THE PETROLOGY OF THE LOWER SERIES VOLCANICS, ODP SITE 642

7/27/2019 THE PETROLOGY OF THE LOWER SERIES VOLCANICS, ODP SITE 642

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Eldholm, O., Thiede, J., Taylor, E., et al., 1989Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 104

22 . THE PETROLOGY OF THE LOWER SERIES VOLCANICS, ODP SITE 642 1L i n d s a y P a r s o n , 2 L o t h a r V i e r e c k , 3 D a v e L o v e , 4 , 5 I a n G i b s o n , 5 A n d y M o r t o n , 6 a n d J a n H e r t o g e n 7

ABSTRACTBetween 1086.6 and 1229.4 m below seafloor at Site 642 on the Outer W ring Plateau, a series of intermediate volcanic extrusive flow units and volcaniclastic sediments was sampled. A mixed sequence of dacitic subaerial flows, an-desitic basalts, intermediate volcaniclastics, subordinate mid-ocean ridge basalt, (MORB) lithologies, and intrusiveswas recovered, in sharp con trast to the more uniform tholeiitic T-type MORB un its of the overlying upper series. Thislower series of volcanics is composed of three chemically distinct groups, (B, A2, Al), rather than the two previouslyidentified. Flows of the dacitic group (B) have trace-element and initial Sr isotope signatures which indicate that theirsource magma derived from the partial melting of a comp onent of continental material in a magma chamber at a relatively high level in the crust. The relative proportions of crustal components in this complex melt are not known precisely. The most basic group (A2) probably represents a mixture of this material with MORB-type tholeiitic melt. Athird group (Al), of which there was only one representative flow recovered, is chemically intermediate between the twogroups above, and may suggest a repetition of, or a transition phase in, the mixing processes.

INTRODUCTIONSite 642 was selected for drilling during ODP Leg 104 because it offered an excellent opportunity to test a number ofcurrently available models for the interpretation of the structureand evolution of the volcanic sequences associated with theocean-continent transit ion (OCT) at passive margins recently reviewed by (Morton and Parson, 1988). These models deal withboth general and specific distribution of continental and oceanic crust at these sites. The identification of continental material at depth beneath Site 642 as reported by Eldholm, Thiede,Taylor, et al. (1987) is of vital importance not only to the posit ioning of the OCT zone and therefore continental reconstruction, but also for an understanding of the early structuring andevolution of passive volcanic margins, in particular those along

the northwestern European margin. The northwestern outer VefringPlateau (Fig. 1A), like many northeastern Atlantic margins northof 55N, is characterized by a thick, margin-parallel sequenceof basaltic lava flows disposed in a seaward-thickening and seaward-inclined wedge readily mappable on seismic reflection profiler data.The seismic horizons which characterize these sequences appear to diverge on profiles towards the ocean basin. A shallowsection of these so-called dipping reflectors was sampled duringLeg 81 of the Deep Sea Drilling Project (DSDP) on the westernRockall Plateau in the northeastern Atlantic (Roberts, Schnit-ker, et al., 1984). These rocks were found to be plagioclase-cli-nopyroxene-olivine phyric tholeiites, and are described as highlylight rare-earth element (LREE) depleted normal mid-ocean ridgebasalt (MORB) in character by Joron et al. (1984). This appears

to indicate an advanced stage of rifting or early drifting in progress at the time of their deposition. During Leg 104, a thick se-

1 Eldholm, O., Thiede, J., Taylor, E., et al., 1989. Proc. ODP, Sci. Results,104: College Station, TX (Ocean Drill ing Program).2 Institute of Oceanographic Sciences, Deacon Laboratory, Brook Road, Worm-ley, Surrey, U.K.3 Ruhr-Universitet Bochum, Bochum, Federal Republic Germany.4 Department of Earth Sciences, University of Waterloo, Waterloo, Ontario,Canada .5 Present address: Department of Geological Sciences, Queens University,Kingston, Ontario, Canada K7L 3N6.6 British Geological Survey, Keyworth, Nottinghamshire, U.K.7 Katholieke Universitet , Leuven, Belgium.

