SAMPLES AND ANALYTICAL TECHNIQUES

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INTRODUCTION

llic di~illsilr. will lie a coii~poneiil of the tectonic sdnfics (Intci.iiiif iotial Strcriiig ('oni~iiittcc. 1994; Qipc Roberls Science 'l'cii m. IOOSSa). The stiiily ofcltist samples froiii tlic core will he an impostant means of providing insiglit into (lie tiniiii;; of up1if.t of tlic 'I'i~ti~istintti~~ctic Mountains.

Tlioli-ii~ic ij>iicous rocks of tlie lurassic ( I S 0 Mii) I:errai~liii~~~,~~ igiwoiis proviiicc (TIJP) tire wiclcsprc;~cl ;ilong the Tr;~ii.s;i~itai'c~ic Mountiiins and have the potential io provide (list inct indicators of erosion ciusing uplift of the iiioundiius. I iitli<~'l'~~a~isa~itiirctic Mo~intaiiis aij,jai.'ent lo llic C;ipe Roberts drill site the

HI

,IP is rcprcscntcd by I a ~ t i s and pymcliislic rocks o f t h e Kirkpiitrick basiilts ;incl by tliicl<

I~errardoli.~ritesills which intrude the Beacon Supergroup sedi~iic~iis iind. occasionally, the granitic basement rocks.

I n the Prince Albert Mountains. the youngest Kirkpatrick xcisalt 1;iv;i is over 150 111 thick, ant:! lias a very distinct high ' i 0 2 c l i e ~ i i i c a l c o ~ i i p o s i t i o ~ i which is unique i n the FLIP. 11' such rocks can be identified in the core they may provide precise t i ming of the initiation of uplift and denudation of [he Trt~~isiintasetic Mountains.

Here

w e

report o n

an

examination of 20 Fen'ardolerite dasts. This brief report is intended as a pilot study to the examination of FLIP clasts from older drillcore.

SAMPLES AND ANALYTICAL TECHNIQUES

Twenty three clast samples werecollected from various intervals along the length of the core. Twenty of the samples were Fen-as dolerites (Tab.

1

j, whereas the other three clasts consisted of a metasedirnent (61.57 metres below sea floor (si~bsf)). a sandstone clast (1 14.76 mbsf) and a porpliyritic intermediate McMurdo Volcanic Group lava (144.17 mbsf). All Fersar dolerite samples were examined in thin section. Nine of the larger dolerite clasts were analyzed by

XRF

for major and trace element compositions (Tab. 2) using standard techniques (see Krissek & Kyle. this volume). Two dolerites (32.82 and

DISCUSSION

i l r o u n d i n i i s \

n l l c w ( l c,::. ( l o l c ~ ~ i l c ~ , c I ~ l ( l r i i c ;III(I liiolilr ic[~liice pyio.\enc

S.;;. c q i i i ~ ~ n i ~ ~ i ~ l i i r tlolcrili.'. ~ I I ~ I I I I O ; I I I ~ i,'ryplo~'rysliillinr

~ r o i n ~ c l i i ~ a s s

C.". tiok'riu-. ilviitrrie i i l ~ r ~ ~ i o n , ~rii~~tipl~yic

mcsosla\is

s1roni:ly ileutcrie iilkrcd I I I , ~ ~ , d o l i ~ i i l ~ ~ , cliIorili/i.'d slro~~glly tlnilcric iillrrcii I I I . ~ dolcrilc

iiltcrcil c.i!. tlolnilc. t;hlori~e am1 liiolilc ~ ' L L [ I ~ : I W

~ p ~ r o x e i i c

['.g. dolcri~c. siili-ophi~ ic tcMnrc. stronp dc~utcric

i l l t e r ~ l l i o l l

c , & (Iok'rilr. Jeiilcric allcri~lioii

illcrcil 111.p. ilolci-ite. clilorite i i i u l biotile re1)l;icc pyroxenc

e.g. ilolcrilc. tleiiteric i~llercd i i i u l ci~~)locryst;illi~ie

Note: e.g.- coarse grained ( > l mm). in.&- medium grained (0.5 t o I mm).

f.g.- fine grained (<0,5 mm).

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Tali. 2 - Analyses of Fcrsar llolcritc clasls froni CRP-l.

Drp111 32.82 55 00 63.12 72 97 10<S.41 123.19 131.28 13463 137.16

Devonian Beacon Sandstone from Taylor Valley. 'I'hey considered convective hydrothermal circulation gericr:ik~l by a Fcrrar dolerite iiitrusion to be the cause of the alteration.

So

i t is apparent that hydrothcrmal sysii'ins were formed (luring the intrusion of the dolerites, ;nul i l

likely that the dolerites themselves were sub.ject 10 alteration.

No elasts of the Jurassic Kirkpatrick basalt were identified in the core, even though these rocks occur i n tlie upper reaches of the Mackay Glacier, a potential source area for sediment and detritus to the Cape Roberts dri 1Isitc.

Pebbles of possible Kirkpatrick basalt were recoyniscd din-ing~"'Ar/'^Ardatingby their Jurassic age

(W.

