Specialized felsic plutonic rocks of the Arabian Shield and their precursors

Journal of African Earth Sciences, Vol. 4. pp. 153-16;4, 19;46 Printed in Great Britain

0731-7247/;46 $3.00 + 0.00 Pergamon Press Ltd.

Specialized felsic plutonic rocks of the Arabian Shield and their precursors COLIN R.

RAMSAY*

Saudi Arabian Directorate General of Mineral Resources, P.O. Box 345, Jiddah, Saudi Arabia Abstract--Metallogenetically specialized felsic plutons are widespread in the Arabian Shield. but are concentrated in several belts and clusters. They are discordant bodies of unfoliated, cquigranular to porphyritic, alkali granite, alkali-feldspar granite, monzogranite or (quartz) sycnite. A computerized data-base containing 159 major-oxide and trace-element analyses representing these rocks has been compiled and statistically analysed. Four types of specialization can be distinguished: ( 1) rocks of agpaitic type are mostly alkali granite. Relative to other specialized types, they have high REE, Nb, Y, Zn and Zr. but low Ba, Li, Pb and Sr. Their precursors were plutons of the alkali granite association, and they are related to mineral deposits containing Nb, Th, Zr, R E E and other granitophile elements; (2) rocks of phtmasitic type are the most numerous, and are generally leucocratic alkali-feldspar granites with traces of muscovite, biotite, fluorite, or rarely sodic amphibole or topaz. They are characterized by high Be. Li, Nb, Pb. Rb, Sn, Ta, Th, U and Y, but low Ba, Ce, Cu, La, Ni, Sr, V and Zr. Their precursors were plutons of the alkali-feldspar granite association and they are associated with S n - T a - N b - - W - F mineralization; (3) rocks of the calcic type are inconspicuous biotitic monzogranites to granodiorites characterized by high Ba, Pb. Sr and V, but relatively low Be, Li, Nb, Rb, Sn, U, Y, Zn and Zr. Their precursors were plutons of the monzogranite association and they are associated with M o - B i - W mineralization; (4) (quartz) syenites and (quartz) monzonites of the miaskitic type are the least common. They are characterized by high Ba, Co, La, Nd, Sc, Sr, V and Zr, and low Li. Nb, Pb, Rb, Sn. U and Y. Their precursors were plutons of the syenite association and they are associated with mineralization of several different types. There appears to be no single parameter which reliably distinguishes specialized rocks as a whole from unspecialized rocks, but the following granitophilc-element concentrations suggest specialized character: F > 1000, Be > 7, La > 70. Li > 40, Nb > 70. Rb > 200, Y > 60, Ba < 200, and Sr < 80 ppm. Ratios of selected elements (notably Rb/Sr > 18, Y/Ba > 4, Rb/Ba > 6, Ba/Be > 10 or K/Rb < 200) may also be indicative. Specialized varieties of an established association can be more readily distinguished by a number of trace-element parameters. Specialized rocks of unknown type can best be classed by evaluating multivariate classification functions based on either major-oxide or trace-element parameters.

INTRODUCTION

Terminology

METALLOGENETICALLYspecialized felsic plutonic rocks, as defined by Tischendorf (1977), are those having a spatial and genetic association with ore deposits of rare elements such as Be, Cs, F, Li, Mo, Nb, Rb, Sn, Ta and W. They are distinguished from ordinary felsic plutonic rocks by a number of geologic, petrographic and chemical peculiarities, of which the most obvious is commonly enhanced contents of Be, F, Li, Mo, Nb, Pb, Rb, rare-earth elements (REE), Sn, Ta, Th, U, W, Y, Zn and/or Zr. These elements include several commercially important metals. As residual magmatic fluids may have been available to concentrate these, the plutons have obvious potential for developing ore deposits. An understanding of the different types of specialized rocks and their precursors would therefore provide useful information on the nature of rare-metal deposits and the environments which are favorable exploration targets. Specialized plutons are numerous and widespread in the Arabian Shield (Fig. 1); several have been described in detail, and four different types have been reported. However, neither the distribution of, nor the variety shown by these rocks has been studied, and criteria for classifying them are rudimentary. * Present address: c/o Meares and Phillips, 33-35 Bligh Street, Box 4675, G . P . O . , Sydney, NSW, Australia.

The terminology used herein is outlined by Ramsay et al. (1986a, this volume). In addition, four chemical terms are used to designate different specialization types. 'Calcic' is used to designate a type comparatively rich in Ca. 'Agpaitic' is applied to a type which commonly has an agpaitic coefficient (Na + K/AI) > 1, and is clearly related to the 'agpaitic evolution series' (Tischendorf 1977). 'Plumasitic' is applied, following Tischendorf (1977), Ramsay et al. (1983 and in press a) and Elliott (1983), to a type characterized by Li-rich micas and a metaluminous to peraluminous composition. 'Miaskitic' is applied to specialized syenitoid rocks. The sense is similar to the usage of Heinrich (1980) in that Ti, P, Ba, Sr and Nb are typically enriched, but differs in that it does not include nepheline-bearing rocks. Granitic types are also grouped on the basis of their Ca contents; the agpaitic and plumasitic varieties are designated 'low-Ca' types to contrast them with the calcic type. Specialization processes

Specialized felsic plutonic rocks are end-products of magmatic differentiation, and have essentially magmatic compositions, except for high concentrations of incompatible trace elements (see below). This is the result of

153

154

C.R.

RAMSAY

!CIALIZED FELSIC PLUTONS OF THE SAUDI A R A B I A N SHIELD 29"N SCALE

28"N

28°N

27"N

35 C4}NORTH DOFA It)SOUTH I~OI

N \\

LIBAN I SnMo

i SU~.AYMI (QUNNAWAT)

.':' U

FAWWARAN{4) *NADHIR(2)

--26"N--

I

E]ITAN( 1 | ~ " S

I AMIRAH~

~KHAZZAZ ~J

-36"E

TUKHFAH

Hmnak,vah

I o ....

