Geology and geochemistry of the upper ………….………….Naela Geology and geochemistry of the upper proterozoic granites of As – said intrusive complex in Lawder – Mudia districts Abyan Governorate

The study area encompasses about 135km 2 located between latitudes 13 ° 51' 35'' and 13 ° 56' 20'' north and longitudes 45 ° 56' 40'' and 46 ° 04' 52'' east, within the south eastern region of Abyan Governorate , situated to the north east of Aden at a distance of 176km. The area is occupied by the geological map sheet D38 80 and D38 81. The main rock types of the intrusion includes in the series:biotite granite, monzogranite, syenogranite , rarely gneissosegranites. The petrographic, mineralogical and geochemical studies revealed that there is no considerable differences between the main massifs;suggesting that these granite massifs represent outcrops of the same magmatic origin. The massifs of As – Suwaiyda ,Am-Salaa, Al – Hamrah and Al – Hafa (um Zareb),and Am Maisam, geochemically originated from peraluminouscalc alkaline magma and considered as post– tectonic granites intruded at pressure between 1-3kb within crustal depth, greater than 30km of the lithosphere. A brief description of the main studied and visited massifs in the area of Lawdar – Mudia within the metamorphosed complex enclosing rocks of Barak Group is given below.


Introduction
The aim of this work is to evaluate the studied post tectonic granite massifs of J. As -Suwaiyda,J.Am-salaa J. Al -Hamra , J. Al -Hafa , J. um Zareb and J.Am-Maisam and their associated pegmatites from the petrographical , geochemical and mineralogical point of view. All these granitic massifscomprise rock units of As -Said intrusive complex of the main area. As -Said intrusion covers about 30km 2 of the total area cross cutting the metamorphic formations of Barak Group.
Granites are Classified according to their locality (Jabal) , or classified by different workers (As -Said intrusive complex, Labounets 1988) , or by their relative age as (syntectonic , inter tectonic , post tectonic (2).
The post tectonic granites of Lawder -Mudia area form successive series of younger discordant , commonly massifs, Jointed , grey to pink colour; contains pegmatite dikes and veins with low content of CaO, MgO and related elements. These Granites represent the last major tectonic and magmatic event at the time of craton stabilization (Pan -African tectogenesis,Windly) (2), in the evolution of the Arabian -Nubian shield at Upper Proterozoiclowerpalaeozoic era.

Methodology:
Chemical analysis was made for 13 samples from different locations within the granitic massifs with determinations of majoroxides(wt %). Oxides include SiO2,Al2O3 , Fe2O3 ,TO2 ,CaO, MgO , MnO , P2O5 , SO3 ,Na2O , K2O and lose of ignition.Petrographic studies of thin sections were carried out on rock specimens taken from each type,including pegmatitic veins, to investigate the mineral composition of different granites. All analysis were done in the laboratories of the Geological Survey and Mineral Resources Board-Aden.

General Geology
The area is composed of Upper Proterozoic-Lower Paleozoic metamorphic rock units, a great number of granitic intrusions besides Jurassiclimestones, Neogene-Quaternary volcanicsand Quaternary deposits .
The lower metamorphic unit (Barak Group) consists of migmatites, granite gneisses and plagiogneisses. Amphibolites and schists with subordinate interbeds of two mica schists and quartezites. The rocks are regionally metamorphosed into amphibolite and epidote amphibolite facies. The upper metamorphic unit (Fahman Group) is composed of metasandstone, quartz sandstones,quartezites,quartz feldespathic rocks with horizons of biotitic and two mica schists,marbles and dolomite interbeds. The rocks are regionally metamorphosed in green schist facies and alamendine amphibolite facies (3).
Jurassic deposits of Amran Group are exposed to the south of the area, gently dipping beds of (10-15°)uncnformably overlying the eroded basement surface forming the lower conglomerates,gritstones and limestones with sandstone interbeds. Neogene-Quaternary volcanics of Yemen volcanic series predominates in the south eastern parts in the form of slagy lava cones and domes, mainly of basic to intermediate composition ,basalts, tuffs and volcanic ashes (4).

