Petrology of Plagiogranites from Suhaylah area in the northern Oman
ophiolite
Kaisuke Yoshida1, Sumio Miyashita2
1
M.S. Graduate School of Science and Technology, Niigata University, Niigata, Japan
Emeritus Professor. Graduate School of Science and Technology, Niigata University,
Niigata, Japan
2
Plagiogranites are leucocratic rocks appearing in oceanic crust or ophiolite,
characterized by high-SiO2 and low-K2O contents (Coleman & Peterman, 1975; Koepke
et al., 2005). In general they consist of diorite, quartz-diorite, tonalite, and trondhjemite.
There are four models for the petrogenesis of plagiogranite as follows (e.g. Nicolas,
1989): 1) Extreme crystallization of Mid-Ocean Ridge Basalt (MORB); 2) Partial
melting of gabbro and/or altered dyke complex; 3) Assimilation and melting of altered
basalt (vein); 4) Liquid Immiscibility.
Stakes & Taylar (2003) studied three plagiogranites bodies including Suhayla
complex of this study and suggested that combined assimilation-fractional
crystallization, together with recharge by injection and quenching of basaltic magma
‘pillows’ into these plagiogranite magma chambers, plays a major role in the
development of these composite intrusions. Rollinson (2009) revealed that there are two
types of plagiogranites in the Oman ophiolite; early type (ridge-stage) and late type
(subduction-stage).
We have performed detailed geological survey of the Suhaylah plagiogranite body.
We show major elements, and minor elements compositions of the body by XRF and
ICP-MS. Based on these results, genesis of the plagiogranites is discussed.
Suhaylah plagiogranite complex cropping out at the southern margin of Fizh
block of the northern Oman ophiolite, intrudes between gabbro and sheeted dyke
complex. The Suhaylah complex extends 2 km width and 3 km length, and one of the
largest plagiogranite complex in the Oman ophiolite. This complex contains numerous
mafic enclaves which are classified to three types, 1) xenolith of gabbro, 2) xenolith of
dyke complex, and 3) diorite. This study focuses on mafic-enclave (ME) composed of
diorite.
The MEs show spherical to oval shapes, and they were wrapped with Qtz-diorite
to tonalite. The boundaries between the ME and host rocks are wavy, and chilled
margins are observed in MEs. The ME becomes coarse-grained inward from the
boundary and shows a dendritic texture consisting of acicular plagioclase and mafic
minerals. This indicates that the ME was rapidly crystallized. Based on above
observations, the ME was not solidified when intruded by leucocratic magma. When the
both magmas were mixed, as the ME magma was hotter than the leucocratic magmas,
the former was rapidly cooled and resulted in chilled margin to the host leucocratic
rocks.
Plagiogranite body and MEs are characterized by decreasing TiO2, MgO, FeO*
(Fe as FeO), MnO, CaO and Al2O3, and increasing Na2O contents with increasing SiO2
wt.%. SiO2 contents of plagiogrenite range from 58 to 78 wt.% and of ME range from
58 to 67 wt.%. Plot of host rocks and MEs follow a same trend in SiO2-variation
diagrams. Rare earth elements (REE) patterns of plagiogranites are nearly flat with Eu
negative anomaly, but there are significant dispersion from middle REE to light REE.
Bulk rock compositions of the ME are not basaltic and regarded as less-evolved
compositions of the plagiogranite complex. However, REE concentrations of some of
the ME are higher than others.
On the basis of bulk rock compositions, this complex is classified into ocean
ridge granites (Pearce et al., 1984), and corresponds to the early type of mid-ocean ridge
origin by Rollinson (2009). Although, the early type plagiogranites usually occur as
small-scale intrusions, the Suhaylah complex occurs as a large complex. This is possibly
due to the location of the Suhaylah complex, because this area is regarded as a segment
center (Miyashita et al., 2003).
References
Coleman and Peterman, 1975, J. Geophys. Res., 80, 1099-1108.
Koepke, 2005, Terra Nova, 17 (3), 286-291.
Miyashita, et al., 2003, Geochem. Geophys. Geosys.,
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Nicolas, 1989, Kulwer Academic Publishers, 367.
Pearce et al., 1984, J. Petrol., 259, 956-983.
Rollinson, 2009, Lithos 112, 603-614.
Stakes and Taylar, 2003, Geological Society, London, Special Publication, 218,
315-351.
Yoshida’s e-mail address: f11e063a@mail.cc.niigata-u.ac.jp