~ i , IA

iN N

O.-y..y. ~' 37"E d:=

/

25"N--

D JABALAH

.,~,T T.Y.. el

~"

\ [

HJl*t=*n WO)

\

O 38"E

,,p

12)

~AIL ¢SHAH (%

LEGEND GEOGRAPHIC F E A T U R E S

,ih

TOWn

0

,,, A! Tabl

Harrst (lava field}

--21"N

--

Mineral occurrence PLUTONS

L,,:,,I

PbJtOflrmme and r~JmbMof I l m p l s l where elX)llcabi~! (ornan~nt indlcetea specialization type. NNA,NNB.etc.-imr~me~

/

4- - 20"N--

39"E Mli'~¢ ~lot'le oem~aly too IIm~tlfto del~Ct.

T|Khhlh

SPECIALIZATION TYPE (and characteristic elements) A~OSltlCtype E4.Ce.F.La,mJRb,~h~TII.Th,U,Y,Zn,Zr t

Plurnsldtl¢type Be.F.LI,Nb,PO,Rb,~n.TI~ Th.U,Y

O

MlaSkttt¢(lylm;tk~) tyPe Bm,Co,F.La,NCI.Ni.P.8¢.Sr .TkV,Zt

I o._1 40"E

A~=I

Celtic type Ss.F.Pt~,S~,V Unknovm type (imomak~= ebmmYm Incqcated)

...... s) •r--~.-.. ..| ~..'.-..-"

STRUCTURAL FEATURES Major laull

-- 17"N - -

Felslc plutohlc ;ock bell

42"E i

"43"E I

44"E L

Fig. 1. Specialized felsic plutons of the Saudi Arabian Shield (including generally unspecialized plutons known to have specialized facies).

5"E

t

Specialized felsic plutonic rocks of the Arabian Shield and their precursors

155

progressive concentration of volatile constituents (CO2, quartz diorite, diorite and gabbro; (6) a trondhjemite H20, F, CI, etc.) and incompatible elements after the association, consisting of trondhjemite, tonalite and magma was first fused from its protolith. The processes minor gabbro; and (7) a tonalite association, consisting involved include removal of anhydrous phases by crystal of tonalite and quartz diorite. Different types of fractionation, transport of volatile-rich phases upwards mineralization are related to each association (Jackson under gravitational and thermal gradients, and complex- 1986a, this volume). ing of incompatible trace elements with these volatile phases (Hildreth 1981, Jackson 1986c, this volume). The Objectives, approach and data result is to concentrate volatiles and incompatible granitophile elements in the uppermost portion of a The purposes of this review are: (1) to assess the crystallizing pluton, particularly in structural traps such distribution of Arabian specialized felsic plutonic rocks; as cupolas. (2) to determine what types of specialized pluton are Although the residual magmas were undoubtedly represented; (3) to characterize these types and establish volatile-rich relative to their precursors, the specialized criteria for classifying them; and (4) to identify the rocks which crystallized from them are generally not precursors of each specialization type. hydrothermal in character, indicating that most of the A series of regional compilations showing the distribuvolatile-rich phases escaped, and that there was not tion of felsic plutonic rocks in the Shield of Saudi prolonged interaction of magma and ground-water. Loc- Arabia has recently been completed (Ramsay and Drysally hydrated phases (such as greisens) and veins formed dal11984, Jackson and Douch 1984, Jackson and Ramsay by escaping fluids are therefore the most likely to be of 1984, Moore 1984, Stoeser 1984, Stoeser and Elliott commercial interest, but unhydrated magmatic rocks 1984, du Bray 1983b, Jackson in press a, Stoeser et al. may, in some cases, contain sufficient concentrations of 1986, this volume). A number of regional petrological commercially interesting elements to be considered and geochemical studies have also been conducted (Jackson etal. 1984, Stuckless etal. 1982a and b, Jackson 'mineralized' rather than specialized. The precursors of specialized plutons are the rocks 1986c, this volume, Ramsay etal. 1986b, this volume, du which genetically preceded them (Tischendorf 1977, Bray 1983a). These reviews (and personal communicaRamsay et al. 1986a, this volume). A precursor magma, tion with most of their authors) were used initially to though not itself chemically distinctive, clearly had the identify plutons which might be specialized, and as the capacity to generate a residuum with high concentrations main sources of analytical data. About 200 analyses of granitophile elements. The character of a specialized which met the following criteria were compiled into a rock is thus a consequence of the original protolith, the computerized data-base: (1) complete data for majorprecursor magma and the differentiation processes oxides and F, and essentially complete trace-element which affected it. There may, therefore, be one speciali- data were available; (2) F content exceeded 900-1000 zation type for each group of genetically related rocks ppm and/or there was obviously enhanced concentration of one or more other granitophile trace elements, typi(rock associations). cally Be, Nb, Rb, Y or Zr; (3) the samples were essentially plutonic in character (i.e. they showed no signs of Arabian felsic plutonic rocks hydrothermal alteration, greisenization, exceptional Felsic plutonic rocks make up a large proportion of SiO 2 content, extraordinary Na20/K20 ratios, etc.). The data-base was statistically analysed using the the Arabian Shield (e.g. Drysdall et al. 1986, this volume). Their distribution, petrographic variety and origin BMDP program package (Dixon and Brown 1977). The are summarized by Stoeser (1986, this volume) and data were first screened by using histograms (program Jackson (1986c, this volume). Most belong to a small P5D). Rocks with high outlier values for elements such number of rock associations which can be broadly as Nb, Zr, REE, etc., were considered to be 'mineralgrouped into two plutonic assemblages (Jackson et al. ized', and excluded; others with low outlier values were 1984, Jackson 1986c, this volume, Ramsay et al. 1986b, considered to be unspecialized, and also excluded. The this volume). The younger plutonic assemblage, aged distribution of each variable in the screened data-base of 680-570 Ma, includes: (1) an alkali granite association 159 analyses was then re-assessed (Fig. 2). Some plutons had previously been established, in composed of alkali granite and associated alkali-feldspar granite with metaluminous to peralkaline chemistry; (2) detailed studies referenced below, as being of particular an alkali-feldspar granite association composed of alkali- types, and samples from each type were grouped in the feldspar granite and syenogranite, with metaluminous data-base. Stepwise discriminant analyses (program to marginally peraluminous or rarely peralkaline chem- P7M) of the major-oxide and trace-element data using istry; (3) a monzogranite association consisting of the pre-assigned groups were then conducted to class metaluminous monzogranite and granodiorite; and (4) a other samples, and re-assign any which were incorrectly syenite association, composed of alkali-feldspar syenite, classed. By applying this technique iteratively, the great quartz alkali-feldspar syenite, syenite, quartz syenite, majority of samples (about 95%) were classed into monzonite and quartz monzonite. The older assem- geologically sensible categories. Further details of this blage, aged about 680-900 Ma, includes: (5) a granodior- method and its application are given by Ramsay et al. ite association, consisting of granodiorite, tonalite, (1986c, this volume).