quence of similar lithologies was recovered from Site 642 between 336.9 and 1086.6 m below seafloor (mbsf) (Eldholm,Thiede, Taylor, et al., 1987; Viereck et al., 1988); these are referred to as the upper series volcanics. However, these MORBsare slightly LREE-enriched, and their consistently parallel convex upward REE pattern suggests different degrees of partialmelting of a relatively homogeneous mantle source (Eldholm,Thiede, Taylor, et al., 1987).It is now recognized that these basalts were deposited duringa period of extensive extrusive activity which dominated themargin evolution during the final stages of rifting and earliestseafloor spreading (Eldholm, Thiede, Taylor, et al., 1987; Skog-seid and Eldholm, 1987). This extrusive period took place over arelatively short time interval of between 2 and 5 m. y , bu t com posite thicknesses of more than 6 km have been recorded for

some sections (Hinz, 1981). Equivalents of the dipping reflectorsequences onshore can exceed this, reaching 7 km in thicknessin the case of the East Greenland Plateau lavas (Nielsen andBrooks, 1981). At their oceanward limit, the smooth dippingseismic reflectors almost invariably give way to the more typically rough and hummocky oceanic basement surface. Dippingseismic horizons typically occur within oceanic crust, but atmore widely spaced and irregular intervals (Parson et al., 1986;Larsen and Jakobsdottir, 1988). On some seismic sections, awayfrom the OCT, the flow units are seen to lap onto poorly-defined structural highs, an example of which is particularly welldeveloped at the Voting Plateau (Skogseid and Eldholm, 1987).An un derstanding of the geology of this particular high was oneof the prime objectives of Leg 104. The basement floor to thissequence has been identified as a readily mapped horizon onseismic records over the western Veering Plateau , b ut similar, distinct bases are only recognized over a few other localities of dipping reflector formation elsewhere in the world (unpublishedIOS and GGU multichannel seismic data; Parson et al . , 1988).This surface on the Voting Plateau has a local nomenclature,(K), but remained unsampled until ODP Leg 104.

Migrated multichannel seismic profile NH-1 shot by the Bun-desanstalt fur Geowissenschaften und Rohstoffe (BGR) was usedto identify a site where the onlapping "feather edge" of the dipping reflector sequence could be readily drilled, and one whichwould allow a good opportunity to penetrate the underlyingbasement surface, K. The sampling procedure and analysis ofboth the sedimentary sequence (0-336.9 mbsf) and the upper se-

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L. PARSON, ET AL.

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PETROLOGY OF LOWER SERIES VOLCANICS

ANAL YT I C AL T E C HNI QUE SWherever possible, samples were taken from each of the geologicalunits recovered from the lower series, and analyzed at a number of laboratories in three countries. Petrographic work was completed at the Institute of Oceanographic Sciences (IOS), United Kingdom; the BritishGeological Survey (BGS), United K ingdom; Ruhr-Universitaet, Bochum ,(FRG); and Waterloo, Canada (CAN). Tables 1-5 are geochemical analyses of lower series volcanics, Site 642. Electron microprobe analyseswere carried out at BGS using a Link Systems energy-dispersive X-rayanalyzer attached to a Geoscan electron microprobe. Major and trace

element data were obtained by X-ray fluorescence spectroscopy andatomic absorption techniques at the University of Newcastle-upon-Tyne(using a Phillips PW1410 system) and at Midland Earth Science Associates in the United Kingdom, as well as at the Ruhr-Universitaet, Bochum, FRG. Rare-earth elements were determined by neutron activationanalysis at the University of Waterloo (Canada), Imperial College ofScience and Technology, Reactor Centre, (UK), and at the KatholiekeUniversitet, Leuven, Belgium. Isotope analyses were obtained from theDepartment of Earth Sciences, Oxford University, United Kingdom.Methods of isotopic analysis are discussed by Taylor and Morton (thisvolume). A blank instead of a value indicates no analysis, rather thanzero content. LAB: analyzing laboratory (see below); D (MBK): depthin meters below reflector K for extrusives. Geochemical analysis sites areidentified in Tables 1-4, and techniques are discussed in the captions.The highly altered state of many of the samples coupled with a locally poor recovery prevented a full analysis being undertaken on every

