p. 858-863. 

Brown, G.F., Layne, N.M., Goudarzi, G.H., and MacLean, 

W. H., 1963, Geologic map of the northeastern Hijaz quadrangle, Kingdom of Saudi Arabia: U.S. Geological Survey 

Miscellaneous Geologic Investigations Map 1-205-A, 

scale 1:500,000. 

Camp, V .E., 1984, Island arcs and their role in the evolution of the 

western Arabian Shield: Geological Society of America Bulletin, v. 95, no. 8, p. 913-921. 

Carmichael, I.S.E., Turner, F.J., and Verhoogen, J., 1974, Igneous petrology: New York, McGraw-Hill Book Company, 

739 p. 

Chevremont, P., 1982, Geologic and mineral reconnaissance of 

volcanosedimentary and mafic plutonic rocks in the Hf il 

area: Saudi Arabian Deputy Ministry for Mineral Resources 

Open-File Report BRGM-OF-02-39, 33 p. 

Darbyshire, D.P.F., Jackson, N.J., Ramsay, C.R., and Roobol, 

M.J., 1983, Rb-Sr isotope study of latest Proterozoic 

volcano-sedimentary belts in the Central Arabian Shield: 

Journal Geology Society London, v. 140, p. 203-213. 

Del four, J., 1977, Geology of the Nuqrah quadrangle, sheet 25E, 

Kingdom of Saudi Arabia: Saudi Arabian Directorate General 

of Mineral Resources Geologic Map GM-28, 32 p., scale 

1:250,000. 

Dodge, F.C.W., Jr., Fleck, R.J., Hadley, D.G., and Millard, 

H.T., Jr., 1979, Geochemistry and 87Sr/86Srratios ofHalaban 

rocks of the central Arabian Shield: Institute of Applied Geology, Symposium on the evolution and mineralization of the 

Arabian-Nubian Shield, Jiddah, Feb. 1978, Bull. 3, v. 2, 

p. 153-163. 

Duyverman, H.J., Harris, N.B.W., and Hawkesworth, C.J., 

1982, Crustal accretion in the Pan African: Nd and Sr isotopic 

evidence from the Arabian Shield: Earth Planetary Science 

Letters, v. 59, p. 315-326. 

Fleck, R.J., Greenwood, W.R., Hadley, D.G., Anderson, R.E., 

and Schmidt, D. L. , 1980, Rubidium-strontium geochronology and plate-tectonic evolution of the southern part of the 

Arabian Shield: U.S. Geological Survey Professional 

Paper 1131, 38 p. 

References Cited 1 5 
Gill, J. B. , 1981 , Orogenic andesites and plate tectonics, Minerals 

and Rocks 16: Springer-Verlag, 390 p. 

Greenwood, W.R., Anderson, R.E., Fleck, R.J., and Roberts, 

R.J., 1980, Precambrian geologic history and plate tectonic 

evolution of the Arabian shield: Saudi Arabian Directorate 

General of Mineral Resources, Mineral Resources Bulletin 24, 35 p. 

Greenwood, W.R., Stoeser, D.B., Fleck, R.J., and Stacey, J.S., 

1982, Late Proterozoic island-arc complexes and tectonic 

belts in the southern part of the Arabian Shield, Kingdom of 

Saudi Arabia: U.S. Geological Survey Open-File Report 83-

0296, 49 p. 

Hadley, D.G., 1976, Geology of the Bi'r Juqjuq quadrangle, 

Sheet 21143D, Kingdom of Saudi Arabia: Saudi Arabian Directorate General of Mineral Resources, Geologic Map GM26, scale 1:100,000. 

Hanson, G.N., 1978, The application of trace elements to the 

petrogenesis of igneous rocks of granitic composition: Earth 

and Planetary Science Letters, v. 38, p. 26-43. 

Hart, S.R., Brooks, C., Krough, T.E., Davis, G.L., and Nava, 

D., 1970, Ancient and modem volcanic rocks; a traceelement model: Earth and Planetary Science Letters, v. 10, 

no. 1, p. 17-28. 

Irvine, T.N., and Baragar, W.R.A., 1971, Aguidetothechemical 

classification of the common volcanic rocks: Canadian Journal of Earth Science, v. 8, no. 5, p. 523-548. 

Jackaman, Barry, 1972, Genetic and environmental factors controlling the formation of the massive sulfide deposits of Wadi 

Bidah and Wadi Wassat: Saudi Arabian Directorate General 

of Mineral Resources Technical Record TR-1972-1, 243 p. 

Jakes, P., and Gill, J.B., 1970, Rare earth elements and the island 

arc tholeiitic series: Earth and Planetary Science Letters, v. 9, 

no. 1, p. 17-28. 

Jakes, P., and White, A.J.R., 1971, Composition of island arcs 

and continental growth: Earth and Planetary Science Letters, 

V. 12, p. 224-230. 

Kellogg, K.S., 1983, Reconnaissance geology of the Qufar quadrangle, sheet 27/41D, Kingdom of Saudi Arabia: U.S. Geological Survey Open-File Report 84-0159, 39 p., scale 

1:100,000. 

