Page 7 - i1052-5173-32-10_FY22_new

saturation nucleation threshold or autocatal-
ysis (Fig. 4D; Fu et al., 1994; Ball, 2015).
Crystal ripening and gradients in intensive
parameters, such as temperature and chemi-
cal potential, can produce banding in igne-
ous and metamorphic rocks (e.g., Thompson,
1959; Boudreau, 2011). Crystallizing granitic
plutons are hydrous, high-temperature reac-
tion vessels that stay hot and juicy over time
scales of 10 –10 years. Whether such pro-
6
5
cesses operate in these vessels is not known
but is testable with experiments.
K-Feldspar Megacrysts
Large K-feldspar phenocrysts (megacrysts)
in calc-alkaline granodiorites (Fig. 1) pro-
Figure 3. Basalt swirls in a matrix of rhyolite from Yellowstone National Park. vide an example of a reasonable field inter-
Iddings (1899) interpreted relationships such as this as clear evidence that
the basalt was melted by the rhyolite. Although experimental petrology in pretation that is contradicted by experimen-
the early twentieth century showed that this is thermodynamically unlikely, tal and analytical data. A common field
Fenner (1938) concurred with Iddings’s field interpretation and appealed to
unknown sources of energy to explain the apparently backward melting interpretation is that megacrysts were phe-
relationships. Wilcox (1944) showed that these are simply mixed magmas, an nocrysts that grew to large size early enough
interpretation that stands to this day (Pritchard et al., 2013). Width of view to be swept around by magmatic currents,
14 cm; photo courtesy of Chad Pritchard.
pile up in jams, and switch magmatic hosts
(e.g., Vernon and Paterson, 2008).
directly contradicted by experimental petrol- outside those of human experience. Granitic This interpretation is firmly ruled out
ogy (Bowen, 1928, p. 175ff.), and appealed to plutons are intruded and crystallized at depths on several grounds (Glazner and Johnson,
unknown sources of energy to explain the ranging from a few kilometers to tens of kilo- 2013), only one of which we discuss here.
conflict. There was no need; Wilcox (1944) meters, over durations of 10 to 10 years, at The phenocryst interpretation requires K-
5
7
showed that the two magmas were molten at temperatures comparable to the melting feldspar to be among the first phases to crys-
the same time and mixed, an explanation that temperature of gold, from magmas at least tallize, but a large and consistent body of
fits the field observations, physical chemis- 10,000,000 times more viscous than water. experimental data and petrographic observa-
try, and geochemistry. Human experience is not relevant to these tion of dacite lavas shows that K-feldspar is
Salt domes are another case where an obvi- conditions and can be highly misleading. the last major phase to begin crystallization
ous and long-accepted interpretation turned in a dacite (= granodiorite) magma, rarely
out to be largely incorrect. It appears self-evi- Bands in Granitic Rocks That even starting to grow before the magma is
dent from field relations that the relative buoy- Resemble Those Produced by half crystallized. At half crystallization, the
ancy of salt drives it upward through overly- Sedimentation geometric state of the magma is akin to that
ing rocks (Nettleton, 1943), and this origin of Banding comparable in scale to bedding of loosely packed fine gravel or coarse sand,
salt domes appeared in structural geology in sedimentary rocks but defined by differ- a touching framework of crystals with ~50%
textbooks for decades. However, seismic ing mineral proportions is common in pore space. Most K-feldspar crystals thus
imaging and borehole data led to recognition plutonic rocks. Such banding is generally grow from the last ~50% of liquid, which is
that the tops of many salt domes along the assumed to result from crystals settling dispersed in a tortuous network of millime-
Gulf of Mexico remained at a fixed depth from a large, slowly crystallizing magma ter-scale pores. There is no space in which
below the sea floor and that the domal shape body (e.g., Wager and Brown, 1968, p. 208ff.). large crystals of K-feldspar can grow, and
results from the flanks being depressed by A common interpretation of intersecting therefore they likely grow and recrystallize
sediment deposited in adjacent “minibasins” mineral layers (Fig. 4C), by analogy with to highly potassic compositions by a dis-
(Worrall and Snelson, 1989). Subsidence of cross-bedded sediment, is scour-and-fill by placive process akin to growth of garnets in
the minibasins is driven by the sediment load currents in a magma chamber (Gilbert, schist or authigenic halite in evaporites
and accommodated by lateral extrusion of 1906; Irvine, 1980). Magmatic liquids in (Glazner and Johnson, 2013).
underlying weak, ductile salt into domes that granitic rocks, however, are so viscous that
grow downward from a fixed roof. current velocities of tens of kilometers per Deposition of Mudstones
second would be needed to produce the tur- An example to which one’s experience
Cases Where Field Observations bulence required for erosion to form cross- with Earth-surface conditions surely ought
Lead to Reasonable Yet Questionable bedding (Glazner, 2014). to apply is the accumulation of mudstone.
or Invalid Interpretations This physical argument makes the sedi- These are assumed to be a continuous record
mentary analogy highly unlikely; cross- of quiescent environmental conditions in the
Trying to Explain the Unimaginable cutting layers in granitic rocks likely form water column directly above (e.g., Gilbert,
Astronomy and geology require contem- by other processes, such as reactions that 1895; Herbert and D’Hondt, 1990). However,
plation of time scales and length scales far involve diffusion coupled with a super- Schieber et al. (2007) showed that classroom

www.geosociety.org/gsatoday 7