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GEOLOGY 235 (2015)
LAB 5 – Due at beginning of next laboratory
MAFIC VOLCANISM
INTRODUCTION
Volumetrically, basalts are the most significant volcanic rocks. They are found in both
oceanic plate (e.g. MORBS, Hawaii, Iceland) and continental tectonic environments (e.g.
Columbia River Plateau, Snake River Plain, Deccan Traps). Basalts are also the volcanic
rock type most likely to be parental to more evolved lavas such as andesites and
rhyolites. In addition, it is thought that many basalts represent primary magmas. Such
magmas have compositions that have resulted solely from melting of deep-seated source
material; and thus, the composition of a primary magma has been unmodified since the
conclusion of melting.
This lab stresses the textural diversity of volcanic rocks. You will see a number of
different basalts from various rock suites. These suites are mainly recognized on the basis
of chemical composition, or mineralogy, or texture, or all three variables. Presumably,
these differences result from different conditions of generation, transport, eruption and
cooling of molten magma.
Basalts generally have plagioclase (generally greater than An60), pyroxene and Fe- Ti
oxides as their main mineral constituents. Many contain olivine, and often contain
groundmass apatite. Alkali feldspar may be present but is subordinate to plagioclase and
is generally confined to the groundmass. The presence of cpx and opx identify tholeiite
basalts (saturated w.r.t. SiO2), while the absence of a second pyroxene and the presence
of olivine characterize alkali olivine basalts.
Assignment
In the process of studying these rocks in thin section and hand sample, please answer
questions for samples and provide the bold definitions in your lab hand-in). Note: no
petrographic report this week!
OCEAN-FLOOR VOLCANISM (MORB – Mid Ocean Ridge Basalt)
Dredging of the sea floor has revealed that basaltic lavas comprise the major portion of
the upper oceanic crust. Glasses of the ocean-floor basalts commonly contain olivine,
Mg-Cr spinel, plagioclase and clinopyroxene as their phenocryst assemblage.
AII-73-14-27A, AII-92-31-42 (Thin section only), AII-77 -76-59, AII-77-76-67, AII-7776-42, AII-73-14-36 (Hand or Thin sections). These samples show substantial evidence
of quenching, and show spectacular devitrification structures, such as spherulites -
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sketch one, and label the phases present (remember your scale bar!). What is the
phenocryst assemblage in these samples?
HOT SPOTS (Hawaii)
The Hawaiian Islands are a linear chain of oceanic islands that were produced from the
migration of a relatively fast-moving oceanic plate over a "hot spot".
HP-28 (hand sample & thin section), HP-44 (thin section), 4133 (thin section), HP-14-xx
(thin section), or CORME 235 (thin section).
These are representative specimens of a major alkali olivine basalt flows, from the
Kilauea shield volcano, Hawaii. Look at their mineralogy (and textures) and convince
yourself that this is true.
HP-23B (hand sample) & HP-23B-2, -3, -4, -5, -7, & -1 (thin sections)
These samples were collected from within the Pohina Flow (Mauna Loa). Give a possible
cause(s) for the diversity in rock types (i.e., HP-23B-4 & HP-23B-7 & HP-23B-9 vs HP23B-1).
HP-46 (hand sample) or HP-21 (hand sample): Ankaramite (Maui)
List the mineralogy (hint – what is an ankaramite?).
HP-7 (hand sample & limited thin sections, plan accordingly):
Rhyodacite (Mauna Kuwale, Oahu)
This is the most evolved member of the suite. Identify the mineralogy of the phenocrysts
in the rock as well as the suspected mineralogy of the groundmass. Is there any sign of
mineral resorption in the section and if so, what mineral(s) show it the best?
MORB
235 – 1 (limited thin sections, plan accordingly)
This specimen represents part of a pillow basalt dredged from the axis of the MidAtlantic Ridge, probably from the ca. 37°N - FAMOUS (French American Mid-Ocean
Underseas Study, 1970's) area. It shares many petrographic characteristics of typical NMORB’s. (observe the sample, no question).
L-1, L-2 and L-3
Hawaiian Islands. How can you identify the stratigraphic tops of these samples? These
samples were taken from a single flow unit. Describe the upper surface features and
based on lectures, briefly propose a reason for these changes in flow surface morphology.
KOMATIITES (Monro Township, Ontario)
Komatiites are Archean basaltic rocks, and are characterized by unusual fabric and
composition. These rocks represent peridotitic magmas. These samples are at least 2.7
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billion years old, and much of the original mineralogy has been replaced by low-grade
metamorphic minerals.
These lavas are mafic to ultramafic in composition and are characterized by spinifex
texture. This texture is thought to result from supercooling the lava upon extrusion. What
minerals would you expect to form these quench crystals? Note that these sections were
cut a bit thin.
ARC-127, 128, 130, & SPINIFEX (thin sections and one large hand sample).
Some of these samples show beautiful cumulate texture, which ones? The rocks
represents the base of a lava flow, which had an accumulation of crystals, and would
seem to imply that the lava must have been thick in order to delay cooling long enough to
allow crystal settling. What is interesting is that in most komatiite flows, only 1-2 metres
separates spinifex textures from cumulate textures.
HIGH ALUMINA OR CALC-ALKALINE BASALTS (Mt. St. Helens, Washington)
Calc-alkaline lavas are typically highly porphyritic, with calcic plagioclase the most
common mineral phase. The most common mineral assemblages in calc-alkaline basalts
are combinations of olivine, plagioclase and augite.
W-90-6 (hand sample) Muddy River porphyritic andesite. Estimate the proportions of
phenocrysts and groundmass. Is this an andesite? How would you tell?