Contact metamorphism and metasomatism at a dolerite-limestone contact in the Gebel Yelleq area, Northern Sinai, Egypt

Summary

At the northeastern flank of Gebel Yelleq, northern Sinai, pure limestones of Upper Cretaceous age were subjected to a thermal overprint, caused by a c. 80 m thick Tertiary olivine dolerite sill. Metasomatic supply of Si, Al, Fe, Mg and Ti was greater to the c. 7 m wide upper than to the c. 25 m wide lower thermal aureole. The greater width of the lower aureole is possibly due to a longer duration of the thermal overprint at this contact. Mineral assemblages in both aureoles are (from the contact outward):

(i) clinopyroxene + garnet ± wollastonite + calcite

(ii) garnet ± wollastonite + calcite;

(iii) wollastonite + calcite.

In places, late stage xenoblasts of apophyllite and witherite overgrow these assemblages. Garnets are grandites to melanites with Grs_56–86Adr_14–42Sch_0–2Sps_0–0.2Prp₀ in the lower, and Grs_29–94Adr_5–64Sch_0–12Sps_0–0.2Prp_0–1.7 in the upper aureole. Close to the upper contact, clinopyroxene is virtually pure diopside with X _Mg = Mg/(Mg + Fe²⁺) = 0.97−1.0, whereas clinopyroxenes farther away from the upper contact and in the lower aureole have X _Mg-values of 0.49 and 0.53, respectively.

The minimum temperatures reached during contact metamorphism in the upper and lower aureole are defined by the lower stability limit of wollastonite. The temperatures are inferred with a calculated T-X(CO₂) projection in the system CMASCH and are estimated at c. 290 °C and 380 °C for X(CO₂) values of 0.05 and 0.25, respectively. A pressure of roughly 100 bar is estimated for the lower dolerite-limestone contact. As indicated by one-dimensional thermal modelling, a maximum temperature of 695 °C was attained at this contact, assuming a magma temperature of 1150 °C. Further modelling results indicate (i) wollastonite, which occurs first 13 m away from the lower contact, formed at a maximum temperature of c. 575 °C, (ii) there, wollastonite formation lasted for approximately 170 years and, (iii) at the outer rim of the lower aureole, the maximum temperature reached was 480 °C, and temperatures sufficient for wollastonite formation lasted for about 140 years.