TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

[118]. No volcaniclastic rocks were recovered at Site 884 [118] (Fig. 3.2). Keller
et al. [78] reported an 40 Ar- 39 Ar age of 81 ± 1 Ma for a single Site 884 basalt
sample. Site 1203 is located on the eastern flank of Detroit Seamount where 453
m of igneous basement was cored [128] (Fig. 3.2). Eighteen flow units were re-
covered at Site 1203 including tholeiitic pillow basalts, tholeiitic simple pahoehoe
flows, and alkalic compound pahoehoe flows [128]. A variety of volcaniclastic and
sedimentary rocks were recovered from Site 1203 [128] (Fig. 3.2). Duncan and
Keller [43] reported an average 40 Ar- 39 Ar age of 76 ± 1 Ma for four separate lava
flows from Site 1203. Basalts in lower portion of the cored section from Site 1203
include tholeiitic pillow basalts and tholeiitic simple pahoehoe flows but are dom-
inated by thick compound alkalic pahoehoe flows. Basalts in the upper portion
of the cored section from Site 1203 are all tholeiitic pillow basalts and tholeiitic
simple pahoehoe flows [128]. The alkalic to tholeiitic transition is between Units
18 and 19 (Fig. 3.2)
3.3.2 Volcanic history of Detroit Seamount
The tholeiitic compositions of Site 884 basalts are consistent with the shield
building stage of a typical Hawaiian shield volcano [29]. Drilling at Site 1203,
however, recovered a more extensive sequence of lavas that exhibit an upward
change from dominantly subaerial erupted alkalic basalt to submarine erupted
tholeiitic basalt, which Huang et al. [72] noted as compositionally consistent
with the transition from the pre-shield to shield stage of magmatism. The entire
Island of Hawaii is made up of five shield volcanoes and has been constructed
over ∼ 1 Myr [29]. The ∼ 5 Myr age difference between Site 1203 and 884 lavas
and the eruption environment differences between Site 884 and Site 1203 lavas
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