عنوان

The role of crustal interactions in the formation of cogenetic silica oversaturated and undersaturated syenites:

TLpq304105005

انگلیسی

The role of crustal interactions in the formation of cogenetic silica oversaturated and undersaturated syenites:

[Thesis]

J. D. Landoll

Evidence from the alkaline complexes of Abu Khruq (Egypt), Mt. Shefford (Quebec), and Marangudzi (Zimbabwe)

K. A. Foland

The Ohio State University

1994

384

Ph.D.

The Ohio State University

1994

This study addresses the formation of associations of cogenetic silica oversaturated and undersaturated felsic igneous rocks. Such occurrences have been a long-standing petrologic problem because they are not explicable by closed-system fractional crystallization in the system quartz-nepheline-kalsilite (Petrogeny's Residua System). Their formation is examined with a survey of occurrences and an examination of the hypotheses proposed to explain them. A model is proposed whereby oversaturated rocks form from an undersaturated magma via incorporation of silica-rich crustal material, whereas the undersaturated rocks form from the same (or a closely related) magma principally by fractionation. This model is tested at three alkaline complexes (Abu Khruq, Egypt; Mt. Shefford, Quebec; Marangudzi, Zimbabwe). The contamination hypothesis is supported by several features that are common to most associations including: cogenetic over- and undersaturated felsic rocks occurring only in continental settings; oversaturated rocks occurring on the perimeter of complexes whereas undersaturated ones are interior; and, crustal contamination is invariably implicated in the formation of oversaturated rocks whereas undersaturated rocks generally are contaminated to lesser degrees, or not at all. For all studied complexes, calculated initialSr/Sr and Nd/Nd ratios display significant variations and are negatively correlated. Isotopic relations show that the parental magmas were generated in the mantle and experienced input of crustal material. Oversaturated syenites invariably have lower Nd/Nd, higher Sr/Sr, and are more contaminated than undersaturated syenites. Contamination is explained by AFC at mafic and evolved stages. Quantitative models in Petrogeny's Residua System are consistent with isotopic models and show that AFC of crustal material can cause an undersaturated magma to evolve across the thermal divide and become oversaturated. The results suggest the following paradigm for generation of the syenites. An undersaturated, mantle-derived, mafic magma evolves at depth to form an intermediate or evolved (syenitic) undersaturated liquid. This magma is emplaced in several batches; the first come in contact with crustal material during intrusion and are contaminated, producing oversaturated compositions. Later batches are intruded within the earlier units and are shielded from the crust. They evolve principally by fractionation and produce undersaturated compositions.

Earth sciences

'ebec

Geochemistry

Geology

Mt. Shefford

Qu

J. D. Landoll

K. A. Foland

p

[Thesis]

276903

a

Y