unit. Regrettably, no interlaboratory standardizations were performedbut the relative accuracy of the mixed data set appears acceptable andconsistent throug hout the range of analyses. Flows F106 and F i l l were

not analyzed due to a lack of satisfactory sample material. Of the remaining flows, analysis of major- and trace-element concentrations confirmed the high degree of alteration throughout (mean loss on ignitionwas approximately 5%, mean K 2 0 and CaO were 1.7% and 4 .5 % , respectively). Nevertheless, the results of all the analyses carried out appear in Tables 1-3, without differentiation between acceptable and non-acceptable alteration states. It would be fair to state that in rigorouscomparative work, the majority of these samples would be treated withextreme care or discarded. In practice, however, there are so few data forthese rocks that they are all included in the discussion.COMPARISON OF L OWE R SE RIE S GROUPS Al ,A 2 A N D B

P e t r o g r a p h i c a l l y t h e Gr o u p B u n i t s a r e c o m p o s e d o f p e r l i t i cg la s sy to mic roc rys ta l l ine dac i t e s , p l ag ioc la se -hype rs thene phyr ic ,w i t h t o t a l p h e n o c r y s t c o n t e n t b e t we e n 2 a n d 1 0 % . P l a g i o c l a s ep h e n o c r y s t s a r e w i t h f ew e x c e p t i o n s we a k l y n o r m a l l y z o n e d b y -t o wn i t e s , w i t h a me a n c o mp o s i t i o n o f An 8 0 and rang ing be tweenA n 6 4 a n d A n 8 4 . I n t e r s t i t i a l p l a g i o c l a s e l a t h c o mp o s i t i o n s a r ea r o u n d An 7 2 . Py roxene compos i t ions f a l l ma in ly wi th in hype r -s thene l imi t s (F ig. 2 ) , be tween E n ^ an d En 4 6 (Fig. 2 and Table 4) .Ad d i t i o n a l p r o b e a n a l y s e s o f g l a s s ma t r i c e s a r e i n d i c a t e d f o rf l o ws F 1 0 9 a n d F 1 1 7 i n T a b l e 4 . P o o r l y t w i n n e d c o r d i e r i t e p h e n o c r y s t s i n Gr o u p B ma k e u p l e ss t h a n 1 % i n p h e n o c r y s t c o n ten t o f f lows F l 16 , F l 17 , and D 4 . Gr ou ps A l a nd A 2 a re r ep re sen ted by mic roc rys ta l l ine l i tho log ie s wi th l i t t l e g la s sy ma t r ix .G l o me r o c r y s t s o f b y t o wn i t e a n d c l i n o p y r o x e n e d o mi n a t e t h ep h e n o c r y s t p h a s e . No e x a mp l e s o f c o r d i e r i t e we r e i d e n t i f i e d .

Table 1. Major-element analyses of lower series volcanics, Site 642. All analyses are included despite high levels of alteration . La bora tory key asfollows: UK A = University of New castle-upon-Tyne; UKB = Mid land Earth Science Associates, University of Nottin gham ; CAN = University of Waterloo, Canada; FGR = Institut fur Mineralogie, Bochum; LOI = loss on ignition.Unit La b Sample D(MBK) SiQ 2 T i 0 2 A 1 2 0 3 F e 2 0 3 FeO FeO T MnO MgO CaO N a 2 0 K 2 0 P 2 0 3 LO IF107F107F108F108F108F108F109F108F109F109F109F110F112F113F113D4D4D4F114F114F114F115F115F115F116F116F116F116D5D5F117F117D6F118F118F119F120F121F121D7D7