156

C . R . RAMSAY Table 1. Average compositions of Arabian specialized and unspecialized felsic plutonic rocks SYENITOID ROCKS

GRANITIC ROCKS

1.

Unspecialized

2.

3.

Specialized

4.

Unspecialized

Specialized

63.56 17.58 2.46 0.74 0.90 1.76 5.63 5.52 0.71 0.74 0.19 0 .O8

64.45 17.16 2.14 1.23 0.87 1.99 5.52 5.10 0.46 0.71 0.15 0.09

ALL SPECIALIZED ROCKS

5.

6.

7.

~,

S

n

Major oxides (weight percent) SiO 2 AI203 Fe203 FeO MgO CaO Na20 K20 H20 TiO 2 P205 MnO

74.10 12.89 1.73 0.72 0.21 0.83 4.12 4.39 0.53 0.25 0.05 0.07

73.63 12.74 1.66 0.94 0.22 0.92 4.16 4.47 0.53 0.26 0.06 0.06

73.907 13.137 1.207 0.763 0.199 0.856 4.256 4.557 0.517 0.206 0.046 0.052

3.293 1.525 0.958 0.505 0.260 0.604 0.588 0.522 0.224 0.201 0.052 0.036

159 159 159 158 159 159 159 159 157 159 159 152

Trace elements (ppm) B Ba Be Bi Ce Co Cu F La Li Mo Nb Nd Ni Pb Rb Sc Sn Sr Ta

Th u V w Y Zn Zr

7 339 3 <5 103 4 II 757 46 23 3 37 43 5 16 125 3
8 293 6 <5 130 4 9 1648 56 90 4 48 56 I0 26 230 4 11 102 <2 20 2 14 <5 63 89 417

<10 800 cal -82 cal0 ca12 747 110 12 <5 cal5 -<5 10 95 12 3 623 ---

10 953 3 <5 134 8 10 969 90 18 3 19 101 16 14 90 9 5 511 <2 13

<9 232 7 <5 117 4 6 2020 50 121 4 49 43

6 326 8 1 106 3 6 1269 36 203 4 42 30 7 28 238 2 16 170 12 15 7 12 4 44 65 262

----

1

46 -18 82 420

39 <5 24 70 531

8 27 285 4 14 96 8 26 7 15 4 73 82 306

0.18 318 0.03 0.09 470

2.97 33 0.31 1.23 133

127 152 149 121 95 130 148 156 158 108 123 156 37 127 135 94 130 156 159 48 67 63 128 70 156 90 158

Ratios Rb/Sr Ba/Be Y/Ba Rb/Ba K/Rb

1.33 113 0.14 0.37 291

2.25 49 0.22 0.78 161

0.15 ca800 0.02 0.12 482

I. Average of mean compositions of granitic rock associations of the Midyan and central Hijaz regions (Ramsay et al. 1986b, this volume, Jackson et al. 1984). 2. Average of mean compositions of the three granitic specialized rock types (Table 3). 3. Average of mean compositions of syenitoids of the Midyan and central Hijaz re/~ions (Ramsay 1982, Jackson and Odel11984). 4--7. This study. ~, mean; s, standard deviation; n, number of values.

Cluster analyses (program P2M) using various combinations of major-oxide or trace-element data, were then conducted to check that all types represented in the data-base had been identified. Having established types and classed samples by these procedures, basic compositional parameters were computed for each specialization type (programs P5D and P7D).

CHEMICAL COMPOSITION

The average composition for all samples in the screened data-base is given in Table 1. Averages are also given for granitic and syenitoid rocks, and compared with averages for unspecialized rocks. The average compositions of specialized and unspecialized rocks differ in

Specialized felsic plutonic rocks of the Arabian Shield and their precursors

SiO2

I

"

•

I 70

,--5--Tm~j--T--F-Ub 72 74 76

"

AI203

,,,?

Fe2031

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~

....

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1.0

MgO

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,

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L

0.2

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0.8

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3.0

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- ....

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100

200

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400

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PERCENT

, 200

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500

-_N

, "-/-'l-~/--F-~,

. . . .

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" ?Lb "L I

~=

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i

100

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~ ........... "[ I I

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-- L

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,.N

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0

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.... 0

....

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.__..'_9(.., ~

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1000

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15

TT-.TI_', N, , 2000 3000 4000

:~-;:.-,-r.

