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Hassan Eliwa Ibrahim
MenouKa University
What is the petrogenesis of anorthosite?
Question
Asked September 25, 2016
l am dealing with dating of anorthosite using U-Pb zircon LA ICP-MS.
Anorthosite is a rock type of the maKc-ultramaKc succession, which is composed of pyroxenite
(bottom), olivine-, and oxyhornblende gabbros, then anorthosite, and porphyritic quartz-diorite
(top). Anorthosite is commonly traversed by felsite veinlets (1-5 cm thick). It intrudes the
surrounding volcanics, older granitoids (615-735 Ma) but are intruded by the younger granites
(610-550 Ma).
Petrographically, anorthosite is composed totally of plagioclase megacrysts with subordinate
interstitial amount of augite, enstatite, and amphibole, biotite, K-feldspar, and quartz. Accessories
are magnetite, ilmenite, apatite and zircon. Plagioclase is of andesine composition. Zircon occurs
interstitially in the groundmass consisting of quartz, K-feldspar, and biotite. It is closely associated
with magnetite and ilmenite. It forms subhedral to euhedral crystals, which are commonly found
as aggregates and as single individual grains. Some zircons are broken and corroded but
embayed forms are not common. Zircons are not found in the plagioclase megacrysts. Fine and
irregular veinlets of alteration products of maKc components traverse the plagioclase (Fig. 2A).
Schiller structure delineated by very Kne opaque lines is characteristic of plagioclase. There is
evidences of transformation of pyroxene to amphibole and then to biotite +/- chlorite+/- opaques.
Geochemically, anorthosite exhibits calc-alkaline afKnity and low/intermediate Mg# (0.29 - 0.38).
They have high contents of Zr, Ba, Rb, Y, and highly fractionated REE pattern with weak negative Eu
anomaly.
The question: Zircon occurs as core-rim structured crystals and homogeneous zoned crystals; the
core gave age of about 625 Ma, and the rim gave 595 Ma, which is similar to age given by
homogeneous zoned zircons in the same rock. The obtained 595 Ma is similar to the age of the
nearby younger granite intruded anorthosite. The age difference between rim and core of zircon is
about 30 Ma, which might be a result of a later magmatic event. But How?