UK BU KAUK BCA NC A NUK ACA NCA NFG RUK BCA NUK BUK BFG RUK AFG RUK AUK BC ANUK AC ANFG RUK ACA NCA NC A NC ANUK AUK AFG RCA NUK AUK AFG RUK BUK AUK BFG RUK AFG RUK A

97-1- 130-13297-1- 133-13597-2- 129-13198-1- 48-5098-1- 58-6098-1- 60-6298-1- 147-14998-2- 16-1898-2- 69-7298-2- 77-7998-2- 81-8399-1- 5- 799-2- 45-47100-1- 31-33100-1- 118-120101-1- 98-100101-1- 109-111101-2- 25-27102-1- 13-15102-1- 20-22102-1- 33-35102-1-2 146-40102-2- 4-6102-2- 23-25102-2- 68-70102-2- 94-96102-2- 113-115102-2- 114-115104-1- 18-20105-2- 84-86106-1- 33-35106-1- 86-88107-2- 82-84108-2- 51-53108-2- 55-57109-1- 17-19109-1- 51-52109-2- 79-81109-2- 90-92110-1- 28-30110-1-44-46

12.0012.8014.9016.2016.5016.8017.5017.7018.0018.3018.6020.0227.8030.6032.1054.5054.9055.0058.2058.5058.8060.5060.8061.4061.7082.8083.00103.50103.60109.80112.50118.00118.20

59.0559.9061.3159.4058.6061.1053.0057.5061.1059.0358.3061.0363.7867.2060.4063.0064.5059.4357.0049.0048.0061.6053.2060.8059.3054.5060.0061.9051.8048.9059.1060.0049.4048.9049.9749.5047.0251.7055.5054.2051.90

1.281.261.321.401.201.281.331.411.171.261.281.211.381.201.261.201.151.451.311.441.431.421.361.261.301.291.121.241.201.071.371.261.501.181.191.171.241.511.321.331.00

15.6915.2015.2217.1014.8014.8016.1017.2014.6114.5815.3014.4916.8514.2516.2014.6814.4017.7719.1017.7017.5019.2616.5018.3017.9016.0013.9014.2013.3013.2817.3015.3018.2017.1116.9717.6016.6614.8716.1015.5515.80

10.289.768.216.959.679.347.647.613.2210.439.989.135.794.889.442.657.9310.051.4514.9011.700.2213.800.473.159.098.839.2413.705.535.039.529.487.2310.4111.3013.647.9310.207.7310.90

5.79

1.625.18

0.42

6.017.56

0.62

7.66 12.64

2.59 9.10

3.42 10.639.64

0.060.110.200.070.130.120.410.120.130.110.210.130.060.050.060.100.100.060.020.120.090.010.120.010.010.040.140.150.230.210.110.160.160.160.180.120.120.140.100.270.24

1.601.761.000.811.541.611.010.981.591.751.471.500.620.430.881.651.571.630.352.402.020.162.980.110.912.761.661.986.378.431.232.085.195.215.146.115.953.073.002.333.39

2.262.443.113.802.782.887.664.953.022.902.592.814.641.623.923.583.484.573.253.503.183.612.843.144.044.411.193.3910.409.844.643.997.527.487.606.756.515.665.576.818.04

2.022.182.162.592.222.312.552.912.212.082.132.632.351.842.362.382.822.732.482.061.681.861.981.982.682.493.083.132.522.092.882.883.612.983.273.173.102.683.122.682.68

1.031.140.490.541.711.610.450.481.751.291.470.811.382.940.520.770.820.568.240.640.408.590.348.604.500.370.191.050.220.112.640.630.341.661.571.200.862.142.161.331.02