, , ,

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:Nb"N

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•

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(3,) 78

°-T--,--7~-T--r--. °Nb, , 12

I -_ ~ :Nb

,Nb 11) • Ub (21

Z

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157

200 PARTS

400

7

Ub

e00

# BOO

1000

PER M I L L I O N

Fig. 2. Typical ranges of major-oxide and trace-element values in ( 1) unspecialized rocks of the Midyan region (Ramsay et al. 1986b, this volume, Ramsay et al. in press b); (2) unspecialized rocks of the central Hijaz region (Jackson e t a / . 1984, Jackson and Odell 1984); and (3) specialized felsic plutonic rocks of the Arabian Shield (this study). Bars show the arithmetic mean and the range containing approximately 70% of the values. N, normal distribution (bar represents mean + 1 standard deviation); L, log-normal distribution (bar represents 1st to 70th percentile); U, indeterminate distribution (bar represents 16th to 85th percentile); b, population may be bimodal.

several important respects though, as pointed out by Tischendorf (1977), their major-oxide compositions are only subtly different. For granitic rocks, the average SiOz content of specialized varieties is slightly lower than that of unspecialized rocks, but there is no significant difference in A1203, Hz O, Na20 or K20 contents. Trace-element characteristics are more distinctive, with specialized granitic rocks having higher average Be, Ce, F, La, Li, Nb, Nd, Ni, Pb, Rb, Th, Y, Zn and Zr, and lower Ba. Averages for B, Bi, Cu, Mo, Sn, Ta, U and W

are not significantly higher in specialized rocks, and Sr is much the same in both. Data for syenitoid rocks is less comprehensive, but average specialized and unspecialized rocks differ mainly in the higher FeO, Ba, Be, Ce, F, Ni and Zr of the former. However, arithmetic means and standard deviations are poor measures of typical concentrations because many data distributions are bimodal or log-normal (Fig. 2). A better estimate of typical concentration ranges is indicated in Fig. 2. There is extensive overlap between

158

C . R . RAMSAY • 20

.15 • 10

.5

N '5 10 15 20 1,0

0.0

--1.0

2.0 25 -

20

• 15

-10 -5

N -5 .10 .15 0.0

1.0

2.0

20

3.0

,20

15

major-oxide compositions of specialized and unspecialized rocks. It is therefore impossible to differentiate these reliably on the basis of univariate major-oxide parameters. However, low MgO values (below 1.5%) and TiOa values (below about 0.2%) might indicate a specialized rock. There is less overlap of trace-element distributions, and the following concentrations would suggest specialized character; Be > 7 ppm; F > 1000 ppm; La > 70 ppm; Li > 40 ppm; Nb > 70 ppm; Rb > 200 ppm; Y > 60 ppm; Ba < 200 ppm; Sr < 80 ppm. Nonetheless, these criteria cannot be used individually to characterize specialized rocks because many have quite ordinary concentrations of some elements (Fig. 2). Specialized rocks are commonly reported as having elevated ratios of some trace-elements (e.g. Tischendorf 1977, Plant et al. 1980, Elliott 1983). Figure 3 shows that this is only partly true of Arabian specialized plutonic rocks as a whole. Trace-element ratios in specialized and unspecialized rocks generally have overlapping ranges, though specialized rocks tend to have more extreme values of some ratios. High values of Rb/Sr (> 18), Y/Ba (>4) or Rb/Ba (>6), or low values of Ba/Be (<10) or K/Rb (<200) would therefore suggest specialized character, but many specialized rocks have ordinary trace-element ratios.

10 5

N

DISTRIBUTION

5 10 15 -2.0

--1.O

0.0

1.0

2.0

2O

20 15 10 5

N 5 10 15

-1.0

20

0.0

1.0

2.0

N

o

100

200

300

400

500

600

Fig. 3. Trace-element ratios of specialized and unspecialized felsic plutonic rocks of the Arabian Shield. Data as in Fig. 2; numbers of determinations in brackets; stippling indicates range of values indicative of specialization.

Specialized felsic plutons and plutons with specialized facies occur in most parts of the Shield (Fig. 1), but are clustered. This is due partly to different intensity of study in different areas, and partly to the distribution of Precambrian outcrop, but also reflects several real features. Except for the cluster in the northern Shield, most of the specialized plutons are in linear arrays ('belts'). In the S and E, there are five N-trending belts. Of these, the Bahah and Ablah belts (Jackson in press b), and the Ranyah belt (Ramsay et al. 1979) are related to prominent fault zones and belts of particular country rocks. The Dahul and Halaban belts (Ramsay et al. 1979) are apparently unrelated to fault zones, but may be related to the Hemal batholith. The Dahul belt lies along the western margin of the batholith; the Halaban belt cuts across it. In the central Hijaz, specialized plutons also define a N-trending belt, apparently unrelated to any linear structure. The Midyan belt trends NNW. Several belts are affected by Najd faulting, with progressive, displacements resulting in NNW alignment. The Halaban belt may be different in this respect, as it appears to cut across a major Najd fault zone with little or no displacement. There are specialized plutons in all the accreted terranes of the Shield (Stoeser and Camp in press), except the Ar Rayn terrane. Other notable voids are in the island-arc provinces of the Asir and Jiddah/Umm Lajj regions, and between Halaban and Jabal Sa'id. There is no apparent relationship between specialized plutons

Specialized felsic plutonic rocks of the Arabian Shield and their precursors

159

-15

~o N

MIASKITIC

5 I

I

I

I

1

•

15 I

!

,o N

CALClC

S I

15

~o N

PLUMA=

15

,o N

AGPAITIC

5

1.3

1,2

1.1

1.O O,9

mol

( Na20 4- K20) /

0.8

0.7

0.6

0.5

AI203

1.2

1.1

1.0

mol A I 2 0 3 / ( C a O

0.9

0.8

0.7

+ N a 2 0 + K20)

Fig. 4. Peralkaline and peraluminous character of Arabian specialized felsic plutonic rock types.

and the suture zones defined by Stoeser and Camp, but both the Ranyah and Najran belts are marginal to the Nabitah mobile belt.

TYPES Only a few of the numerous chemically extraordinary plutons reported (e.g. by Kanaan 1979, Elliott 1980 and 1983, Ramsay 1982, 1983a and b, du Bray etal. 1982, du Bray 1986a, this volume, Jackson et al. in press, Jackson and Odell 1984, Stuckless et al. 1982a and b) have been studied in sufficient detail to establish their nature, but four types have been recognized. Three are granitic (the agpaitic, plumasitic and caicic types) and one is syenitoid (the miaskitic type). The characteristics of these types are summarized in Tables 2 and 3.

