Petrological and Geochemical Constraints on the Evolution of El-Kahfa Alkaline Ring Complex ...

22 Pages·2016·1.16 MB·English

D
Journal of Geological Resource and Engineering 6 (2016) 283-304
doi:10.17265/2328-2193/2016.06.004
DAVID PUBLISHING
Petrological and Geochemical Constraints on the
Evolution of El-Kahfa Alkaline Ring Complex, South
Eastern Desert, Egypt
A. M. Bishady1, A. M. El-Sherif2 and M. E. Darwish2
1. Geol. Dept., Fac. Sci., Menoufiya Univ., Shebin El-Kom, Egypt
2. Nuclear Materials Authority, Cairo, Post Office Box No. 530 Maadi, Egypt

Abstract: El-Kahfa complex locates at the intersection of Lat. 24°8′18″ N and Long. 34°38′55″ E, South Eastern Desert of Egypt. It
comprises an outer ring composed of alkaline syenites, ranging from barren- to quartz-bearing syenites, an inner ring which varies in
composition from leucocratic to melanocratic syenites, and a central stock formed essentially of alkali gabbros. These alkali gabbros
are cut by small bodies of alkaline syenites, which occasionally grade into nepheline syenites. The country rocks are represented mainly
by metavolcanics and epidiorites. El-Kahfa syenites lack any primary hydrous mafic silicates indicating that their magma was
anhydrous. Major and trace elements geochemistry of El-Kahfa rocks reflects the crystallization of early plagioclase followed by Fe-Ti
oxides. Salic members in the complex are co-magmatic and related to crystal liquid fractionation. The REE (rare earth elements)’s
tetrad is obvious on the primitive mantle normalized pattern of El-Kahfa rocks. The M-type tetrads clearly appear in the more evolved
rocks. These tetrad effects are either a feature of magma-fluid system before crystallization or inherited from external fluid during or
after the emplacement of the magma. Generally, El-Kahfa rocks parental magma was probably derived from Nb-enriched or at least
under-plate mantle source within the continental East-Africa rift zone, where it may be subjected to minor contamination through their
fractional crystallization.

Key words: Ring complex, El-Kahfa, tetrad effect, South Eastern Desert.

1. Introduction of development of the ring nature in addition to the
complexity of their structure. El-Kahfa ring complex is
Hunting Geology and Geophysics [1] introduced for
encountered in a variety characterized by well-defined
the first time El-Kahfa as a ring complex. It is located
ring structure, wide range of rock types and the
at the intersection of Lat. 24°8′18″ N and Long.
occurrence of nepheline.
34°38′55″ E, with its peak rising 1,018 m above sea
The genesis of this ring complex had been discussed
level.
in association with the other Egyptian and African ring
Lockwood [2] surveyed a segment of Egypt
complexes by several workers; El-Ramly et al. [3, 4]
including El-Kahfa area magnetically and
suggested two branches of magmatic differentiation.
radiometrically and revealed a radioactive anomaly
The first branch would be the result of an original
that attains an amplitude of 200 cps.
trachy-basalt magma, while the second trend
El-Ramly et al. [3, 4] and El-Ramly and Hussein [5,
supposedly resulted from the fusion of sialic crust
6] gave some details about the geology and
under the influence of the high temperature of the
petrography of El-Kahfa ring complex. They grouped
basaltic magma. They confirmed also that the ring
the Egyptian ring complexes into five groups on the
complexes of the Egyptian Eastern Desert (Fig. 1)
basis of their magmatic differentiation and the degree
represent the northward continuation of the belt of ring

complexes associated with the East African rift system.
Corresponding author: Ahmed Bishady, professor,

research fields: mineralogy and petrology.
284 Petrological and Geochemical Constraints on the Evolution of
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt

Fig. 1 Key map showing the distribution of the main ring complexes in the South Eastern Desert, Egypt (after El-Ramly [7]).