0.090.140.190.050.180.280.210.090.200.180.130.180.610.160.110.190.180.190.240.230.250.290.160.280.260.220.190.190.090.100.160.230.130.180.180.150.170.190.180.160.11

6.795.707.030.166.934.928.546.006.157.545.663.323.001.775.855.3513.805.624.235.858.235.082.830.254.623.162.253.242.004.622.172.27

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L. PARSON, ET AL.Table 2. Trace-element abundances of lower series volcanics, Site 642. Laboratory key as for Table 1.Unit Lab Sample D(MBK) Cr Co Ni Cu ZnF107F107F108F108F108F108F109F109F109F109F109F110F112F113F113D4D4D4F114F114F114F115F115F115F116F116F116F116D5D5F117F117D6F118F118F119F120F121F121D7D7

UK BUK AUK BC ANCA NUK ACA NC A NFG RUK BCA NUK BUK BFG RUK AFG RUK AUK BC ANUK ACA NFG RU K ACA NCA NCA NCA NUK AUK AF G RCA NUK AUK AFG RUK BUK AUK BFG RUK AF GRUK A

97-1- 130-13297-1- 133-13597-2- 129-13198-1- 48-5098-1- 58-6098-1- 60-6298-1- 147-14998-2- 16-1898-2- 69-7298-2- 77-7998-2- 81-8399-1- 5- 799-2- 45-47100-1- 31-33100-1- 118-120101-1- 98-100101-1- 109-111101-2- 25 -27102-1- 13-15102-1- 20 -22102-1- 33-35102-1-2 146-40102-2- 4- 6102-2- 23-25102-2- 68-70102-2- 94-96102-2- 113-115102-2- 114-115104-1- 18-20105-2- 84-86106-1- 33-35106-1- 86-88107-2- 82-84108-2- 51-53108-2- 55-57109-1- 17-19109-1- 51-52109-2- 79-81109-2- 90-92110-1-28-30110-1-44-46

12.0012.8014.9016.2016.5016.8017.5017.7018.8018.3018.6020.0227.8030.6032.10

54.5054.9055.0058.2058.5058.8060.5060.8061.4061.70

82.8083.00103. 50103.60109. 80112. 50118. 00118. 20

117.00 116. 00 1 5. 0 0159. 00 65. 00 1 0. 0 0119. 00 76. 00 1 2. 0 0

165.00

137.00139.00117. 00154.00116.00160.00116.00153. 00154.00

120. 00186. 00

165. 00461.00362.00166.00595.00229.00250.00300. 00261.00i 7 0. 0 0233.00200.00285.00

5.002.003.0012.0013.0010.00

100.00165.0057.0059.0061.0034.00

102.00119.00158.0028.0034.0082.00

270.00197.00276.0020.0020.0023.00

31.0026.0030.00279.00228.00 17.00

16.0028.0014.00

10. 00 1 2. 0 0 122.00 84 .00 143. 00 82 .00 285. 00 1 9 . 0 0 31 . 0 0 12. 00 2 8. 0 0

58.00136. 00 7 .0021.00 14.0013. 00 8.0012. 00 126.00123.00 87.0071.00 148. 00132. 00 4 8. 0 051.00 251. 00258. 00 16. 0019. 00 30. 00 407.00 390.00 2 1. 008 4 . 0 0177. 006 5. 0 066.008 7 . 0 05 9. 0 07 8. 0 0

10.002 9. 0 03 .0015. 007 .0013. 0018. 00

11. 0017. 009.0018. 0018.0012.0016. 00

2 1. 0 011. 0030.0015. 0021.0013. 0016. 00

112. 00109.0075.0078.00117. 00115.007 3 . 0 0

8 3. 0 02 6. 0 0120.002 0. 0 063.007 0. 0 016.00

141. 00214. 00159.00213. 00212. 00187.00230.00

4 7 . 0 0118.004 3. 0 031.004 8 . 0 04 3 . 0 06 7. 0 0

277.00320. 00270.00324. 00263.00296.00331. 00

21.002 4. 0 019. 002 2. 0 017. 002 0. 0 024.00

2 7 . 0 03 4. 0 0

16.003 2. 0 0

506.00 2 5 . 0 0516.00 2 7. 0 0530.00 22.00 20. 00434.00 21. 0 0 2 0 . 0 0429.00 2 6 . 0 0203.00 85 .00 34 .00 25 .00 26 .00 128. 00 25 .00 134. 00 5 1 . 0 0 363. 00 24. 00 28 . 00 86.00 24. 00