Agpaitic specialized granites

Specialized granites of agpaitic type are well known in the Arabian Shield (e.g. Harris and Marriner 1979, Drysdall 1979 and 1980, Stoeser and Elliott 1980, Turkistany and Ramsay 1982, Elliott 1983). They are wide-

spread (Fig. 1), but less numerous than those of the plumasitic type. Characteristics. These rocks are generally alkali granites and microgranites containing sodic pyriboles, and occur typically as the specialized facies of large posttectonic alkali granite plutons (Tables 2 and 3). Chemical compositions are distinctive, with low contents of CaO and MgO, high FeOt, and sub-equal amounts of Na20 and K20 (Table 3). Relative to other specialized granitic rocks, the agpaitic type has: high FeOt, Ce, Cu, La, Nb, Nd, Ni, Y, Zn, Zr, Rb/Sr; and low A1203, MgO, CaO, Pb, Sr. If no modal or major-oxide information is available, this trace-element signature can be confused with that of miaskitic specialized syenitoids, as both types tend to have the same enhanced contents of La, Nd and Zr. They can be distinguished, however, by the higher Ba, Co, Sc, Sr and V, and lower Nb, Rb and Y of the miaskitic type. These rocks are commonly peralkaline, or more rarely metaluminous (Fig. 4). Though they have previously been designated the peralkaline group (e.g. EIliott 1983), this can be misleading as some plumasitic varieties are also peraikaline. Agpaitic specialized granites are probably A-type rocks (e.g. Collins et al. 1982, Jackson 1986c, this volume).

160

C . R . RAMSAY

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162

Table 3. Average compositions of Arabian specialized granite types and their precursors LOW-CALCIUM GRANITE

Alkali sranite assoc.

Agpaitic specialized

Precursor

HIGH-CALCIUM GRANITE

Alkalilfeldspar granite assoc.

Moozogranite

assoc.

Plumasitlc specialized

Precursor

Calcic specialized

Precursor

75.41 12.82 0.77 0.59 0.09 0.62 4.19 4.51 0.I0 0.04 0.02 0.52

75.54 12.84 1.13 0.38 0.II 0.53 4.20 4.52 0.16 0.06 0.03 0.47

71.29 14.29 1.36 1.01 0.50 1.65 4.04 4.29 0.36 0.05 0.11 0.62

72.94 14.02 1.21 0.74 0.42 1.39 4.14 4.06 0.30 0.06 0.09 0.58

Ma~or oxides(weight percent) SiO 2 AI203 Fe203 FeO MgO CaO Na20 K20 TiO 2 MnO P205 H20

74.18 ii.I0 2.86 1.21 0.08 0.50 4.25 4.61 0.32 0.09 0.04 0.44

Trace elements B

Ba Be Bi Ce Co Cu F La Li Mo Nb Nd Ni

Pb Rb Sc Sn Sr Ta Th o V

9

8

265 5 -181 4 i0 844 77 21 5 60 88

13

8

14 169 3 I0 20 I 14 4 I0

15 Ii0 3 <12 38 <2 15 3 I0 -84 120 747

<5

Y

91 137 795

Zr

(ppm)

99 6 <5 182 4 13 1572 71 51 5 62 91

W Zn

73.81 11.81 2.85 1.05 0.09 0.56 4.20 4.59 0.30 0.09 0.05 0.54

10 79 9 <5 95 3 5 2343 41 184 4 56 36 6 32 386 4 16

4 16 151 3 <12

38 9 31 8 9 <5 62

47 <2 20 4 13 -39

83 211

60 195

8

182 3 -75 4 7 773 35 18 2 31 17

6 700 4 <5 112 4 9 1029 46 36 3 26 40 12 31 135 4 8 248

6 571 2 -54 4 15 653 28 30 2 21

I

<2

15 2 22

10 2 21

35 48 245

21 42 169

23 4 18

113 4 <6 196

Equivalent data for syenitoid rocks are given in Table 1. Values for precursors are the average of mean compositions for rock associations in the Midyan and central Hijaz regions (Ramsay et al. 1986b, this volume, Jackson etal. 1984).

Mineralization. Specialized agpaitic granites are associated with substantial polymetallic N b - Z r - R E E deposits, particularly in the Hijaz region (Drysdail et al. 1984, Jackson 1986a, this volume; Fig. 1). Large tonnages with disseminated Nb, Ta, Sn, Y, Th, U, Zr and REE occur in alkali microgranite stocks at Ghurayyah (Lalande 1977) and Umm al Birak (Jackson et al. 1983), in layered aplite/pegmatite at Jabal Sa'id (Hackett 1986, this volume) and in microgranite-microsyenite at Jabal Tawlah (Drysdall and Douch 1986, this volume). Plumasitic specialized granites

The first Arabian example of a specialized granite

type enriched in Be, F, Li, Nb, Pb, Rb and Sn, and commonly peraluminous, was recognized by Elliott (1980) at Jabal al Gaharra. Detailed work at Wadi Ratamah (Ramsay et al. 1982 and in press a), Huqban (du Bray and Ramsay 1985), and elsewhere (Eiliott 1983, du Bray et al. 1982, Jackson and Odell 1984) has shown that this is a widespread type with a plumasitic chemical signature. It has recently been established that such plutons are genetically related to intrusions of the alkali-feldspar granite association (Ramsay et al. 1982 and in press a, Jackson and Odell 1984). Characteristics. This is the most common type of specialized felsic plutonic rock in the Arabian Shield

