skaergaard - 1 The Skaergaard Intrusion A natural lab for...

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1 The Skaergaard Intrusion A natural lab for studying closed system fractionation? The Skaergaard intrusion is a large 2.5 km - thick intrusion of tholeiitic magma into Precambrian quartzofeldspathic gneisses in eastern Greenland. It was emplaced in the Eocene during the opening of the N. Atlantic, and represents a single surge of magma related in time and space to voluminous fissure eruptions of flood basalts. The magma crystallized slowly to produce a layered structure which can be used to test the different models and mechanisms of differentiation that we have been discussing so far. It is particularly important because it has been studied extensively since the 1930's by Wager and Deer (at the same time that Bowen was publishing his experimental results on the crystallization of basaltic magmas), then Wager and Brown (1968), and McBirney and Noyes (1979). Structure of the Skaergaard intrusion: The structure of this intrusion is given in three cross sections in Figs. 1 and 2. Fig. 1 shows Wager and Brown's (1968) version of a N-S section, whereas Figs. 2a & b show the revised N-S and E-W sections based on the work of McBirney and Noyes (1979). From these figures, it can be seen that this intrusion consists of four groups or series: 1- Layered series (LS) 2- Marginal border group (MBG or MBS) 3- Sandwich horizon (SH) 4- Upper Border Group (UBG or UBS) It also contains several small bodies of granophyre (mainly within the UBG), as well as an unexposed "root" known as the Hidden zone (HZ). Four types of layering can be identified in the Skaergaard intrusion: 1- Modally graded layering: consisting of layers with different mineralogies or modal contents. 2- Cryptic layering: which is represented by the chemical changes recorded in composition of one or more solid solution phases going upsection, and which cannot be identified in the field. 3- Phase layering: Is a type of layering defined on the basis of the appearance or disappearance of a particular mineral. 4- Rhythmic layering: where the modally graded layering occurs in several repeated cycles. If the cyclicity of this type of layering is not ideal, then it is called “intermittent layering”, and tends to be similar in some cases to the “modally graded layering”!. In Skaeregaard, this type of layering occurs on a scale of 50 cm, and is not as well developed as in some of the other huge layered intrusions such as the Bushveld Complex. In addition to the observed mineralogical layering, the Skaergaard rocks show a systematic (and rather extreme) Fe enrichment going up-section.
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2 I- The layered Series: Consists of cumulates which have their crystals oriented with their long axes parallel to what appears to be the base of the intrusion or magma chamber. From the bottom (1) Lower zone a: Ol + Plag as cumulus phases, Cpx as an intercumulus phase (Fig. 5b)
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.

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skaergaard - 1 The Skaergaard Intrusion A natural lab for...

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