The distribution of these ring complexes follows either the modern Red Sea. They also suggested that the
NW or NE trends, where the largest ring complexes are shallow emplacement and short time span of
those found as the two trends intersect. Garson and Krs emplacement indicate that the differentiation probably
[8] suggested that the distribution of the ring occurred in a magma chamber deep in the crust or
complexes in Egypt is controlled by N60°E crustal upper mantle, rather than in situ.
block faults and shear zones, and N30°W deep-seated Hashad and El-Reedy [10] mentioned that the
tectonic zones related to the opening of the Red Sea. anorogenic alkalic rocks in the South Eastern Desert of
Serencistes et al. [9] mentioned that the immediate Egypt (including El-Kahfa) reflect within-plate
cause of the intrusions of the Egyptian ring complexes tensional environment. Höhndorf et al. [11] on the
may be a heat source in the mantle. This heat may be other hand mentioned that the Egyptian ring complexes
the same, that cause swelling and updoming in the have been intruded through the oceanic crust of the
upper crust, leading to fracturing and rifting. They Nubian Shield.
added that the genesis of the Paleozoic complexes is Vail [12] referred to three groups of complexes in
more likely to be linked to the large-scale faulting Egypt based on their ages. The oldest and more
pattern of Eastern Africa. These complexes appear to saturated massifs 229 ± 5 Ma (El-Gezira) to 221 ± 12
predate any activity connected with the development of Ma (Silaie), the second and more alkaline ring
Petrological and Geochemical Constraints on the Evolution of 285
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt
complexes between 148 ± 12 Ma (Mishbeh) and 132 ± inner ring and a central stock (Figs. 2 and 3a). The ring
10 Ma (Mansouri) and the youngest at 95 ± 5 Ma ridge is not complete and open to the south in the form
(El-Kahfa) to 89 ± 16 Ma (Abu khruq). Vail [12] of a horse shoe ridge. Near its entrance there is a central
through his investigation of the ring complexes and stock which rises for about 180 m above the level of the
related rocks in Africa, concluded that none of these ring wadi.
complexes seem to be directly connected with the El-Kahfa country rocks encounter various rock types.
present African rift systems, although that pattern of The epidiorites surrounded the outer ring. The contact
faulting is itself related to earlier Precambrian between these epidiorite rocks and the syenites
structures. He added that the heterogeneous nature of encloses gray rhyodacite xenoliths embedded in
the ring complex provinces, with the exception of the generally homogenous syenitic matrix (Fig. 3b).
Cretaceous meridional group, does not support the Metavolcanics are fine-grained, hard and compact
view of Razvalyayev and Shakhov [13, 14] that the basic, intermediate and acidic varieties, exhibiting
locations of most of the NE Africa and Arabia were various colours. They are dissected by some trachytic
controlled by submeridional rift faults associated with dykes.
east African rift structures. The outer ring is composed of alkaline syenites,
Abdel Khalek et al. [15, 16] concluded that the inner which gradually vary from leucocratic syenites barren
ring syenites in El-Kahfa ring complex is more alkaline of quartz to quartz enriched syenites. These rocks are
and shows a higher differentiation status than those of associated and invaded by numerous trachytes.
the outer ring. Their study revealed also that the The inner ring seems to possess a complicated
syenodiorites composition locates between the structure and its rocks vary in composition from
examined diorites and syenites of the outer ring, where leucocratic to melanocratic syenite varieties. In
they are formed by the assimilation of the country equigranular alkaline syenites with quartz and a
diorite-masses by the firstly ascending syenite magma. relatively high percent of dark minerals form its
Ragab et al. [17] concluded that the trace elements western and northern parts. The inner contacts are
covered by wadi alluvium, but highly cataclased and
contents in El-Kahfa alkali syenites indicate that they
ferruginated rocks are developed at the very foot of its
were derived from an enriched mantle source in the
slope.
sub-continental lithosphere, where they originated in
The central stock is formed essentially of
an extensional region in a within-plate environment.
alkali-gabbros. Relics of volcanic cones represented by
From the previous work, it is clear that the origin of
alkali basalts and their pyroclastic equivalents are
El-Kahfa syenites and their geological, geochemical
preserved in the eastern part of the stock. The
and tectonic implications are not fully understood.
alkali-gabbros are cut by small bodies of alkaline
In the present article we present comprehensive
syenites, which may occasionally grade into nepheline
petrological and geochemical data, from the different
syenites.
rock unites of El-Kahfa complex in a trial to resolve the
controversy on their petrogenesis. 3. Petrography
2. Geologic Setting 3.1 Country Rocks
The geologic setting of El-Kahfa ring complex is The country rocks of El-Kahfa ring complex are
dictated from the present authors’ investigations and represented by metavolcanics and epidiorites.
El-Ramly et al. [3]. 3.1.1 Metavolcanics
El-Kahfa ring complex comprises an outer ring, an El-Kahfa metavolcanics are represented mainly by
286 Petrological and Geochemical Constraints on the Evolution of
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt

Fig. 2 Geological map of El-Kahfa ring complex, South Eastern Desert, Egypt (modified after El-Ramly et al. [3]).

meta-basalts, meta-andesites and meta-dacites. They acidic varieties.
are fine grained, exhibiting commonly porphyritic and The groundmass is commonly very fine-grained and
glomeroporphyritic textures. In the more basic mostly trachytic (Fig. 4a). The metabasalts
varieties the groundmass varies from microdoloritic to groundmass is formed of devitrified glass and
hyalopilitic groundmass. spheriolitic feldspar aggregates with doloritic and
Plagioclase phenocrysts (mostly andesine ≈ An ) in apodoloritic textures. Amphiboles and biotite
40
the porphyritic varieties are anhedral to subhedral, predominate among the mafic constituents. In the more
zoned, cracked and relatively saussuritized. Porphyritic basic varieties the groundmass is turbid feldspathic
kaolinized K-feldspars can be recorded in the more with fine laths of colorless to greenish clinopyroxene,
Petrological and Geochemical Constraints on the Evolution of 287
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt

Fig. 3 Photograph of: (a) Panoramic view showing the relation between the inner (I), outer (O) rings and the central stock (C)
of El-Kahfa ring complex; (b) Dark gray rhyodacite xenolith embedded in syenitic matrix.

whereas the common amphiboles are greenish Few brown biotite may occur after hornblende.
hornblende, actinolitic amphibole and occasional Subhedral clear quartz grows on the expense of the
kaersutite. In the more acidic varieties the groundmass plagioclases and hornblende. Interstitial clear
is mostly quartzo-feldspathic, with fine grains of orthoclase grains may compose 5% to 10% of the rock
greenish clinopyroxene, amphiboles and uncommon constituents. Opaque Fe-Ti oxides minerals grains
biotite flakes. dispose most of the rock constituents. Zircon and
The accessories all over these rocks are opaque apatite are the common accessories.
Fe-Ti oxides minerals, apatite and sphene.
3.2 Ring Complex Rocks
3.1.2 Epidiorites
El-Kahfa epidiorites (meta-gabbros), show 3.2.1 Ring Volcanics
hypidiomorphic coarse grained texture. They are The extrusives associating El-Kahfa ring complex
constituted mainly of relatively saussuritized zoned varied from porphyritic trachytes, alkali trachytes to
andesine plagioclases (An ). Mafics are mainly of trachy-andesite (latite).
40
yellowish-green to green partially epidotized Trachytes are dense (aphanitic) holocrystalline rocks,
hornblende, that mostly after augitic pyroxene, which which are often porphyritic (Fig. 4c). The phenocrysts
occur as relics crowned by that hornblende (Fig. 4b). are mostly of the sanidine alkali-feldspar, whereas
288 Petrological and Geochemical Constraints on the Evolution of
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt

Fig. 4 Photomicrograph of: (a) Metavolcanic showing very fine-grained doleritic groundmass; (b) Epidiorite thin section
showing hornblende after pyroxene (sheller texture); (c) Phenocryst of zoned plagioclase in a trachytic groundmass in the
porphyritic texture; (d) Trachytic texture in the ring complex trachytes; (e) Aegirine altered to arvedsonite amphibole and
aenigmatite surrounded by perthitic feldspars; (f) Syenodiorites showing hornblende altered to chlorite and scarce carbonates.