Geologic outlines
As -Said intrusive complex is composed of rocks of three phases of intrusion. The first and the second phases comprise monzogranites and syenogranites. The first phase is having varieties of granites while the second is represented by syenogranites. The rocks of both phases have massive or gneissose structure and represented by granite gneiss of biotitic composition. Pegmatoid granites and pegmatites, aplites are the latest rocks of the complex. Granite domes, swells, dykes are abundant in the studied area. Geological map of the studied granites is shown in Fig (1). Univ. Aden J. Nat. and Appl. Sc. Vol. 22 No.1 -April 2018 A brief description of the main studied and visited massifs in the area of Lawdar -Mudia within the metamorphosed complex enclosing rocks of Barak Group is given below.
Strong weathered ,biotitizedvarieties of Jointed granite are observed amidst the massif to different extents. The contacts are distinct. Granites are medium grained,jointed,weathered rocks containing fine flakes of biotite gives the schistosicgenissose texture of the granite around the boundaries. Granites are intersected by different oriented pegmatite veinlets and veins having steep (70 -90)°dip angles, with thickness of 1 to 5cm ,veinlets and veins are traced for tens of meters. Jointesare found and are caused probably by magma cooling, solidification and weathering(1). Am -Sala'a granite massif The massif is situated 1.5 km to the north east of the same named village with coordinates center 13° 54' 40'' north latitude and 46 0 40' 40'' east longitude , being represented by the massif of Um Zareb mountain with absolute elevation from 920 to 1357 meters above sealevel. The massif occupies an area of abaut 1.5 sq km.
The body mainly is composed of jointed biotitized , medium grained granites of light grey colour having biotite schlieren,impersistent veins and veinlets of pegmatites occassionaly quartz bearing from (1 -3) up to (50 -80) cm in thickness. Joints are confined to systems of northwest 340° with dip angle 80°.
The granite massif is irregularly biotitized schistosed , sheet jointed and being crowned by cone -like peak with steep relief.

Al-Hamra granite massif
The granite massif is located to the east of Um Shehabiah village at a distance of 1. Km, having an elevation ranging from (840 to 1319) meters above sea level with coordinate scenter13 0 53' 52'' and 46 0 04' 52''. Granites are biotitic, fine to medium grained,massive, light grey in colour, jointed and weathered from the lower levels of the massif with marks of gneissose structure. Joints found of different sizes and lengths,empty or filled by earthy materials. The top of the massif is steep cone-like peak rising 160 meters above the crest of the same mountain.Veins and veinlets of pegmatites and quartz are reported and being traced in various directions at a distance from afew meters to tens of meters.

Al-Hafa granite massif
The massif is located 0.1 km to the south of Al-Hafa village, covering an area of about 0.3 sq km with coordinates center 13 0 54' 40'' and 45 0 56' 50''. Steep sided granite cone-like peak of up to 100 meters high;It,s elevation is in the range of (900-1400) meters above sea level. The granitic massif is light grey ,locally pinkish, medium grained contains biotite schlieren and pegmatite veins and veinlets. The granite is weathred,jointed,massive,gneissose. Pegmatites are differently oriented in thickness attaining 20 cm in average. Alamendine crystalsis clearly encountered in some veins. Am-Maisam granite massif.
The massif is located nearby the village of Am Maisam with coordinates center 13 0 53' 50'' north latitude and 45 0 57' 00'' east longitude, with absolute elevation ranging from (800 to 1040) M, stretches from west to east for 1km with its width being 0.3km.
It is completely composed of monzogranits, jointed, weathers and biotitized to different extents. Granites are massive medium grained, strong, slightly jointed, light grey colour. Granitesare characterized by sheet jointing with gentle slopes. Veins and veinlets of pegmatites (2-3-to15-40 cm thick) and occasionally quartz-bearing (up to 10-20 cm thick) are reported and traced for tens of meters, rarely biotite schlieren recognized.

Petrographical study
A petrographical description of the rocks collected from different outcrops in the studied granites is given below.

