​​​Precambrian
 
The Precambrian of western Saudi Arabia: Geologic history, tectonic concepts, and issues for further research
 
Introduction
 
Although Precambrian rocks underlie the entire Arabian Peninsula, they are best known where exposed in the Arabian shield, in the western part of the Kingdom.   In the shield, the Precambrian rocks are mostly Neoproterozoic, but in total record three thousand million years of Earth history.  This history began with the creation of Archean-Paleoproterozoic continental crust, fragments of which crop out in Yemen and west-central Saudi Arabia, continued through Mesoproterozoic processes which culminated in the assembly of the 1000-750 Ma supercontinent of Rodinia, and passed into the Neoproterozoic, which spanned the unprecedented tectonic, biological, biochemical, and climatic changes that accompanied rifting and reassembly of supercontinents, the formation and destruction of oceanic basins, world-wide glaciations, and the beginning of the proliferation of animal and plant life forms that exploded in the Cambrian.  With the c.750 Ma rifting of Rodinia, the Mozambique Ocean was created representing an oceanic basin flanking Rodinia in which most of the rocks of the Arabian shield were deposited, and passive margins came into existence, some of which are represented by epiclastic-carbonate sequences deposited on the margins of the East Sahara craton and preserved at depth in Oman.  During the ensuing 200 million years, the rifted cratons of Rodinia rearranged and converged to form Gondwana, the Southern Hemisphere supercontinent that existed for all the Paleozoic and half the Mesozoic. In these processes, the Mozambique Ocean collapsed by the process of sea-floor subduction, and East and West Gondwana converged, sweeping together and deforming the oceanic assemblages of the Mozambique Ocean and reworking the margins of the older cratonic blocks. 
 
 
 
Arabian-Shield-1.jpg
From the Cambrian to the Cretaceous, these newly formed crystalline rocks were almost continuously covered by Phanerozoic sedimentary rocks, until uplift and erosion associated with the separation of Arabia and Africa resulted in exposure of the Arabian shield as found today.  Juvenile Neoproterozoic rocks predominate in the Arabian shield and extend distances of 200-300 km from the edge of the shield into central Arabia, to the east, and as far as the Iraqi border, to the north.  Farther east, east of about lat 47° E, rocks in the crystalline basement of Arabia may be part of an older continental crust, although isotopic data from Oman indicate that some Neoproterozoic material is certainly present.
 
 
 
TECTONIC CONCEPTS
 
The rocks of the Arabian shield form part of the Arabian Plate, which is currently drifting away from Africa and colliding with Eurasia.
Starting in the mid 1960s, geologists interpreted the Precambrian rocks of the shield in terms of Wilson-cycle plate tectonics, making analogies between the structural and chemical characteristics of rock assemblages found on the shield and rock assemblages found at present-day plate margins. Diverse tectonic settings were recognized, as varied as ocean-plateaus, spreading ridges, intraoceanic island arcs, continental-margin island arcs, and passive margins. Ophiolite/serpentinite-decorated fault zones were identified as sutures representing the remnants of collapsed ocean basins between convergent crustal blocks.
 
 
                                                                                                          Areas of outcropping Precambrian rocks in
                                                                                               the Arabian plate and adjacent areas of the African plate
 
In the mid-1980s, geologists modified this tectonic model by the introduction of terrane concepts, recognizing that many crustal blocks in the shield have particular stratigraphic, structural, and or geochemical/isotopic features that distinguish them from adjacent blocks and allow them to be interpreted in terms of tectonostratigraphic terranes. The terranes converged and amalgamated over a period of about 100 million years between about 780-680 Ma, forming a neocontinental crust that was overlain by younger sedimentary and volcanic basins and intruded by enormous volumes of granite, completing a cycle of continental growth that culminated at the end of the Precambrian in the formation of continental lithosphere 45 km thick.
 
 
The past ten years has witnessed a new development in the tectonic model applied to these rocks as a result of paleomagnetic-pole studies of rocks in cratonic areas of what used to be Gondwanaland and new isotope dating along the eastern margin of Africa and parts of India and Madagascar. It is now universally recognized that the Precambrian rocks of western Saudi Arabia have a larger context than merely forming parts of the Arabian-Nubian shield; they are part of the East African orogen, one of the great collision zones of the Earth that, prior to Gondwana breakup, stretched from Antarctica to Jordan along the present-day eastern margin of Africa, and through Madagascar, parts of India, and the Arabian Peninsula. There is also an increasing awareness that the degree of tectonic complexity observed in other, more recent orogenic belts is likewise found in the Arabian shield and that oceanic closure and mountain building in the shield was not the end, but merely the first act in a tectonic story that terminated in extension and collapse.
 
 
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The perspective of the East African orogen entails cognizance of the fact that the Proterozoic rocks of western Saudi Arabia (1) have along-strike correlations that extend south into the Mozambique belt, and east and west to cryptic sutures with older cratons; (2) have been affected by the global tectonic, biological, and climatic, and geochemical processes that affected Proterozoic rocks elsewhere; (3) are part of an orogenic belt caught up between the converging masses of East and West Gondwana; and (4) local structures and rock types may have formed because of plate movements thousands of kilometers away.
 
 
This template of expanded orogenic complexity entails acknowledgment of the fact that the rocks of the Arabian shield evolved through processes of (1) supercontinent rift and drift; (2) subduction collapse of ensuing ocean basins; (3) continental collision and orogeny; and (4) orogenic extension and collapse. It constitutes a tectonic paradigm that can be summarized as orogeny-extension-collapse and carries the implication that particular rocks and structures may have formed in one part of the orogen contemporary with very different events elsewhere, because of the likelihood that tectonic processes were diachronous.
 
 
 
                                                                                                  Reconstruction of the Rodinia super continent prior to
                                                                                                                     Neoproterozoic  break-up
  
 
 
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GEOCHRONOLOGIC AND ISOTOPIC FRAMEWORK
 
The rocks of the Arabian shield are mostly Neoproterozoic in age, but some date from the Archean and Paleoproterozoic, and a few intrusions yield Cambrian ages.  The sources of the Archean and Paleoproterozoic isotopic ages in the shield are rare exposures of intact old rock and detrital or xenocrystic grains of zircon eroded from old continental crust and inherited by Neoproterozoic sedimentary and plutonic rocks.  The shield in Yemen locally contains 2300 Ma Archean gneiss, and some areas in the east-central shield in Saudi Arabia contain Paleoproterozoic igneous rock, parts of a microplate indicated by an inherited continental isotopic signature that was largely destroyed by later intrusions.  The rocks in these areas are characterized by elevated initial strontium ratios that plot above the standard mantle growth curve.
  
 
They yield elevated 208Pb/204Pb and 206Pb/204Pb ratios in feldspar and galena grains that plot above the average orogene growth curve (Type I and III lead) and together with other rocks in the northeastern part of the Arabian shield were evidently derived from mixed oceanic and evolved continental sources. Rocks in the western part of the shield, in contrast, date only from the Neoproterozoic. They have low initial strontium ratios, have Type I Pb ratios that plot below the orogene growth curve, and were deposited in juvenile oceanic environments.
 
 
TERRANE ANALYSIS
This bipartite oceanic/mixed continental-oceanic division of the Arabian shield is a fundamental tectonic feature of the region and reflects creation of the shield by the amalgamation of terranes derived from different tectonic settings. Terrane amalgamation is a key concept used now days in the analysis of orogenic belts, and is applied in the tectonic analysis of deformed rocks ranging from the Archean to the Recent in many parts of the world.
 