oligoclase-andesine feldspars present in the more basic brown flakes are common in its groundmass,
trachy-andesites (latite). which is disseminated by fine opaque Fe-Ti oxides
The groundmass is dominated by homogenous or minerals, whereas accessory apatite grains are also
cryptoperthitic sanidine and orthoclase, which form common.
slender laths or microlites in parallel orientation 3.2.2 Syenites
exhibiting the trachytic texture, or may form more stout El-Kahfa syenites vary petrographically from alkali
laths giving an orthophyric texture (Fig. 4d). syenites, which form the bulk of the ring syenites, to
Aegirine-augite and aegirine, in addition to the alkali quartz bearing syenites and mafic syenites
arfvedsonite and aenigmatite occur in the groundmass (syenodiorites). These varieties are generally
as interstitial needles or as fealty aggregates. hypidiomorphic granular, coarse to medium grained,
Fine interstitial clear quartz grains are uncommon, whereas pegmatitic and porphyritic varieties can be
but cryptocrystalline silica mosaics are noticed e ncountered.
in the trachy-andesites. Fine hornblende grains and (1) Alkali quartz bearing syenites
Petrological and Geochemical Constraints on the Evolution of 289
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt
Subhedral microperthitic orthoclase exhibiting green, Y = grass green, Z = yellowish green), is the
micro-string perthitic blebs, represent the main alkali main recorded pyroxene, whereas aegirine-augite is not
feldspar (modal percent of 85-65%). Antiperthites are uncommon. Acmite with its brownish to greenish
less common. Feldspars are mostly subjected to yellow colors, can hardly been noticed especially in the
kaolinization, but relatively free from micro-inclusions. more alkalic varieties.
The rarity of these micro-inclusions in the feldspars of These alkali pyroxenes are mostly altered to
these hypersolvus alkaline rocks is indicative of the arfvedsonite, which may uncommonly be altered to
very low fluid content in the liquid, and under the riebeckitic.
thermodynamic crystallization conditions of low pressure Aenigmatite represents another main alteration
(less than 2 kbars), and high temperature (≈500 °C) and product after the aegirine and may acmite.
less content of F-bearing fluids [18]. Kataphorite with the pleochroic formula (X = pale
Clear subhedral quartz, which represents about 5% brown, Y = deep brown, Z = greenish brown), is
of the rock constituents, is interstitial or forms crystals recorded in the rarely present nepheline-bearing
in microlitic cavities. syenites.
Aegerine, with a modal percent range from 10% to Scarce nepheline mostly altered to sodalite can be
30%, is strongly pleochroic of deep grass-green to detected associating the perthitized potash feldspar.
brownish-green. Nevertheless aegirine is relatively Apatite and opaque Fe-Ti oxides with the less
fresh, it is generally altered to arfvedsonite, where this common zircon prevail specially in association with
alteration may continue the development of riebeckite the mafic constituents.
(Fig. 4e), thus forming the inseparable (3) Mafic syenites (syenodiorites)
aegirine-arfvedsonite-rebeckite alteration series. Mafic syenites or syenodiorites according to Le
Aenigmatite can also be noticed after aegirine. These Maitre [20] are subsolvus syenites intermediate
Na-pyroboles enclose poiklitically grains of the alkali between syenites and diorites. They are constructed of
feldspars and occasionally quartz grains. orthoclase, plagioclase, augitic pyroxene, scarce
Fine-grained apatite is the main accessory, while hornblende, biotite and accessories.
zircons are also recorded. Subhedral opaque Fe-Ti Microperthitic orthoclase represents about 45% of
oxides minerals are in common association with the the rock constituents. It is mostly kaolinized. Andesine
Na-pyroxenes-Na-amphiboles alteration series. plagioclase (An ) is uncommonly zoned with
30-32
(2) Alkali syenites corroded margins. Greenish augitic-pyroxene, with
Alkali syenites are generally barren of quartz. occasional aegirine-augite are the main mafics, which
Microperthitic to cryptoperthitic, partially kaolinized are maybe altered to greenish brown hornblende and
orthoclase is the main alkali feldspar. These feldspars yellowish-brown biotite. Chlorite and scarce
show string, bleb and occasionally patch perthitization. carbonates represent the main alteration products of
Antiperthites can be recorded protruding perthites, these mafics (Fig. 4f).
exhibiting crenulated composition planes. Scare interstitial secondary clear quartz can be
