Geology and petrology of the plutonic complexes in the Wadi Fizh area: Multiple magmatic events and segment structure in the northern Oman ophiolite

Abstract

Multiple magmatic events are recorded in the gabbroic unit in the Fizh area of the northern Oman ophiolite. Gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event. Gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro. Wehrlitic intrusions (wehrlite I) mark the third event of magmatism. These three magmatic events occurred at the retreating (dying) ridge axis because all these rocks are intruded by dolerite dike swarm, which is generally regarded as a precursor of advancing ridge axis. The next stage of magmatism is a main phase of oceanic crust generation in this area. Wehrlite II and then gabbronorite dikes intrude the still hot main gabbro unit. All of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals, though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit. After considerable cooling of the main gabbro unit, primitive basalt dikes intrude the main gabbro unit, which may correspond to the Lasail unit. Finally, the Fizh‐South complex intrudes into considerably cooled crustal sequence, being below the brittle‐plastic transition temperatures. The Fizh‐South complex, which was regarded as a common wehrlitic intrusion, is significantly different from all of the above mentioned rocks, with respect to the covariation between plagioclase and associating mafic minerals, crystallization order, and clinopyroxene compositions. The clinopyroxenes are characterized by extremely low Ti and Na contents, comparable with those of the V2 unit (Alley volcanics), suggesting that the Fizh‐South complex correlates with the plutonic facies of the V2 unit during arc stage. Layered gabbros in the Wadi Zabin area, about 10 km north of the Fizh area, may be a northern extension of the gabbro blocks of the Fizh area, because they are intruded by numerous dolerite dikes. On the other hand, basin‐like structure of the main gabbroic unit in the northern end of the Fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion. On the basis of these lines of evidence, we propose that the Fizh area indicates the northward propagation tip of the ridge axis.

Root zone of the sheeted dike complex in the Oman ophiolite

“…They were mapped as ''late intrusive'' [Lippard et al, 1986; Ministry of Petroleum and Minerals, 1986], encompassing, wehrlites and gabbronorites, uralitic gabbros, diorites and tronjhemites, the last one making up magmatic breccias with diabase dikes. Olivine gabbros have not been reported among these intrusions These ''late intrusive'' have been related to domains where accretion was under the influence of major tectonic and magmatic activity related to ridge propagation [Juteau et al, 1988;MacLeod and Rothery, 1992;Nicolas and Boudier, 1995;Boudier et al, 2000;Adachi and Miyashita, 2003;Miyashita et al, 2003;Umino et al, 2003]. Here, they are referred to as domains of ''ridge segmentation'' and opposed to domains where the accretion should have been steady state, called here ''normal'' domains, considering their continuity and homogeneity.…”

Along‐strike variation of the sheeted dike complex in the Oman Ophiolite: Insights into subaxial ridge segment structures and the magma plumbing system

“…The southern end of the segment is inferred to be located in Wuqbah massif, to the south of Wadi Sadm, where Girardeau et al [2002] proposed a fossil overlapping spreading center comparable to a thirdorder segment boundary on the basis of structural analysis of mantle peridotite. Detailed description of this paleoridge segment is presented by Adachi and Miyashita [2003]. The sheeted dikes generally strike N-S with steep dips to the west, but locally they dip as gentle as 40°-50°W and in some places to the east.…”

“…[7] Miyashita et al [2001] and Adachi and Miyashita [2003] proposed a trace of fossil segment boundary along upper Wadi Fizh, where an intense dike penetration zone is formed: Layered gabbro is intruded by numerous NNW-striking dikes subparallel to the overlying sheeted dike complex, and abundant xenoblocks of the layered melagabbro are embedded in leucocratic gabbro below. Sr isotopic evidence suggests that the crust section in Wadi Fizh was a part of recharge (downflow) zone located at a segment boundary of the paleoridge [Kawahata et al, 2001].…”

“…The studied block of the ophiolite does not include the paleoridge axis itself, but was formed slightly off a northwardpropagating rift. Crust and mantle sections in Wadi Thuqbah, north of Wadi Jizi, are considered to have formed at the center of the 60-70 km long segment by the presence of the least differentiated bulk and mineral compositions of the layered gabbros and sheeted dikes, and a Moho transition zone attaining 300 m, which is much thicker than elsewhere along the studied paleoridge segment Adachi and Miyashita, 2003]. The southern end of the segment is inferred to be located in Wuqbah massif, to the south of Wadi Sadm, where Girardeau et al [2002] proposed a fossil overlapping spreading center comparable to a thirdorder segment boundary on the basis of structural analysis of mantle peridotite.…”

“…Discontinuities in the paleospreading axis and loci of mantle upwelling comparable to second-or third-order segments [Macdonald et al, 1991] have been proposed (Figure 1) [e.g., Nicolas, 1989;MacLeod and Rothery, 1992;Adachi and Miyashita, 2003]. These segments extend ca.…”