HR: 0830h
AN: V51I-0392    [PDF]
TI: Absence of Cryptic Zonation in Modally zoned Granitic Pegmatites: Petrogenetic Implications
AU: * Larsen, R B
EM: rune.larsen@ngu.no
AF: Rune B. Larsen, Geological Survey of Norway, Trondheim, N-7491 Norway
AU: Prestvik, T
EM: tore.prestvik@geo.ntnu.no
AF: Tore Prestvik, Norwegian University of Science and Technology, Trondheim, N-7491 Norway
AU: Jacamon, F
AF: Tore Prestvik, Norwegian University of Science and Technology, Trondheim, N-7491 Norway
AB: The Evje-Iveland and the Froland granitic pegmatite fields in South Norway share many similarities and are nearly indistinguishable in terms of their mineralogy, radiogenic isotope signature and accessory mineral assemblage. Both fields comprise REE-Nb-Ta type pegmatites with overlapping Mesoproterozoic ages close to 900 Ma and low 87Sr/86Sr ratios of only 0.7062-0.7064. However, a recent study of the REE distribution in K-feldspar (Larsen, 2002, The Canadian Mineralogist, 40, 137-151) unequivocally demonstrates that the parent melts forming the two pegmatite fields were derived from different source regions. The trace and major element distribution in feldspar follows well defined igneous trajectories during igneous evolution of the granitic melts from typical K/Rb ratios in perthite of 360 in the most primitive pegmatites to 100 at the most evolved localities. Altogether the pegmatites in both fields feature a well-defined igneous evolution from primitive to progressively more evolved compositions. The individual pegmatites are modally zoned from the margin towards the interior. The wall zone is dominated by plagioclase and perthite, accessory biotite and muscovite, and quartz. The intermediate zone (IZ) comprises metre size crystals of biotite, perthite, plagioclase, quartz and minor muscovite. The core-zone (CZ) features rafts of perthite and minor plagioclase in large masses of quartz. Given this well-defined modal zonation and the regional igneous evolution, it would be expected that the major minerals feature a cryptic zonation from relatively primitive compositions in wall zone to more evolved compositions in the core-zone. However, sampling and analysis of perthite from IZ to CZ from 20 pegmatites failed to confirm the presence of a cryptic zonation when comparing the major and minor element compositions. Key element ratios including the K/Rb, Sr/Rb, Eu/Rb and the LREE/HREE ratios that all are supposed to decrease from IZ to CZ mostly remained unchanged or were increasing. Similar to perthite, the trace element distribution in quartz change systematically from primitive to progressively more evolved pegmatites throughout the pegmatite fields. However, the analysis of trace elements in quartz from 120 localities also failed to document systematic changes in key trace element ratios (e.g. the Ti/Ge ratio) from the margin towards the interior of individual pegmatites. The absence of a cryptic zonation of major minerals from contact to core in the otherwise modally zoned granitic pegmatites is a challenge to normal petrogenetic models. Some possible models that will be discussed at the AGU 2003 Fall Meeting are: A) The modally distinctive zones of the granitic pegmatites solidify simultaneously in compositionally distinctive domains that may be maintained by double diffusive convection cells. B) Following a recent proposal by M. Taylor, the hydrous pegmatite melts transform into gels upon emplacement or during crystallization. Gels facilitate the evolution of large modally distinctive domains that crystallize simultaneously (Taylor M., 18th IMA Meet. 2002, Prog. Abstr. P. 260).
DE: 3620 Crystal chemistry
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting