CHAPTER 19 BASEMENT GROUP
Published:January 01, 2010
The basement of Oman comprises a northeast-trending collage of accretionary wedge, ophiolitic, magmatic arc and microcontinental terranes that accreted onto the southeast margin of the East Arabian Terrane during the Pan African Orogeny. The available age dates suggest that the main phase of accretion and associated igneous activity was completed in Oman at around 800 Ma (Mercolli et al., 2006; Allen, 2007; Bowring et al., 2007). Romine et al. (2008) suggest that the whole process commenced at ca. 830 Ma and that a final phase of terrane amalgamation was completed by ca. 720 Ma, prior to Abu Mahara Group deposition. The limited number of exposures of basement rocks in Oman indicate a major gap in the record, potentially related to unroofing and erosion some time between approximately 800 Ma and 713 Ma, associated with the intrusion of younger granites/granodiorites and late dyke swarms. Detrital zircon suites show prominent peaks at about 800 Ma, consistent with derivation from the volcanic-plutonic complex exposed in North Oman. The full suite of detrital zircons recorded in Abu Mahara sediments range predominantly from 720 Ma to 1 Ga, but zircons as old as Archean (>2.5 Ga) have been noted and these are evidence of multiple recycling of original melt rock components.
Much of the discussion on basement rocks can only focus on outcrop data as subsurface material is rare and difficult to place in context.
Type sections: Outrcops in Mirbat Area, Dhofar (Mercolli et al., 2006) and Al Jobah, Al Huqf Area (Dubreuilh et al., 1992a; Leather, 2001; Allen and Leather, 2006). It is impossible to define a single or series of type wells for the large variety of rocks that are potentially combined in the Basement Group. Many wells just manage to penetrate a thin section of igneous/metamorphic rocks at their base. These are assumed to be basement, but without reliable dating this is difficult to prove (e.g. probable extrusive basalt in Hathnar-1, Figure 18.3). Note also the ca. 699 Ma (Bowring et al., 2007) syenite in Makarem-2H3 (Figure 19.3). This would be rather young for a basement interpretation given our current understanding.
Examples of subsurface sections that may have drilled basement are Zafer-1 in Central Oman (Figure 19.2) and Makarem-2H3 in North Oman (Figure 19.3). See also Hathnar-1 (Figure 18.3) and Ghadir Manqil-1 (Figure 18.6).
Lithology: Oman’s basement outcrops comprise high-grade metamorphic (amphibolite facies) sediments and basic igneous rocks that are complexly deformed and intruded by undeformed dioritic to granodioritic and granitic rocks. In turn these are intruded by mainly basic dyke swarms. The geochemistry of the igneous suites indicates a volcanic arc/subduction setting, with possibly an ophiolitic sequence on Juzor Al Hallaniyyat (offshore Dhofar).
Most knowledge on the pre-Huqf Supergroup geological history of Oman is derived from the Mirbat area in Dhofar, as summarised by Mercolli et al. (2006). They recognise four major units:
(1) The Juffa Group is a siliciclastic sedimentary protolith composed of metasedimentary mica-gneisses and amphibolites with an age >1,300 Ma (Mesoproterozoic).
(2)The Sadh Group comprises highly deformed and metamorphosed banded gneisses intruded by slightly deformed diorites and tonalities (Mahall Complex).
(3)The Tonalite Group is composed of three large calc-alkaline plutons dated 800–780 Ma.
(4)The Granite Group comprises different types of dykes and small intrusive bodies including the Pegmatite Complex (770–750 Ma), and the Mirbat Granodiorite, Leger Granite and Shaat Dyke Swarm (750– possibly 700 Ma), see Worthing (2005).
Boundaries: Basement rocks are generally unconformably overlain by sediments of the Huqf Supergroup. Without reliable age dates igneous basement rocks can be difficult to distinguish from some of the volcanics in the Abu Mahara Group.
Distribution: Metamorphic and igneous basement is exposed in South Oman in the Mirbat (Dhofar) region (Platel et al., 1992a; Roger et al., 1997; Hauser and Zurbriggen, 1994; Mercolli et al., 2006) and on the Juzor Al Hallaniyyat Islands (Platel et al., 1992b). In east-Central Oman it is exposed in small outcrops at Al Jobah in the north of the Al Huqf area (Dubreuilh et al., 1992a; Leather, 2001; Allen and Leather, 2006), and in North Oman near Sur at Jabal Al Jaalan (Wyns et al., 1992; Würsten et al., 1991).
A small number of exploration wells have potentially encountered crystalline basement.
Age: There is a principle ‘Cryogenian’ accretionary/formative phase at ca. 830–800 Ma with minor intrusive emplaced ongoing until ca. 720 Ma. Note however, younger intrusive bodies are also present, e.g. the ca. 699 Ma intrusive syenite in Makarem-2H3 (Bowring et al., 2007; Figure 19.3).
The implications of a wide range of radiometric ages, mainly associated with U-Pb techniques applied to both primary melt and derived detrital zircons, are discussed extensively in Bowring et al. (2007) and Allen (2007). Stuart-Smith and Romine (2003) and Romine et al. (2008) have detailed the tectonostratigraphical evolution of the basement rocks and associated Neoproterozoic sediments.
Interpreted ages range from the ca. 824 Ma for the granodioritic basement at Al Jobah (Bowring et al., 2007) to ca. 726 Ma for various generations of granitic intrusions and dykes. The Leger Granite (ca. 726 Ma, Bowring et al., 2007) and associated Mirbat Granodiorites in Dhofar mark the end of magmatic and metamorphic activity associated with the final phase of terrane amalgamation, as interpreted by Romine et al. (2008). These pre-date the deposition of undeformed Abu Mahara Group diamictites and associated sediments, e.g. the potential maximum age of the basal Ayn Formation at ca. 722 ±12 Ma (Figure 18.2, Rieu et al., 2006) in the Mirbat area and the Ghubrah Formation age at ca. 713 Ma in Jabal Al Ahkdar (Bowring et al., 2007).
Other published geochronologic data, not necessarily specific to the Basement Group but which are of general interest, include a K-Ar date of 654 ±12 Ma for trachytic volcanic rocks (Gorin et al., 1982) and a Rb-Sr whole-rock date of 562 ±42 Ma (Dubreuilh et al., 1992a) on samples of rhyolite from the Halfayn Formation. However, these dates should be viewed with caution as they are probably influenced by significant phases of thermal resetting (Stuart-Smith and Romine, 2003) and they are indeed rejected by Bowring et al. (2007) in favour of more robust U-Pb/zircon based data. A 662 ±13 Ma age for the basement rocks in Zafer-1 (Hughes Clarke, 1988), based on whole rock Rb-Sr analysis, is similarly interpreted as a thermal reset age by Stuart-Smith and Romine (2003).