Geology topics
Publications and source records attributed to Anna Hietanen.
Tourmaline (dravite) from a layered two-plagioclase anorthosite in the Boehls Butte quadrangle, Idaho, has the following structural formula: (Na 0.55 Ca 0.17 ) 0.72 (Mg 1.96 Fe 0.57 Mn 0.01 Ti 0.04 V 0.03 Al 0.47 ) 3.08 Al 6 (BO 3 ) 3.02 Si 5.98 O 18 (OH,F) 3.35 . The indices of refraction are ω = 1.647(1) and ε = 1.621(1), and the unit cell parameters are a = 15.9425(6) and c = 7.1883(3).
In the Boehls Butte area, Idaho, three large and numerous small lenses of layered to massive anorthosite consisting of two, and locally three, types of plagioclase and minor hornblende and micas occur in aluminum silicate-rich garnet mica schist. In most of this anorthosite, andesine megacrysts with bytownite inclusions are embedded in a fine-grained groundmass of bytownite or anorthite. In places labradorite occurs instead of andesine. Some labradorite laths show Carlsbad twinning and rims of andesine around anorthite inclusions. Along the contacts, lenses of fine-grained bytownite anorthosite with some hornblende or garnet and quartz are common. These lenses could represent calcic parent rocks that were changed to two-plagioclase rocks by partial replacement of bytownite by andesine.
In the Feather River area, California, and in the McGrath quadrangle, Alaska, axinite-bearing veins occur as fracture fillings along or near the fault zones, suggesting that boron was introduced along the fractures. An unusual occurrence of axinite as a possible primary constituent of a plutonic rock is in the Russian Mission quadrangle, Alaska. The four analyzed axinites from these widely different localities and from different host rocks are surprisingly similar in chemistry and optics. All are ferroaxinites, having high iron and low manganese and magnesium contents. The number of calcium ions is very close to two, which is in agreement with the idealized formula Ca 2 (Fe,Mn,Mg)Al 2 BSi 4 O 15 (OH). The indices of refraction increase slightly with increasing FeO:MgO ratio over the small range studied.
Blades of olivine in random arrangement crystallized in a small body of serpentine just north of the Grizzly pluton as a result of contact metamorphism. Interstitial minerals are talc, chlorite, and minor amount of cummingtonite.
Comparison of the evolution of magmas in the Precambrian of southwestern Finland with that in the Paleozoic and Mesozoic of the northern Sierra Nevada brings out features that may clarify the origin of potassium-poor silicic magmas. In the northern Sierra Nevada, Paleozoic sodarhyolitic effusive rocks and associated trondhjemite represent silicic differentiates of andesitic magmas formed near a Benioff zone. These potassium-poor magmas were formed early, before thickening of the crust, and were followed by basaltic and rhyolitic magmas with normal potassium content. In southwestern Finland, where 70 percent of the area is covered by silicic and intermediate plutonic rocks, the early synkinematic intrusive masses are trondhjemitic and the later ones are granitic with eutectic ratios of quartz, plagioclase, and potassium feldspar. The latest granites are exceptionally rich in potassium feldspar. The oldest rocks, cordierite-garnet-sillimanite gneisses and interbedded metavolcanic rocks, are folded on gently plunging axes that steepen diapirically around large late-kinematic plutonic masses. Trondhjemite occurs as thin sheetlike masses parallel to the folded bedding and could not have traveled far without losing its initial heat. Therefore it seems that the trondhjemitic magma was formed at shallow depths. By analogy with the shallow depth of early magma generation in the northern Sierra Nevada, it is suggested that the trondhjemitic magmas in Finland formed at mantle depths near a Benioff zone or at the base of the early thin crust at pressures where phlogopite or biotite was stable. Later, after thickening of the crust above, potassium from the biotite was released, making the late kinematic magmas rich in potassium. A plate-tectonic model of an island-arc environment explains the coeval age of the Svecofennian and "Karelian" foldbelts and the increase of potassium with decreasing age in the extrusive and intrusive magmas.
The early magmas of the northern Sierra Nevada, calc-alkaline andesite of island-arc type and its derivatives, all low in potassium, were generated during the Devonian(?) period, possibly along an eastward-dipping sub-duction zone. These magmas could have been derived from mantle peridotite of the continental plate by introduction of water from the descending oceanic plate. Later, during the Permian(?) period, the magmas became basaltic, with potassium-rich silicic derivatives indicating anhydrous conditions and a deeper level of magma generation. Plutonism began in Jurassic time, at the end of a period of intense deformation and metamorphism. The earliest intrusive rocks are gabbro and diorite. At the end of the Jurassic period, large granitic plutons were emplaced. These grade from hornblende quartz diorite at the borders to monzotonalite at the centers. Trondhjemite occurs as the latest product of crystallization differentiation of plutonic magmas. Exchange of elements between plutonic and metamorphic rocks suggests that the plutonic magmas were composite. The partial melts of the downfolded volcanic and sedimentary rocks were modified by partial melts from the mantle and the subducted oceanic lithosphere below. Relative amounts of material contributed by each of the three sources of plutonic magma changed with time, and these changes, along with differentiation processes, were responsible for the diversity in composition of magmas.
S capolite is a common rock-forming mineral in parts of the Belt Series of Precambrian age in southern Shoshone County and adjoining parts of Clearwater County, Idaho. It is most abundant in moderately metamorphosed calcareous shaly layers of the Wallace Formation but occurs also in their highly metamorphosed equivalents and in the lowest part of the Prichard Formation. The mode of occurrence varies with distance from the Idaho batholith, with bulk composition, and with grade of metamorphism. In the northern part of the area where rocks were metamorphosed to the epidote-amphibolite facies, the highest concentration of scapolite is in layers rich in calcite, biotite, or hornblende and diopside. In biotite-rich layers interbedded with quartzite, scapolite is in round holoblasts; in carbonate granofels, crystals are euhedral to subhedral; and in hornblende- and diopside-bearing layers, small anhedral grains are common. In rocks metamorphosed to the amphibolite facies, such as diopside gneiss and calcium-magnesium-aluminum silicate rocks (here named “camalsite”), scapolite occurs in small anhedral grains. In diopside gneiss, scapolite is in thin layers that extend long distances parallel to the bedding. In camalsite, small masses exceptionally rich in scapolite are common. The mode of occurrence and the distribution parallel to the bedding suggest that scapolite crystallized from a sedimentary rock that contained saline minerals. The scapolite contains much Cl but only a little SO 3 , suggesting that halite was the chief source mineral. The local high concentrations of scapolite occur in a zone where elements have been redistributed, either because of metamorphism of dispersed or layered saline minerals with accompanying migration of chlorine or because of metamorphism of primary local masses of such minerals.
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