Geochemistry
A group of samples were collected from different granitic outcrops for chemical analysis in order to identify their geochemical behavior and petrochemical characteristics including magma type and tectonic setting.
The chemical analysis for major oxides (wt%), for 13 samples is given in Table ( 1). No evidence of any valuable differences in mineralogical and chemical composition of the granites are shown. Generally, the granites are characterized by their relatively normal contents of major oxides .
On the variation diagram of SiO2 versus major constituents are shown in (Fig 2 and on Fig 3 a). In the monzogranites,the SiO2 a negatively correlated withFeO ,AI2O3 , CaO,TiO2 and Positively correlated with MgO,Fe2O3 and Na2O and compatible with k2O, P2O5 and MnO. In Syenogranites, the SiO2 is positively correlated with CaO,MgO, and k2O ,negatively correlated with Na20,FeO,Fe2O3,MnO and compatible with K2O,P2O5.
On the total alkalies-silica diagram of Cox et al (1979) (6),monzogranites and syenogranites are poltted within the granite field and are classified them as acidic subalkaline igneous rocks(Fig3) The molar (Al/Na+k) versus(Al/Ca+Na+k) diagram of White and Chapel, (7)shows that the granites are disturbed between peraluminous and per alkaline fields, the studied monzogranties are plotted within the field of peraluminousaffinity, while the syenogranites are discriminated to the peralkaline field affinity (Fig4a).
On the AFM ternary diagram (FeO,totalalkalies,MgO), of Beard (7),all the two types of granites are plotted in the calc alkaline field (Fig4b). Hence, these rocks are characterized by Calc-alkali trends and have calcalkali indices. The agpaitic index (Na2O + K2O \ Al2O3) of the rocks varies from (0.47) to ( 0.74).
The ternary diagram ( Or -Ab -An )of ( Barker , 1979) (7) shows that the studied granites are plotted in the field of granites (Fig4c), on the same diagram according to (5) and (7)   Tectonic setting : The granites of Lawdar -Mudiaarerepresentative part of the main granite of As-Said intrusive complex originated in the last major tectonic event in the evolution of the crystalline shield of Yemen and Saudia Arabia which were formed at the time of the stabilization (Pan African period of the continental crust (Rogers and Greenberg 1983,Greenwood 1968,Stosser and Camp 1985) (2). This resulted in the initiation ofperaluminousand peralkaline within plate granitic intrusions, they occur as ring structures or as great batholiths which are almost circular. A wide range of theories about the gneisses of peralkaline granites exist (Stosser and Elliott,1980) (2). A tendency toward a peralkaline geochemistry of the post tectonic igneous rocks is mentioned by several workers in connection with the comparable situation in Saudi Arabia, but it is accepted that they are predominantly emplaced with peraluminous granites with which they are genetically related inside within the plate association as products of partial fusion.
Chapman and Hall, 1979 (7) suggested that post tectonic granites are charactrized by the following: Peraluminousleucogranite. Collision granites. Crustal type granites. Crystallized at low pressure (less than 2.5 kb) The data on the radiometric age of the granites (K-Ar method), collected from massif rocks, has offered figure of 670m.y, late proterozic by ( Labounetes et al 1988)(5), The age of the similar granites from Saudia Arabia has been established by Pb-Sr method fluctuated from 660 to 550m.y (5).

Conclusions and Recommendations
1-The granites of As-Said intrusive complex in the Lawder-Mudia area are classified intomonzogranites and syenogranites The petrographic mineralogical and geochemical studies revealed that there is no considerable difference between these massifs which represent outcrops of the same originated magma. 2-Age determination by K-Ar method revealed that these granites are related to post -tectonic type of granites originatedat the latest stages of the pan-African tectogenesis in the Late Proterozoic -Lower Palaeozoic period. 3-The granites is characterized by low potentiality of rare elements and rare earths. 4-The granites are of great importance in the association of pegmatites of different widths and extensions that must be included for further studies for rare elements and radioactive minerals. 5-High decorative properties of the granites for the production of facing and floor tiles is recommended for future studies. 6-Further field studies for pegmatites as ceramic raw material is required in future.