hyaloclastite


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hyaloclastite

[¦hī·ə·lō′kla‚stīt]
(geology)
A tufflike deposit formed by the flowing of basalt under water and ice and its consequent fragmentation. Also known as aquagene tuff.
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has been reported from mafic hyaloclastite beds of the Sawyer Brook Member about 200 m stratigraphically above the sampled Campbell Point tuff horizon (Fyffe et al.
On Campobello Island it consists of locally pyritiferous, black to grey siltstone, lithic sandstone, shale and argillite intercalated with thick (up to several tens of metres) units that contain vesicular flows, agglomerate and/or well-developed hyaloclastite sequences.
These include abundant dark green, fine-grained, amygdaloidal, porphyritic basalt flows, and lesser pillow lava, basalt breccia and bedded hyaloclastite.
The basaltic hyaloclastite occurs in beds from 3 cm to 1 m thick, interlayered with maroon mudstone beds from 1 to 15 cm thick.
The bedded hyaloclastite (Samples ER-7, -9) has a trace-element and rare-earth-element content similar to the sparsely porphyritic basalt flows (Table 1).
1993) consists of volcanic flows, hyaloclastite and volcaniclastic sandstone and siltstone.
Hyaloclastite contains a wide range of decimetre-sized blocks in a cleaved matrix.
The other mafic sample, 30, is a hyaloclastite composed of angular, formerly glassy, fragments, devitrified to microlite around coarse vesicles.
Classification of samples with respect to rock types and alteration stage Rock type Alteration stage Unaltered Smectite/ Mixed Chlorite zeolites layer clay Basaltic Lavas 122 44 22 2 Hyaloclastites 30 21 8 3 Intrusions 24 12 6 Andesites 5 Acid rocks 25 Sediments 2 3 Total 154 122 42 11 Rock type Alteration stage Chlorite/ Epidote/ Total epidote amphibole Basaltic Lavas 42 36 268 Hyaloclastites 6 1 69 Intrusions 30 36 108 Andesites 2 2 9 Acid rocks 11 12 48 Sediments 1 1 7 Total 92 88 509 Table 2.
At present, the collection contains more than 500 samples spanning the basaltic to rhyolitic composition including lavas, hyaloclastites, intrusions, and a few sedimentary rocks of volcanic origin.
The rocks recovered include variably palagonitized glassy lavas (pillows, clinker, sheet flows) coated with manganese crusts, and hyaloclastites.
5 Ga microbial biomarkers in pillow lavas and hyaloclastites from the Barberton Greenstone Belt, South Africa: Earth and Planetary Science Letters, v.