تکامل گرانیت مبتنی بر جداسازی ماگما غیر تعادلی
Abstract
Introduction
Geological setting
Analytical methods
Petrographic description
Description of zircon
Results
Discussion
Conclusions
References
Abstract
We present 77 new granite whole-rock analyses from the Qattar and Gharib areas, Eastern Desert, Egypt. Both areas include a “normal” granite and either a hypersolvus (Gharib) or an almost plagioclase-free granite (Qattar) enriched in fuorite. According to earlier results, F infuences element distribution in granitic melts forming complexes with specifc elements as Nb, Ta, Ga, Hf, Th, Zn, Sn, whereas F excludes Ba and Sr. We use principal component analyses to split the granite into chemical groups allowing an unbiased study of the inter-group element distribution. This adds the heavy REEs and Y to the earlier lists of elements with an afnity for F. The light REEs show a decreasing afnity with decreasing atomic mass; fuorine separates Sm from Nd, whereas Zr follows La. Opposite to some, but in accordance with other earlier results, the ratio Nb/Ta is higher in the fuorite-enriched than in the other granite. Weak tetrad efects are present. Zircon in the hypersolvus granite is high in common lead. We suggest F to be instrumental for separating Pb2+ from Pb4+. Two hypotheses may explain the occurrence of the two contrasting granites: they have either diferent sources, or they are co-magmatic, but the magma was split into two discrete types. We apply the second hypothesis as our working hypothesis. The liquidus has a gentler slope with pressure than the diapir requiring crystallisation to be most important in the lower part of the magma chamber. Our hypothesis suggests that globules of magma, enriched in volatile components, form during crystallisation due to slow difusion rates in the crystallizing magma. Elements accompanying F are distributed into this magma batch, which has a lowered density and viscosity than the rest of the magma due to its increased contents of volatile components. A mushroom-formed diapir rises, forming the hypersolvus (or almost plagioclase-free) granite. Due to an edge efect, it is concentrated close to the wall of the magma chamber. The size and form of the outcropping granite depend on the intersection of the diapir with the erosion surface. Fluorine only makes it possible to follow the process. The model may be generalised to explain the diversifcation of non-F enriched granite, since the buoyancy of a magma batch several thousand m3 in size has a much larger impact on the system than the small negative buoyancy of crystals or small crystal aggregates. A-type granite classifed merely from its trace element content may form from separated F-enriched magma batches. This may be the reason for their high frequency in the Eastern Desert.
Introduction
Several aspects of granite evolution are still contradictory and poorly understood. The mineralogical defnition of granite limits the variation of its major element composition, whereas the trace elements often show larger variations. A number of various mechanisms are suggested to explain trace- and major-element diferences in granite: i. Crystal diferentiation is often advanced to explain compositional variation of magmatic rocks. After having reached a near eutectic composition, thermodynamic laws prevent melts to evolve further, and they restrict any evolution of melts close to a minimum melting composition. Thus, if crystals continue to separate after this stage, major minerals separate in granitic proportions. ii. For a given source rock, limited major- and trace element variation can be explained by varied degrees of melting. iii. Protolith heterogeneities explain signifcant regionalscale compositional diferences (for a recent example, see Lindh 2014), but if applied to single intrusions restrictions are imposed both on the size of these heterogeneities and on homogenisation following melting. iv. Mixing or, due to viscosity and temperature diferences, rather mingling of melts is often suggested as explanations. v. Magmas may split due to volatile enrichment during the crystallisation process followed by separation of the magmas due to their diferent densities and viscosities (Lindh 2012). This is a non-equilibrium process; there is no need for liquid immiscibility.