Specialized felsic plutonic rocks of the Arabian Shield and their precursors (Fig. 1). It generally occurs as small, post-tectonic plutons or specialized facies of batholiths, and is typically leucocratic alkali-feldspar granite containing only a few percent of biotite and Li-rich white mica in addition to essential quartz and perthite, albite and/or microcline (Tables 2 and 3). Some varieties are micaceous, and many plutons have greisenized facies. Rare plumasitic rocks are alkali granites, as sodic pyriboles occur in minor, highly differentiated phases as at Ratamah (Douch 1986, this volume). The major-oxide composition is subtly different from that of the agpaitic type. Both types have the same very low MgO and CaO and sub-equal Na20 and K20 contents, but plumasitic rocks commonly have higher SiOz contents (in the 75-77% range) and substantially less FeOt (Table 3). Plumasitic rocks differ from those of the calcic type in their substantially lower CaO contents. The geochemical signature, relative to other specialized granite types is: high SIO2, Be, F, Li, Pb, Rb, Sn, Ta, Th, U, Rb/Sr; and low FeOt, MgO, CaO, TiO2, P2Os, Ba, Ce, Cu, La, Ni, V, Zr, K/Rb, Mg/Li. This type has previously been designated as the peraluminous group to contrast it with the generally peralkaline agpaitic type (e.g. Elliott 1983). It is therefore important to note that plumasitic rocks are not peraluminous S-type specialized granites like those described in other areas (e.g. Chappell and White 1974, Beckinsale 1979). Most plumasitic rocks are almost or slightly peraluminous (Fig. 4), with molecular A1203/ (CaO + Na20 + K2O) ratios mostly between 0.9 and 1.1. They also differ from S-type rocks in their sub-equal contents of Na20 and K20, high SIO2/A1203 ratio, low B, high Y and moderate Rb. They are probably A-type granites (Collins et al. 1982, Jackson 1986c, this volume). Mineralization. Plumasitic specialized rocks are associated with the most important examples of Sn-WTa-Nb mineralization in the Arabian Shield. Accessory cassiterite occurs in a number of plutons in the southeastern Shield (Elliott 1980, du Bray 1986a, this volume); Sn-rich quartz veins are related to the Rawa South (Ghanah) pluton (Ramsay 1982, Douch 1983); pegmatites and alteration zones rich in Pb, Nb, Th, U and W are associated with the Huqban and Ratamah granites (du Bray and Ramsay 1985, Douch 1986, this volume); and there are substantial Sn-bearing greisens at Jabal Silsilah (du Bray 1986b, this volume) and Jabal Akash (Kellogg and Smith 1986, this volume). Disseminated Nb-Sn-Ta mineralization occurs at Jabal Umm al Suqian and Jabal al Ghorah (Fig. 1) (Bokhari etal. 1986, this volume), and Mo-Sn veins at Jabal Jabalat (A1Tayyar et al. 1986, this volume). Calcic specialized granites

Mineralization is associated with relatively calcic monzogranitic plutons at several localities (Fig. 1). These are not strongly specialized in the chemical sense, but constitute a specialized type because they have a distinct trace-element signature (Table 3) and are associated with mineralization.

163

Characteristics. Calcic specialized granite plutons appear to be less common than those of Ca-poor types (Fig. 1), possibly because they are less conspicuous, having lower radioactivity and more ordinary geochemistry. They generally consist of biotitic monzogranite to granodiorite and occur as moderate-sized, late-syntectonic to post-tectonic plutons (Tables 2 and 3). Rocks of this type have an average CaO content of 1.65% (Table 3). They also contain more MgO and less SiO2 than other specialized granite types, but have the same sub-equal amounts of Na20 and K20. They are metaluminous to marginally peraluminous (Fig. 4). The main geochemical features, compared to other specialized granite types, are: high A1203, CaO, MgO, P205, Ba, Pb, Sr, V; and low Be, F, Li, Nb, Rb, Sn, U, Y, Zn, Rb/Sr. Mineralization. Calcic specialized granites are associated with mineralization of the Mo-Bi-W type, such as the vein systems at Jabal Thaaban (Dodge 1979), Wadi Sidarah (Jackson 1986b, this volume) and probably Bir Tawilah (Sabir and Labbe 1986, this volume). Miaskitic specialized syenitoids

Specialized syenitoid rocks have only recently been recognized in the Arabian Shield, notably at Wadi Mowasse (Fig. 1; Ramsay 1982, Douch 1983), Jabal Abu ad Dud (Jackson and Douch 1986, this volume), and Jabai Martabah (Douch et al. 1986, this volume). The analytical data-base for these rocks is still small, and only a preliminary characterization is possible. In particular, the distinction of specialized and unspecialized rocks is uncertain, as the latter intrinsically have extraordinary compositions. Further, there is some evidence that highCa and low-Ca types might be present. Characteristics. Specialized miaskitic syenitoids are the least common of the specialized felsic plutonic rocks (Fig. 1). They are typically massive, post-tectonic (quartz) syenite and (quartz) alkali-feldspar syenite, occurring as small to moderate sized homogeneous plutons and complexes (Table 2). In contrast with the nepheline syenite at Jabal Sawda (Liddicoat et al. 1986, this volume), they lack feldspathoids. The composition is entirely different from that of the specialized granitic rocks (Table 3). The miaskitic rocks are generally metaluminous (Fig. 4) and saturated to slightly over-saturated with respect to silica. The main geochemical features are: high A1203, MgO, CaO, Na20, K20, TiO2, P205, Ba, Co, La, Nd, Sc, Sr, V, Zr, K/Rb, Mg/Li; and low SIO2, Li, Nb, Pb, Rb, Sn, U, Y, Rb/Sr. In some respects, notably the high Zr and La, this signature is similar to that of the agpaitic type. Mineralization. A variety of important mineral deposits are related to miaskitic plutons, including disseminated N b - Z r - R E E - Y - U - T h in a silexite at Jabal Hamra (Drysdali et al. 1984, Jackson 1986a, this volume,

164

C . R . RAMSAY

Jackson and Douch 1986, this volume), and fluorite with Cu-Pb-Zn-Ag-Bi-Mo in a breccia pipe at Ablah (Jackson 1986a, this volume).

