TY - JOUR
T1 - Granitoid compositional zoning by side-wall boundary layer differentiation
T2 - Evidence from the palisade crest intrusive suite, central Sierra Nevada, California
AU - Sawka, W. N.
AU - Chappell, B. W.
AU - Kistler, R. W.
PY - 1990/6
Y1 - 1990/6
N2 - Compositionally zoned plutons are an important feature of the Sierra Nevada batholith, California. Two such plutons have been examined to determine the mechanism by which crystals separate from a magma. The Tinemaha pluton shows continuous compositional variation from 58 to 67% SiO2, whereas the McMurry Meadows pluton is bimodal, with an outer margin of mafic granodiorite (59-60% SiO2) and an inner core of granite (66-69% SiO2). Extreme differentiates also occur as small isolated masses within the suite and may contain up to 76% SiO2. Both plutons are uniform in strontium isotopic composition but are different from each other, with initial 87Sr/86Sr values of 0·70719 and 0·70651 respectively. The Tinemaha pluton is both horizontally and vertically ({reversed tilde} 1000 m) zoned, with fractionation occurring both inward from the contacts and upward. The vertical trends in relative mineral proportions are not consistent with crystal settling. Both the vertical and horizontal variations in the chemical composition of 50 elements, in mineralogy, and in accessory mineral light rare-earth element zoning, are all directly relatable to side-wall crystallization which created a less-dense melt that buoyantly moved upward along the wall towards the top of the magma chamber. The different rates for diffusive heat exchange and compositional diffusion within the magma initiated the double-diffusive gradient in the magma chamber. Compositional variations in the side-wall crystal accumulation zone occur as boundary layer melts evolve, reflecting changes in the bulk convecting magma. The compositional gap in the McMurry Meadows pluton is the result of a similar but more efficient side-wall fractionation process, related to a higher proportion of melt to crystals in the initial magma and a slower rate of side-wall solidification as a result of the thermal blanket created by the enclosing Tinemaha pluton.
AB - Compositionally zoned plutons are an important feature of the Sierra Nevada batholith, California. Two such plutons have been examined to determine the mechanism by which crystals separate from a magma. The Tinemaha pluton shows continuous compositional variation from 58 to 67% SiO2, whereas the McMurry Meadows pluton is bimodal, with an outer margin of mafic granodiorite (59-60% SiO2) and an inner core of granite (66-69% SiO2). Extreme differentiates also occur as small isolated masses within the suite and may contain up to 76% SiO2. Both plutons are uniform in strontium isotopic composition but are different from each other, with initial 87Sr/86Sr values of 0·70719 and 0·70651 respectively. The Tinemaha pluton is both horizontally and vertically ({reversed tilde} 1000 m) zoned, with fractionation occurring both inward from the contacts and upward. The vertical trends in relative mineral proportions are not consistent with crystal settling. Both the vertical and horizontal variations in the chemical composition of 50 elements, in mineralogy, and in accessory mineral light rare-earth element zoning, are all directly relatable to side-wall crystallization which created a less-dense melt that buoyantly moved upward along the wall towards the top of the magma chamber. The different rates for diffusive heat exchange and compositional diffusion within the magma initiated the double-diffusive gradient in the magma chamber. Compositional variations in the side-wall crystal accumulation zone occur as boundary layer melts evolve, reflecting changes in the bulk convecting magma. The compositional gap in the McMurry Meadows pluton is the result of a similar but more efficient side-wall fractionation process, related to a higher proportion of melt to crystals in the initial magma and a slower rate of side-wall solidification as a result of the thermal blanket created by the enclosing Tinemaha pluton.
UR - http://www.scopus.com/inward/record.url?scp=0025227290&partnerID=8YFLogxK
U2 - 10.1093/petrology/31.3.519
DO - 10.1093/petrology/31.3.519
M3 - Article
AN - SCOPUS:0025227290
SN - 0022-3530
VL - 31
SP - 519
EP - 553
JO - Journal of Petrology
JF - Journal of Petrology
IS - 3
ER -