Magma-host interactions during differentiation and emplacement of a shallow-level, zoned granitic pluton (Tarcouate pluton, Morocco): implications for magma emplacement

The Tarcouate pluton (Anti-Atlas, Morocco) is an inversely zoned laccolith emplaced 583 Ma ago into low-grade metasediments, with the following succes sion: leucocratic granites, biotite-granodiorites (+/- monzodiorites), horn blende-granodiorites ( +/- monzodiorites) and monzodiorites syn-plutonic dy kes. These rocks form two distinct, chemically coherent, units: (1) A main unit consists of layered (57 < SiO2 < 59 wt.%) and homogeneous ( 63 < SiO2 < 67%) hornblende-granodiorites, biotite-granodiorites (67 < SiO2 < 72%) and aplites (70 < SiO2 < 76%) All these rocks are metaluminous to p eraluminous and display fractionated HREE depleted patterns (La/Yb-N = 14-6 1; Yb-N = 0.7-6.8). Initial Sr-87/Sr-86 ratios (0.7072 to 0.7080) increase, whereas epsilon Nd(t) values ( - 1.7 to - 2.8) decrease from the hornblend e- to the biotite-granodiorites. Monzodiorites occur as mafic microgranular enclaves or syn-plutonic dykes. (2) A subordinate unit consists of leucocratic, distinctly peraluminous, mu scovite-bearing granites (72 < SiO2 < 75%) occurring at the northern edge o f the pluton and as dykes in the surrounding schists towards the top of the pluton. These rocks are free of monzodioritic enclaves. They display less fractionated patterns with higher HREE contents (La/Yb-N = 2-19; Yb-N = 11 - 18), a distinct epsilon (Nd(t)) value (-11.8) and a Sr-87/Sr-86 initial r atio (0.7480) within those of the surrounding schists (0.7393-0.7819). Magma-host interactions are closely related to differentiation and occurred at different levels, but mainly before emplacement. Field relationships an d petrogenetic modelling show that the bt-granodiorites formed at levels de eper than the level of emplacement, by fractional crystallisation (0.65 < F < 0.85) with limited amounts of contamination by crustal material ( simila r to 5 wt.%). The hbl-granodiorites differentiated at the level of emplacem ent by fractional crystallisation leading to residual melts (F = 0.55) slig htly contaminated by the country rocks ( - 2 wt.%) and represented by aplit e dykes. The leucocratic granites are likely to derive by partial melting o f lithologies similar in composition to the country metasediments. These data preclude any significant material transfer process for the empla cement of the Tarcouate pluton, but rather suggest assembly of successive p ulses of variably differentiated crystal-poor magmas. These shallow level g ranitic plutons can be considered as an end-member of magma emplacement wit h minimum interactions with the country rocks. (C) 2001 Elsevier Science B. V. All rights reserved.