Secondary fresh, interstitial albite intergrowths grow recorded. Epidote and opaque Fe-Ti oxides minerals
at the peripheries of the alkali feldspars, which may be are the main accessories, in addition to some sphene
formed by the unmixing of the primary high and zircon.
temperature alkali feldspars or by late magmatic Generally, El Kahfa syenites lack primary hydrous
albitization [19]. mafic silicates. This petrographic observation clearly
Aegirine with the pleochroic formula (X = deep indicates that the syenite magma was anhydrous [21].
290 Petrological and Geochemical Constraints on the Evolution of
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt
3.2.3 Alkali Gabbro Madhaven et al. [21] referred that hedenbergite
The nomenclature of this rock variety is after Ref. pyroxene frequently alters (Topotaxial alteration) to an
[22]. In the literature, it was identified as essexite orange-yellow coloured montronite, which is formed
gabbro, which is a miss nomenclature where no by the oxidation of Fe2+ to Fe3+ with the simultaneous
nepheline can be noticed in El-Kahfa alkali gabbro, release of Ca as observed by Eggleton [26] in the
which is essential in essexite gabbro [22]. The rock is Australian skarns:
medium to coarse grained, hypidiomorphic granular. Ca Fe+2 Si O + 2H→ Fe+3Si O (OH)2+ + 2Ca
2 2 4 12 4 10
Plagioclases (An ) compose 45-60% of the rock Barkevikite, with the pleochroic formula (X =
45-55
constituents. They are stumpy, tabular, fractured yellow, Y = yellowish-brown and Z = brown), is
strained, zoned grains, which are generally recorded as subhedral to anhedral grains, which are
saussuritized with occasional carbonitization. Second disseminated with opaques.
phase of plagioclase (An ), more fresh, exhibiting Medium to coarse grained apatite is a ubiquitous
50-60
definite lamellar twinning and zoned can be accessory mineral. It is mostly intermixed with the
encountered. pyroxenes alteration products.
Microperthitic orthoclase, which hardly reach up to
4. Geochemistry
20% of the rock feldspars, are interstitial, partially
kaolinized and enclose poiklitically plagioclases. Total seventeen representative samples from
Mafics representing ≈40% of the rock constituents El-Kahfa ring complex and its country rocks were
are represented mostly by Ca-rich augite (salite), which analyzed for major elements (Table 1) at NMA
shows pale-green to violet tinted varieties. (Nuclear Materials Authority) laboratories, using
Aegirine-augite can occasionally be noticed on the conventional wet chemical techniques of Shapiro and
peripheries of the Ca-rich augites. This is in accordance Brannock [27] with some modifications given by
with what mentioned by Ref. [22] that the initial stage El-Reedy [28]. Ten representative samples of the
of Na-Fe3+ enrichment is represented by violet tinted or syenites and associated trachytes were investigated for
pale green soda augites (Na O > 1.5%; [23-25]) either their trace elements and REE at ACME analytical
2
rimming more diopsidic cores or forming discrete laboratories, Vancouver, Canada, using the ICP-Ms
crystals which may be rimmed by aegirine-augite or techniques (Table 2).
aegirine.
4.1 Major and Trace Elements
These pyroxenes are uncommonly mantled by
uralitic green amphibole, with bluish tint, and partially The distribution of the major oxides and the trace
disposed with opaques. This amphibole is associated elements of El-Kahfa ring complex (Tables 1 and 2)
with a yellowish cryptocrystalline to microcrystalline indicates that El-Kahfa rocks are represented by
undefined. Bonin [18] noticed that the calcic and ferric undersaturated alkaline varieties. It can steadily report
clinopyroxenes are frequently altered to that, the silica content in El-Kahfa complex exhibit a
cryptocrystalline substance that shows a clear distinct gap separating the undersaturated alkali-basalts
green-yellow pleochroism, which is always enclosed in (47.88%) and alkali gabbros (49.16%) from the alkali
green calcic amphibole. Bonin [18] added that in the trachytes (57.91%) and alkali syenites (64.44%).
wake of the work of the subcommision on Amphibole Bryan [29] and Wilkinson [25] have pointed out that
of the IMA (Leake, 1978), the optical variation within silica-gaps are characteristic of differentiated
the amphiboles is revealed to be so complicated and so intrusions, which provide some of the best evidence for
little linked to compositional changes. fractional crystallization.
Petrological and Geochemical Constraints on the Evolution of 291
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt

Table 1 Chemical analyses of major oxides (wt.%) and computerized CIPW-normative minerals for El-Kahfa ring
complexes samples.
Rock type Alk. Basalt Alk. Gabbro Epidiorite Trachytes
S. No. K14 K11 K1 K10 K13a K13b K3 K7
SiO 47.88 49.16 60.63 66.22 69.41 63.18 61.26 57.91
2
TiO 1.32 2.22 0.96 0.40 0.51 0.24 0.36 0.73
2
Al O 15.34 17.43 15.52 12.96 19.56 15.40 19.07 18.59
2 3
Fe O 1.82 7.26 1.92 1.50 0.73 2.58 2.96 3.04
2 3
FeO 8.02 7.19 4.71 2.30 1.63 2.81 3.12 4.25
FeO* 11.46 13.72 6.44 3.65 2.29 5.13 5.78 6.99
Fe O* 12.73 15.25 7.15 4.06 2.54 5.70 6.43 7.76
2 3
MnO 0.12 0.16 0.12 0.06 0.10 0.09 0.15 0.12
MgO 4.01 2.82 3.02 0.32 0.32 1.02 1.60 0.40
CaO 14.00 9.23 7.89 1.56 1.56 2.80 2.68 2.57
Na O 2.97 2.02 2.15 7.65 2.46 6.52 4.03 4.97
2
KO 0.31 1.15 0.63 2.41 1.19 3.18 2.02 3.79
2
PO 0.24 0.37 0.21 0.04 0.60 0.04 0.20 0.50
2 5
L.O.I 0.16 0.61 0.30 1.11 0.48 0.46 0.84 1.03
Total 96.19 99.62 98.14 96.52 98.55 97.86 98.29 97.90
CIPW-normative values
Mg# 47.13 41.15 13.15 19.88 25.93 39.29 47.76 14.37
Q 0.00 10.16 24.33 43.41 48.41 6.84 21.69 7.99
C 0.00 0.00 0.00 0.00 12.92 0.00 5.95 2.78
Or 1.87 0.87 3.81 14.67 7.18 19.22 12.26 23.14
Ab 23.88 17.24 18.59 0.00 21.20 56.30 34.95 43.36
An 28.12 35.39 31.50 0.00 4.30 3.41 12.45 10.14
Ne 0.93 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Lc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Ac 0.00 0.00 0.00 4.46 0.00 0.00 0.00 0.00
Ns 0.00 0.00 0.00 14.32 0.00 0.00 0.00 0.00
Di 34.11 7.12 5.98 11.54 0.00 8.75 0.00 0.00
Wo 17.24 3.67 3.05 5.71 0.00 4.40 0.00 0.00
En 8.59 2.24 1.67 2.06 0.00 2.09 0.00 0.00
Fs 8.27 1.21 1.26 3.78 0.00 2.25 0.00 0.00
Hy 0.00 7.51 10.61 0.00 2.58 1.07 7.15 5.47
HyEn 0.00 4.88 6.05 0.00 0.82 0.51 4.10 1.03
HyFs 0.00 2.63 4.56 0.00 1.76 0.55 3.04 4.44
Ol 2.35 0.00 0.00 0.00 0.00 0.00 0.00 0.00
OlFo 1.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00
OlFa 1.21 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Mt 5.65 10.63 2.85 0.00 1.08 3.79 4.40 4.55
Hm 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Il 2.56 4.26 1.87 0.78 0.99 0.54 0.70 1.43
Ap 0.53 0.82 0.47 0.09 1.34 0.09 0.45 1.13
292 Petrological and Geochemical Constraints on the Evolution of
El-Kahfa Alkaline Ring Complex, South Eastern Desert, Egypt
(Table 1 to be continued)
Rock type Syenite Syenodiorites Alkaline syenite
S. No. K15 K30 K30a K5 K9 K1a K25 K26 K32
SiO 63.81 60.68 61.35 60.20 60.80 62.05 59.76 65.46 64.44
2
TiO 0.41 0.80 0.48 0.44 0.40 0.31 0.80 0.02 0.19