 
 
                                                                                                               Sketch map of the East African Orogen
                                                                                                                (EAO) at the end of the Precambrian
 
 
Tectonostratigraphic terranes are fault-bounded geologic entities of regional extent, each characterized by a geologic history that is different from the histories of contiguous terranes. It is envisaged that, during an orogenic cycle, crustal growth is largely achieved by the convergence and amalgamation of terranes along sutures represented by the fault zones between the terranes and by the accretion of terranes, either individually or jointly, with adjacent continental crust. Overlap assemblages are sequences of post-amalgamation layered rocks deposited in volcanic and sedimentary basins develop unconformably on the newly amalgamated or accreted terranes. The clast composition of clastic grains at different stratigraphic levels in overlap assemblages have been used to reveal the history of unroofing and erosion of the mountain belt that may form during amalgamation, and the structural relations and ages of the assemblages help to date completion of the suturing process. The age of post-amalgamation (post-tectonic) plutons likewise help to date the amalgamation process, signaling the completion of suturing (or “docking”) and the “stitching” together of the terranes.
 
 
In Precambrian orogenic belts, terranes are identified mainly by differences in geochronology, stratigraphy, and structure. However, the ease with which such differences are recognized is heavily dependent on the quality and quantity of available geologic control. In some orogenic belts, the task is straightforward and the result is unambiguous; in other belts, including unfortunately the Arabian shield, the task is difficult and the results are provisional, pending further geologic mapping and analysis. Nevertheless, it is possible to recognize discrete crustal units in the Arabian shield that are distinguished by differences in stratigraphy, structural trends, and isotopic characteristics so that they appear to be valid terranes within the meaning of the term.
 
 
 
Arabian-Shield-5.jpg 
Numerical ages for the Arabian-Nubian shield,
showing the preponderance of Neoproterozoic events but a
significant number of older events back to the late Archean
 
 
 
 Arabian-Shield-6.jpg
Isotopic characteristics of the Arabian shield showing the
presence of a "ghost" continental microplate in Afif terrane,
continental characteristics in the eastern shield, and
unevolved juvenile, oceanic characteristics in the west.
 
  
 
 Arabian-Shield-7.jpg
 
 
 The principles of terrane, showing terrane concepts at various stages in a simple cycle
 
 
 
 Arabian-Shield-8.jpg
 
The terrane concept has been applied to the Arabian and Nubian shields since the mid 1980s, developing out of earlier plate-tectonic/subduction-zone and accreted island-arc concepts, and leads to the interpretation that the exposed and immediately adjacent concealed Proterozoic rocks of western Arabia and northeastern Africa are a collage of crustal blocks, or terranes that, with varying degrees of confidence, can be correlated across the Red Sea thereby linking the now separated Arabian and Nubian shields.  Given the reconnaissance nature of geologic mapping in the Arabian shield, details about the origins, ages, structure, and convergence of the terranes remain to be established, and the terrane-collage model is, strictly speaking, a working hypothesis.
 
Nonetheless, the terrane paradigm broadly accounts for the known chemical and isotopic diversity, structural complexity, and varied ages of the rocks exposed in the shield, and provides a framework for the analysis of the shield comparable to tectonic analyses done in other orogenic belts.  The sutures between the terranes may be conspicuous zones of intense deformation decorated by lenses and slivers of serpentinite, but cryptic or unimpressive sutures are not uncommon both in the Arabian shield and elsewhere.
 
Lacking paleomagnetic data, the positions of the Arabian terranes at the time of their formation are unknown and they are “suspect”, but it is generally accepted that they variously amalgamated between about 780 Ma and 640 Ma, and amalgamation and suturing events, dated by the crystallization age of syntectonic intrusions along the sutures, and the ages of overlap assemblages and stitching granites, range from 780-760 Ma for the Bi’r Umq suture, >696 Ma for the Yanbu suture, about 680 Ma for the Halaban suture, 680-630 Ma for the Hulayfah-Ad Dafinah-Ruawah suture, and about 670-640 Ma for the Al Amar suture.
 
                                                                                                         Structural trends in the Arabian-Nubian: one of the
                                                                                                  geologic features that helps to divide the shield into terranes