/A

FeOt

Other types

Several dendrograms, based on various combinations of major-oxide and trace-element data, were generated to determine whether other, unrecognized types are represented in the data-base. Although clustering was commonly imperfect, all distinct clusters could be correlated with one of the four established types. Considering also the large proportion of samples which can be confidently assigned to these types (above), it is concluded that there are no unrecognized types in the data-base. In particular, there appear to be no S-type rocks, presumably as a consequence of the scarcity of pelites. A number of muscovitic specialized granites near Bishah (Fig. 1; Ramsay 1983a) were initially thought to constitute a separate type and perhaps be of S-type. However, these proved inseparable from plumasitic rocks in all the discriminant and cluster analyses.

P

°

'

•

Na20+K20

MgO K20

Na20 PRECURSORS

CaO FeOt

Although specialized rocks have higher granitophile trace-element contents than their precursors, majoroxide and modal compositions are only subtly different (Tables 2 and 3; Figs. 5 and 6). These parameters therefore serve to identify each specialized type with its precursor, as follows: Specialized type Agpaitic Plumasitic Calcic Miaskitic

Precursor Alkali granite association Alkali-feldspar granite association Monzogranite association Syenite association

Roof-zones, contacts and particularly cupolas of plutons belonging to these precursor associations are favorable areas for mineralization. The metals most likely to be found are indicated by the nature of the relevant specialized type, which can be determined in several ways.

CLASSING SPECIALIZED ROCKS

A

CaO+MgO

A120a

Fig. 5. Average composition of Arabian specialized granite types compared to fieldsof felsicplutonic rock associations(Jackson et al. 1984,Jacksonand Odel11984, Ramsayet al. 1986b,thisvolume).A, P and C are average compositionsof agpaitic, plumasitic and calcic specialized granite types;Ag, Af, Mg, Gd and Tr are the fieldsof the alkali granite, alkali-feldspargranite, monzogranite,granodioriteand trondhjemite associationsof the central Hijaz and Midyan regions combined.

alkali-feldspar granites by using SiO 2 and Rb. Further work is necessary to refine these functions and derive Recognizing specialized varieties of a n association comparable ones for the calcic and miaskitic types, but the necessary data-bases do not yet exist. Until approIt can be difficult to recognize specialized rocks, if priate furjctions are derived, differentiation of specialthey are treated as a single population, because of their ized and unspecialized varieties of an association must compositional overlap with unspecialized rocks (above). therefore be based on diagnostic univariate criteria and In contrast, there are generally clear differences ratios (Table 3). between specialized and unspecialized varieties of any one rock association. Jackson and Odell (1984) have Alkali granite association. The average major-oxide shown that, in the central Hijaz region, these differences composition of agpaitic specialized rocks is almost idencan be quantified using discriminant analysis. Special- tical to that of unspecialized rocks of the alkali granite ized alkali granites were differentiated from unspecial- association (Table 3). The trace-element compositions, ized equivalents using linear classification functions however, are markedly different. Diagnostic features of incorporating A1203, P205, Nb, Rb and Y; specialized specialized rocks are: higher F (>1000), Li (>35), Rb

Specialized felsic plutonic rocks of the Arabian Shield and their precursors AMALGAMATED

,;

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165

DISTANCE

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I AGPAITIC SPECIALIZED

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ALKALI GRANITE ASSOCIATION GRANODIORITE ASSOCIATION (Midyan) MONZOGRANITE ASSOCIATION (Central Hijaz ) CALCIC SPECIALIZED

GRANITE

MONZOGRANITE ASSOCIATION (Midyan) ALKALI- FELDSPAR GRANITE ASSOCIATION PLUMASITIC SPECIALIZED

GRANITE

TRONDHJEMITE ASSOCIATION ( Midyan ) GRANODIORITE ASSOCIATION (Central Hqaz) TRONDHJEMITE ASSOCIATION (Central Hijaz ) SYENITE ASSOCIATION MIASKITIC SPECIALIZED

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Fig. 6. Relative similarity of Arabian specialized felsic plutonic rock types and rock associations, each represented by its average composition. The dendrogram is based on standardized, unweighted values of SiO2. A1:O3. Fe,O3, FeO. MgO. CaO, Na20. K20. TiOa and MnO. and uses Euclidean distance as the measure of similarity.

(>140) (and possibly Cu, Ni, Zn and Zr); and lower Ba (<180), Sr (<35). (The numbers in parentheses are suggested cut-off values in ppm.)

are very similar (Table 3), but specialized rocks appear to have: higher Ba, Be, Ce, F, Ni, Y and Zr; and lower Sr.

Classing samples of unknown type Alkali-feldspar granite association. Plumasitic specialized granites have an average major-oxide composition distinguishable from that of unspecialized rocks of the alkali-feldspar granite association only by higher FeO/ Fe203 ratios and possibly lower TiO2 (Table 3). Traceelement compositions are very different, specialized rocks having the following diagnostic features: higher Be (>5), F (>1000), Li (>30), Nb (>40), Nd (>30), Pb (>30), Rb (>200), Y (>50), Zn (>70) and others; and lower Ba, possibly Sr and V.