6 4 . 0 06 8 . 0 0 3.0010.00 16. 0010. 00 31.0015. 00 139. 00125. 00 201.002 1 . 0 0 230.00115. 00 85.003 9. 0 0 322. 00345.00 22.002 2. 0 0 189.00 81 . 00 2 0. 0 0

6 3. 0 0127.002 6 8. 0 016.005 0. 0 053.CSO

10. 0058.00163. 0015. 006 3. 0 065.00

112.00113.00111. 0091.004.002.00

183.0065.00121.0042.0032.0034.00

292.00 20.00 26.0068.00 3.00 48.0067.0 0 1.00 17.00 12.0010.00

61.00206.00166.00261.00165.00235.0089.0094.00108.00152.00

15.0063.0035.0041.0042.0067.0031.0029.0035.007.00

14.0076.0011.0011.0012.004.0015. 006.0016. 006 .00

16. 0086.003 0. 0 02 1. 0 02 5 . 0 02 8 . 0 02 8. 0 014. 0014. 0017. 00

122. 00158.00129. 00109.00115.00101.00125. 00156. 00120. 00106. 00

5 4. 0 08.004 4 . 0 04 6. 0 03 0. 0 02 4. 0 080.0079.0052.004 0 . 0 0

213. 00107.00185. 00144.00160.00165. 00198.00158.00145. 00136. 00

4 7. 0 04 1. 0 03 6. 0 04 5. 0 04 1. 0 04 7. 0 04 5. 0 04 3. 0 04 2. 0 03 1. 0 0

306. 009 7. 0 0127. 00126.00151.00126.00194.00223. 00146.00130. 00

2 0. 0 04 .006.008.008.007.0012. 0014. 009.008 .00

29.0065.0049.0051.00158.0044.00 280.0050.00

242.00343.00344.00382.00

36.00 20.0028.00 10.0014.00 8.00 12.00

42.00 12.0023.00 4.00

No clear indication of tectonic setting of magma types is evident from the variations in clinopyroxene compositions such asproposed by Nisbett and Pearce (1976). A fairly broad spread ofanalyses throughout volcanic arc and within plate basalts is indicated without clear distinctions between upper and lower series samples (Fig. 3).The major, trace and rare-earth characteristics of the groupshave been summarized in Eldholm, Thiede, Taylor, et al. (1987),but means and ranges have been here recalculated to incorporateadditional analyses from more altered lithologies. These calculations appear in Table 5.Trace-element chemistry of the Group B lavas can be used todifferentiate them from Grou ps Al and A 2 (Tables 1 and 2, andFig. 4). Zirconium levels in f lows F106-F117 (Group B) rangebetween 251 and 363 ppm (Al and A2: Zr = 126-306 ppm ),Group B levels of Vanadium (116-203 ppm) contrast with Aland A2 (166-300 ppm), and Chromium (B: 58-177 ppm) differentia tes A2 (165-235ppm) and Al (89-94 ppm).Concentrations of incompatible trace elements niobium, zirconium, Strontium, and Rubidium fur ther suppor t a subdivision of the lower series lavas into the three chemical gro ups. TheZr/Nb plot (Fig. 5A) indicates the relative enrichment of theGroup B lavas in both Zr an d Nb with respect to those of Gro upA2 and a clear separation and clustering into their respectivedistr ibutions. This separation is bridged by values for F121, thesole sampled representative of Group A l. A plot of Zr againstY (Fig. 5B) again clearly separates Grou ps B, A2, and A l , a ndconfirms the closer affinity of the Group A2 lavas with the upper series lithologies (hachured in Figs. 5A and 5B) than withGroup B lavas. Zr/Y ratios as indicated on the plot suggest amixing trend between Group A2 and the uppe r ser ies rocks; R b/Sr and Zr/Rb plots (Figs. 5C and 5D) confirm this trend, de