---1984, Reconnaissance geology of the Rak quadrangle, 

sheet 27/42C, Kingdom of Saudi Arabia: U.S. Geological 

Survey Open-File Report 84-0374, 39 p., scale 1:100,000. 

Kellogg, K.S., and Stoeser, D.B., 1983, Reconnaissance geology 

of the Hf il quadrangle, sheet 27/41B, unpub. data on file at 

the U.S. Geological Survey Mission, Jiddah, Saudi Arabia, 

scale 1:100,000. 

Kemp, J., Pellaton, C., and Calvez, J.-Y., 1982, Cycles in the 

chelogenic evolution of the Precambrian shield in part of 

northwestern Saudi Arabia: Saudi Arabian Deputy Ministry 

for Mineral Resources Professional Paper PP-1, p. 27-42. 

Leo, G.W., 1984, Reconnaissance geology of the AI 'Awshaziyah 

quadrangle, sheet 26/41B, Kingdom of Saudi Arabia: U.S. 

Geological Survey Open-File Report 84-0497, 29 p., scale 

1:100,000. 

Leo, G.W., and Kellogg, K.S., 1984, Proterozoic magmatic-arc 

sequences in the northern Arabian shield (abs.): EOS (Transactions, American Geophysical Union), v. 65, no. 45, 

p. 1130. 

16 Geochemistry of the AI 'Awshaziyah Quadrangle 

Miyashiro, A., 1974, Volcanic rock series in island arcs and active 

continental margins: American Journal of Science, v. 274, 

p. 321-355. 

Quick, J.E., 1983, Reconnaissance geology of the Ghazzalah 

quadrangle, sheet 26/41A, Kingdom of Saudi Arabia: U.S. 

Geological Survey Open-File Report 83-0331, 48 p. 

Quick, J .E., and Doebrich, J .L., 1986, Geology of the Wadi Ash 

Shu'bah quadrangle, sheet 26E, Kingdom of Saudi Arabia: 

Open-File Report USGS-OF-04-11, 41 p. scale 1:250,000. 

Roobol, M.J., Ramsay, C.R., Jackson, N.J., and Darbyshire, 

D.P. F. , 1983, Late Proterozoic lavas of the Central Arabian 

Shield-evolution of an ancient volcanic arc system: Journal 

Geology Society London, v. 140, p. -185-202. 

Schmidt, D.L., Hadley, D.G., Greenwood, W.R., Gonzalez, 

Louis, Coleman, R.G., and Brown, G.F., 1973, Stratigraphy 

and tectonism of the southern part of the Precambrian shield 

of Saudi Arabia: Saudi Arabian Directorate General of Mineral Resources Bulletin 8, 13 p. 

Schmidt, D.L., and Brown, G.F., 1984, Major-element chemical 

evolution of the Late Proterozoic shield of Saudi Arabia: 

Proc. first symposium of the Pan-African crustal evolution in 

the Arabian-Nubian shield, IGCP no. 164, King Abdul-Aziz 

Univ., Jiddah, no. 6, p. 1-21. 

Schmidt, D.L., Hadley, D.G., and Stoeser, D.B., 1979, Late 

Proterozoic crustal history of the Arabian shield, southern 

Najd province, Kingdom of Saudi Arabia: Institute of Applied 

Geology Bulletin No. 3, Evolution and mineralization of the 

Arabian-Nubian shield, v. 2, p. 41-58. 

Stacey, J. S., and Stoeser, D. B., 1983, Distribution of oceanic and 

continental leads in the Arabian-Nubian shield: Contributions 

to Mineralogy and Petrology, v. 84, p. 91-105. 

Stoeser, D.B., Stacey, J.S., Greenwood, W.R., and Fisher, L.B., 

1984, U/Pb zircon geochronology of the southern part of the 

Nabitah mobile belt and Pan-African continental collision in 

the Saudi Arabian shield: Saudi Arabian Deputy Ministry for 

Mineral Resources, Technical Record USGS-TR-04-5, 

88 p. 

Stoeser, D.B., and Camp, V.E., 1985, Pan-African microplate 

accretion of the Arabian shield: Geological Society of America Bulletin, v. 96, p. 817-826. 

Stuckless, J.S., and VanTrump, G., 1979, A revised version 

Graphic Normative Analysis Program (GNAP) with examples 

of petrologic problem solving: U.S. Geological Survey OpenFile Report 79-1237, 115 p. 

Stuckless, J.S., Knight, R.J., VanTrump, G., Jr., and Budahn, 

J.R., 1982, Trace-element geochemistry of postorogenic 

granites from the northeastern Arabian shield, Kingdom of 

Saudi Arabia: U.S. Geological Survey Open-File Report 83-

0287, 37 p. 

Stuckless, J.S., VanTrump, G., Jr., Bunker, C.M., and Bush, 

C. A., in press, Preliminary report on the geochemistry and 

uranium favorability of the postorogenic granites of the northeastern Arabian shield, Kingdom of Saudi Arabia: PanAfrican Crustal Evolution in northeast Africa, IGCP Project 

164. 

Wood, D.A., 1980, The application of the Th-Hf-Ta diagram to 

problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the 

British Tertiary volcanic province: Earth and Planetary Science Letters, v. 50, p. 11-30. 

( 

r 

! 

I 

...