Monzogranite association. Calcic specialized granites have an average major-oxide composition similar to that of unspecialized rocks of the monzogranite association, but have slightly lower SiO2, higher FeO, Fe203 and CaO contents, and a higher K20/Na2 O ratio (Table 3). The diagnostic trace-element features of specialized rocks are: higher Ba (>600), Ce (>100), La, Nd, Ni (>9), Pb (>40), Sn (>6), Sr, Th (> 12), Y; and lower Cu. Syenite association. The differences between specialized and unspecialized syenitoid rocks are not yet properly established. The average major-oxide compositions

The two low-Ca types (plumasitic and agpaitic) are, in some cases, superficially similar, but differ in many respects from the caicic type. Miaskitic syenitoids are readily distinguishable. Thus the modal character or geochemical signature (Tables 2 and 3) of an unclassed sample may be sufficient to assign it to a particular specialized type. However, a more reliable procedure is to evaluate and compare the classification functions in Table 4, or evaluate and plot the canonical variables (Fig. 7). Both procedures are described by Ramsay etal. (1986c, this volume). CONCLUSIONS Specialized felsic plutonic rocks are widespread in the Arabian Shield (Fig. 1) and concentrated in belts. They are of four types: agpaitic, plumasitic, calcic and miaskitic. The precursors of these types are respectively the alkali granite, alkali-feldspar granite, monzogranite and syenite associations. No specialized rocks related to the trondhjemite or granodiorite associations have yet been

166

C. R. RAMSAY Table 4. Multiw~riate functions for classing specializcd felsic plutonic rocks (a) Functions based on major oxides

K,,~o = Koh, = Km~k= K~,t =

19.7(A1.~O~) + 22.0(A1_,O3) + 27.2(AI,Oa) + 24.6(AI,O3) +

5.1(MgO) 26.4(MGO) 69. I(MgO) 43.2(MGO)

+ + + +

16.6(Na,O) + 13.5(Na_~O) + 21.6(Na_~O) + 12.8(Na,O) +

25.9(K_~O) 27.6(K~O) 31.0(K_,O) 26.1(K_,O) -

72.7(TIO,) + 91.6(TiO_,) + 65. I(TiO~) + 77.6(TIO_,) +

21.9(FeO,) 15.5(FeOt) 16.6(FeO~ ) 15.7(FEO,)

-

235.0 239.8 406.9 273.3

(b) Functions based on trace elements

V,,~o = Vow,, = Vr,,= = V,;,~ =

0.002(Ba) 0.O02(Ba) 0.031(Ba) 0.034(Ba)

-

0.(IOI(Bc) 0. I I S ( B e ) 0.4(II(Be) O.020(Bc)

+ + + +

0.474(Ni) 0.180(Ni) 0,065(Ni) 0,034(Ni)

+ + + +

0.367(Sc) t).787(Sc) 0.834(Sc) 0.922(Sc)

-

O.007(Sr) (l.O00(Sr) f).O22(Sr) t).t)10(Sr)

+ + + +

l).031(Zr) 0.O07(Zr) 0.022(Zr) 0.01 l ( Z r )

- 17.6{1() - 4.997 - 38.848 - 15.746

agp, agpaitic; plm, plumasitic: cal, calcic: msk. miaskitic. To class a sample• either major-oxide (wt%) or trace-clement data (ppm) may bc used. Four numerical scores (K, V) are calculated by substituting concentration data in the classification functions. The highest score indicates the type to which the sample belongs. For further details, see Ramsay et al. (1986c. this volume).

identified. Cupolas, daughter stocks, roof-zones and contacts of plutons of the granitic associations are favorable sites for N b - Z r - R E E , S n - W - T a - M o - B e and M o W - B i - P b - Z n - A g mineralization, respectively; several different varieties of mineralization are related to the syenitic association. I

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Variability of specialized felsic plutonic rocks as a whole is such that there is no single univariate or bivariate parameter which can reliably differentiate them from unspecialized rocks. Concentrations of Be > 7 ppm, F > 1000, La > 70, Li > 40, Nb > 70, Rb > 200, Y > 60, Ba < 200, Sr < 80, or ratios of Rb/Sr > 18, Y/Ba > 4, Rb/Ba > 6, Ba/Be < 10, or K/Rb < 200 suggest specialized character, but many specialized rocks have ordinary values of these parameters• Recognizing specialized varieties of an association is more feasible, and can be done by comparing diagnostic granitophile trace-element concentrations or selected ratios, though pilot studies suggest that evaluating classification functions derived from multivariate discriminant analysis may eventually prove the most efficient method. A specialized rock of unknown type can, in some cases, be classed simply by considering its trace-element or modal composition; a more effective method is to evaluate the multivariate classification functions or canonical variables given in Table 4 and Fig. 7. Acknowledgements--Many of the ideas presented here evolved in the course of close co-operative work with Norman Jackson (Saudi Arabian Directorate General for Mineral Resources, DGMR), and developed from concepts introduced by Jim Elliott and Doug Stoeser (U.S. Geological Survey Saudi Arabian Mission; USGS) in the mid1970s. Analyses and other information on specialized plutons were generously made available by Robert Agar, Colin Douch, Alan Drysdall, Norman Jackson, Bill Liddicoat, Najeeb Rowaihy, David Woolf (all of DGMR), and Jim Cole, Ed du Bray, Jim Quick, Bill Moore and Doug Stoeser (all of USGS). Mohammed Jamil lbrahim assisted patiently with seemingly endless data-processing. The manuscript was improved after reviews by Norman Jackson and Alan Drysdall.

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-10.5

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CV 1=-0.639AI203 - 5.144MGO - 0 • 6 2 8 N a 2 0 . 0 ~ . 9 0 K 2 0 . 2.501TIO2 + 0.062FeO t ÷ 13.797

Fig. 7. Template for classing samples of specialized felsic plutonic rock using canonical variables based on (A) major-oxide analyses and (B) trace-element data. A query indicates a sample which appears to be miselassed.

AI-Tayyar, J., Jackson, N. J. and AI-Yazidi, S. 1986. Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region, Kingdom of Saudi Arabia. J. Afr. Earth Sci. 4,183-188. Beckinsale, R. D. 1979. Granite magmatism in the tin belt of southeast Asia. In: Origin of Granite Batholiths--Geochemical Evidence (Edited by Atherton, M. P. and Tarney, J.), pp. 34-44. Shiva Publishing, Orpington, U.K. Bokhari, A. M., Jackson, N. J. and AI Oweidi, K. 1986. Geology and mineralization of the Jabal Umm AI Suqian albitized apogranite, southern Najd region, Kingdom of Saudi Arabia. J. Afr. Earth Sci. 4, 189-198.

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