spite wide spreads in both the rubidium values (10-200 ppm)and more surprisingly the yttr ium values (25-125 ppm). Ternarydiscr iminant plots such as the Hf /T a/T h diagram indicate theirconcentration at the Thorium-rich limit of the volcanic arc basaltic field (Fig. 6).LREE-enr iched pat terns of MORB-normalized concentrations are a clear and consistent characteristic of the lower seriesflows (Fig. 7). They compare well with those published by Morton et al. (1988) for the similar lithologies recovered from deepdrilling in the Rockall Trough (Fig. 7), but con trast with th e flatter MORB patterns of the upper series (Viereck et al. , this volume).Isotope geochemistry of the lavas presented by Taylor andMorton (this volume) serve to reinforce the tr ipartite division ofthe lower series. 2 0 7P b /2 0 4Pb plots against 2 0 6P b / 2 0 4Pb for theupper series units show a strong correlation with the North Atlantic MORB field. The Group A2 sample (F119) also fallswithin this f ield, confirming its closer affinities with the upperseries. Pb-iso tope ratio plots for Al and B samples fall outsid ethis f ield, and occupy a steep trend away from that of theMO RB rocks . Ini t ia l 87 Sr/ 86 Sr ratios for Group B range between0.71225 and 0.71309 (mean of 0.71218), contrasting with thelower values , 0.70947, of the Grou p A2. G roup A l (one sample,0.71097) again lies almost exactly between the distr ibutions ofA 2 and B.

DISC USSIONThe presence of cordierite as a phenocryst phase in at leastthree of the Group B units is unusual , but not unique. Pa-laeocene dacites recovered from the northern Rockall Troughhave yielded a str ikingly similar petrography and composition(Morton et al. , 1988) but they are rare in oceanic or transitional

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PETROLOGY OF LOWER SERIES VOLCANICSTable 3. Rare-earth element abund ances, lower series volcanics, Site 642. Laboratory key as for Table 1, except FGR = Katholieke Universitet,Leuven, Belgium.

Unit Lab Sample D(MBK) La Tb Nd Gd Th Ta Sm Yb Lu Eu Ba Hf Ce Ho U W12.00 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _12.80 23.00 0.78 28.50 12.20 1.16 5.66 3.46 0.60 0.92 7.23 63.50 1.43 48 .0014.90 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _16.20 0.43 16.56 11.58 1.36 2.58 1.35 0.17 1.32 149.91 7.99 4 2.54 16.50 1.36 38.33 9.30 13.04 1.18 8.35 5.21 0.75 1.21 301.49 7.02 79.01 16.80 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _17.50 1.35 41.80 6.98 13.31 1.29 9.10 6.27 0.90 1.55 129.77 7.39 86.24 17.70 0.58 27.82 3.36 14.45 1.45 5.18 2.80 0.36 1.49 154.94 7.71 63.68 18.00 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _18.30 _ _ _ _ _ _ _ _ _ _ _ _ _ _18.60 0.78 26.74 13.35 1.42 6.11 3.30 0.46 0.87 7.43 61.11 20.02 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _27.80 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _30.60 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _32.10 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 44.30 1.07 46.60 14.50 1.34 9.11 4.23 0.65 1.48 5.70 107.00 2.27 2.90 14.60