McIntosh, personal communication). The lack of significant Kirkpatrick basalt clasts suggests that by Miocene time erosion had reached deeper stratigraphic levels than (lie Kirkpatrick basalts. This would indicate some stability in the Transantarctic Mountains, and little or no signii'icaiit uplift and down-cutting since the Miocene, unless the ice was confined to an existing-drainage system.

Petrogi-aphically, the dolerite samples are similar to those described in the Dry Valley area. Textures typically range from ophitic to doleritic, with sub-ophitic textures predominant. The grain size is mainly 0.5 to

4

mm but finer grained varieties also occur (Tab.

1).

All samples show deuteric alteration which mainly affects the pyroxenes.

Sericitic alteration of the plagioclase is common. Several samples (e.g. 32.82, 131.28, 132.62 mbsf) show strong hydrothermal alteration and the pyroxenes are replaced by biotite, hornblende and chlorite. The main mineral phases are plagioclase, augite, pigeonite and opaque oxides with mesostasis of quartz and feldspar. The pigeonite was rarely observed to be inverted to orthopyroxene.

Overall, the nine analysed samples (Tab. 2) are geochemically typical of Ferrar dolerites from the Transantarctic Mountains (Hamilton, 1965; Gunn, 1966;

Hergtet al., 1989; Morrison &Reay, 1995). Geochemically the rocks would be classified as basaltic andesite using the TAS classification (LeBas et al., 1986). Sample55.00 mbsf has higher SiO2 and lower MgO, and is typical of granophyric material from the upper zone of a thick sill.

The two analysed samples (32.82, 131.28 mbsf), which show alteration of the pyroxenes to biotite, hornblende and chlorite, are enriched in K 0 and Rb (Tab. 2) and have been subjected to potassic alteration. Although secondary biotite has been previously noted in Ferrar dolerites, the extent of the alteration of the two samples recorded here is significant. It is possible that these Ferrar clasts are derived from a larger intrusive body which had a hydrothermal system associated with it. The occurrence of hydrothermally altered Fesrar clasts in both the Quaternary and Miocene sections of the core suggest that the volume of these rocks has to be significant. No field occurrence of such altered Ferrar dolerites and evidence of significant hydro-thermal alteration has been previously noted in the Fesrar dolerite. Craw & Findlay (1984) have described hydrothermally altered Lower Ordovician granitoids and

A C K N O W L E D G M E N T S

I a m very grateful to D r John Smellie, British Antarctic Survey for obtaining t h e 23 clast samples from the CRP- 1 drillcore. Shari Bauman and Chris Hai-pel assisted with t h e X R F analyses. Philip Kyle w a s supportedby Office of Polar Programs, N S F grant OPP-9527329. T h e XRFfacility w a s partially funded by N S F grant E A R - 9 3 16467. Reviews b y D r s John Smellie and Franco Talarico helped to improve t h e manuscript.

REFERENCES

Cape Roberts Science Team. 1998a. Background to CRP-l. Cape Roberts Project, Antarctica. Terra Antartica, 5(1), 1-30.

Cape Roberts Science Team, 1998b. Miocene Strata in CRP-l. Cape Roberts Project. Antarctica. Terra Antartica, 5(1), 63-124.

Cape Roberts Science Team, 1998c. Quaternary Strata in CRP-l. Cape Roberts Project, Antarctica. Terra A m t i c a . 5(1), 3 1-61.

Craw D. & Findlay R.H.. 1984. Hydrothermal alteration of Lower Ordovician granitoids and Devonian Beacon Sandstone at Taylor Glacier. McMurdo Sound, Antarctica. ;V. Z. J. Geol. Geophys.. 27.

465-475.

Elliot D.H., Fleming T.H., Kyle P.R. & Poland K.A.. submitted. Long distance transport of magmas in the Jurassic Ferrar Large Igneous Province. Antarctica. Earth and Planetmy Science Letters.

Gunn B.M., 1962. Differentiation in Ferrar Dolerites. Antarctica. N. Z.

J. Geol. Geophys., 5. 820-863.

Gunn B.M., 1966. Modal and element variation in Antarctic tholeiites.

Geocliimica Cosmochimica Acta. 30. 881-920.

Hamilton W.. 1965. Diabase sheets of theTaylor Glacierregion, Victoria Land, Antarctica. U S . Geological Siimey Professional Paper. 456-B, B1-B71.

Hergt J.M., Chappell B.W., Faure G. & Mensing T.M.. 1989. The geochemistry of Jurassic dolerite from Portal Peak. Antarctica.

Contributions to Mineralogy and Petrology, 102, 298-305.

International Steering Committee, 1994. Cape Roberts Project - coring for Antarctic tectonic and climatic history. AGUEOS Transactions.

75, 2-3.

LeBas M.J.. LeMaitre R.W., Streckeisen A. & Zanettin B.. 1986. A chemical classification volcanicrocks basedon thetotalalkali-silica diagram. Journal of Petrology, 27,745-750.

Morrison A.D. & Reay A.. 1995. Geochemistry of Ferrar Dolerite sills and dykes at TerraCottaMountain, south VictoriaLand, Antarctica.

Antarctic Science. 7. 73-85.