2
Al O 14.51 16.40 15.00 11.31 16.80 16.32 17.05 17.92 16.98
2 3
Fe O 3.02 4.08 3.88 3.72 4.82 1.16 3.08 0.73 1.65
2 3
FeO 2.06 2.52 3.40 3.62 3.81 4.59 3.51 1.61 3.19
FeO* 4.78 6.10 6.89 6.97 8.15 5.63 6.28 2.27 4.67
Fe O* 5.31 6.88 7.66 7.74 9.05 6.26 6.98 2.52 5.19
2 3
MnO 0.15 0.13 0.12 0.11 0.15 0.23 0.08 0.08 0.05
MgO 1.10 1.90 2.02 3.02 0.83 6.07 3.18 1.20 1.40
CaO 2.80 2.80 2.80 4.20 2.24 3.36 6.56 2.84 2.40
Na O 7.20 7.20 7.20 6.97 6.74 3.65 2.04 1.46 2.31
2
KO 3.35 1.86 2.41 4.69 3.10 1.33 0.84 1.26 2.06
2
PO 0.03 0.18 0.23 0.10 0.08 0.20 0.60 0.05 0.18
2 5
L.O.I 0.15 0.09 0.09 0.42 0.12 0.64 0.39 0.84 0.12
Total 98.59 98.64 98.98 98.80 99.77 96.23 97.89 93.47 94.97
CIPW-normative values
Mg# 48.77 57.34 51.44 59.8 28.46 70.22 61.76 57.06 43.90
Q 35.02 4.62 3.17 19.95 3.65 16.74 26.82 46.72 36.28
C 0.00 0.00 0.00 0.00 0.00 3.21 2.18 9.81 7.34
Or 20.13 11.16 14.41 28.20 18.17 7.92 5.10 8.05 12.85
Ab 0.00 61.75 61.53 0.00 57.08 31.08 17.68 13.32 20.58
An 0.00 7.02 1.49 0.00 6.41 15.62 29.79 14.90 11.45
Ne 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Lc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Ac 8.86 0.00 0.00 10.92 0.00 0.00 0.00 0.00 0.00
Ns 12.05 0.00 0.00 11.06 0.00 0.00 0.00 0.00 0.00
Di 11.60 4.69 9.05 16.90 3.55 0.00 0.00 0.00 0.00
Wo 5.82 2.51 4.67 8.59 1.78 0.00 0.00 0.00 0.00
En 2.60 2.11 2.94 4.59 0.79 0.00 0.00 0.00 0.00
Fs 3.19 0.08 1.44 3.72 0.98 22.71 0.00 0.00 0.00
Hy 0.44 2.81 3.22 5.58 2.99 15.29 10.95 2.90 8.19
HyEn 0.20 2.71 2.16 3.08 1.34 7.43 8.15 3.24 3.69
HyFs 0.24 0.10 1.06 2.50 1.65 0.00 2.79 2.66 4.50
Ol 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
OlFo 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
OlFa 0.00 0.00 0.00 0.00 0.00 1.69 0.00 0.00 0.00
Mt 0.00 6.00 5.69 0.00 7.00 1.69 4.58 1.14 2.52
Hm 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Il 0.79 1.54 0.92 0.85 0.76 0.59 1.56 0.04 0.38
Ap 0.07 0.40 0.51 0.22 0.18 0.44 1.34 0.12 0.41

Le Maitre [30] referred that the paucity of salic magma.
intermediate lavas in basalt-trachyte sequences may be MacDonald [31] mentioned that these gaps may
due to a discrimination in the eruptive process, and not reflect the following:
to the scarcity of intermediate magma, i.e. which (1) Some sort of physical control in which rise to the
trachy-andesitic magma formed at depth has not been surface is prevented by high viscosity and/or density
erupted at the surface as readily as more basic and more [32, 33].