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A l lO, and TiO l in clinopyroxene inc rease. (ivl the c« of spinel decreases, and
tv ttne Mg' o f spinel increases (Fig. 4 .91.
4.2.5a HAL Gabbro
No clea r f ractionation t rend is ap parent f or t he HA LPPG. The HAL
gabbros do not repres ent ma gma com pos itions because o f t heir mhig" whole -
rec k Mg', (ii) high M g' o f oliv ine. and (iiil positive Eu anomal y which is
sup portive o f tex t ural evid ence fo r th e accumulation 01 plagioc lase . Using
sam ple l 294 as an example , the anorthite content lAo = 691 of plagioclase is
much lowe r than pred ic ted for olivine o f M g ' : 8 3 with which it coexists. As
a ccmcanscn.jeuccqabbrcsin Nort h Arm Mou nta in with oli vine of M g# = 76 -88
have plagioclas e o f A n = 78 -88 (Komor til ei.. 19 B7). This r.ssccrancn of hi gh
M gO in oliv ine and relatively high Na10 in plagiocl ase is portrayed in t ile AFM
di agram bv t he lack of FeO enrtcnmen t witll inc reasirl Q orcp ortto n of Na 20 (Fig .
4.41. c c neec ceouv . the HAl PPG do not exh ibit a t noientrc or calc-alkaline
trend , but a tre nd w here enrichment in FeO is suppresse d below that of t he
norma l calc-alkal ine tr end . This trend ma y be expla ined bv combined
assimilation and crv sta t fract ionat ion. Th e effect o f ass imilation of roc ks ric h
in MgO cv a f ractionating magm a, is to prod uce a les s reu-enncned. mo re
alka line deriv ati ve mag ma t han would be produ ced bV crvstar fract ionat io n
alone (Kelem en , 19861. Such a process is believed to have operated dur ing
fo rmation of th e HAlPPG an d is consi dered in secti on 4 .3 .