54.50 1.70 49.31 10.07 15.80 1.42 10.19 8.24 1.23 1.73 259.47 8.52 105.32 54.90 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _55.00 1.58 53.53 11.21 14.76 1.33 10.85 5.22 0.72 1.73 62.14 8.07 108.36 58.20 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _58.50 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _58.80 2.00 56.55 14.98 1.37 12.48 9.06 1.27 1.74 172.26 8.29 121.95 60.50 1.69 54.63 10.99 15.02 1.34 11.21 8.04 1.19 1.84 161.76 7.98 119.84 60.80 1.29 42.56 13.65 1.20 10.36 4.86 0.71 1.43 128.37 7.36 88.14 61.40 1.16 39.80 12.17 1.11 9.22 4.58 0.67 1.35 374.64 6.74 86.20 61.70 36.40 1.41 41.90 12.50 1.22 7.98 5.24 0.65 1.39 7.37 91.70 5.18 2.80 4.93 0.97 8.14 1.56 0.25 3.28 4.43 0.69 0.89 2.15 8.25 1.49 0.23 24.1082.80 1.03 37.69 16.32 1.82 7.95 5.55 0.76 1.66 8.71 87.35 83.00 53.60 1.61 56.40 16.00 1.66 11.10 5.06 0.83 1.72 9.36 129.00 4.77 11.50 1.14 9.82 1.47 0.25 3.93 5.41 0.80 1.35 3.56 14.70 1.74 0.37 12.80103.50 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _103.60 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _109. 80 20 .00 1 .2 0 2 4. 2 0 5. 25 0. 55 5. 67 5. 69 0. 74 1. 50 4. 52 4 9. 5 0 2 .3 8 1 .4 1 112.50 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _118.00 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _118.20 30.70 1.24 36.10 8.42 1.01 7.67 5.29 0.74 1.62 7.90 75.80 2.26 2.13 16.70 17.20 0.89 22.10 4.58 0.53 4.84 3.04 0.50 1.36 3.70 42.10 1.40 1.17 15.10

F107F107F1 0 8F1 0 8F1 0 8F108F1 0 9F1 0 9F1 0 9F1 0 9F1 0 9F1 1 0F1 1 2F1 1 3F1 1 3D 4D 4D 4F114F1 1 4F1 1 4F115F115F1 1 5F1 1 6F116F1 1 6F1 1 6D 5D5F117F117D 6F1 1 8F1 1 8F1 1 9F1 20F1 21F1 21D 7D7

U K BU K AU K BC A NC A NU K AC A NC A NF G RU K BC A NU K BU K BF G RU K AF G RU K AU K BC A NU K AC A NF G RU K AC A NF G RC A NC A NU K AU K AF G RC A NU K AU K AF G RU K BU K AU KBF G RU K AF G RU K A

97-1- 130-13297-1- 133-13597-2- 1 29 -1 3 198-1- 4 8 - 5 098-1- 5 8 -6 098-1- 6 0 - 6 298-1- 147-14998-2- 1 6 -1 898-2- 6 9 - 7 298-2- 7 7 - 7 998-2- 8 1 -8 599-1- 5- 799-2- 4 5 -4 7100-1- 3 1 -3 3100-1- 1 1 8 -1 20101-1- 9 8 -1 0 01 0 1 -1 -1 0 9 -1 1 1101-2- 25-27102-1- 13-15102-1- 2 0 - 2 2102-1- 3 3 -3 5102-1-2 146-40102-2- 4-6102-2- 2 3 - 2 5102-2- 68-70102-2- 94-96102-2- 113-115102-2- 114-115104-1- 1 8 -20105-2- 84-86106-1- 3 3 -3 5106-1- 8 6 -8 8107-2- 82-84108-2- 51-53108-2- 55-57109-1- 1 7 -1 9109-1- 5 1 -5 2109-2- 79-81109-2- 90-921 1 0 - 1 - 2 8 - 3 01 1 0 - 1 - 4 4 - 4 6

MgSiO: F e S i 0 3Di DiopsideEn End iops ideSa Sal i teF s F e r r o s a l i t e

H d H e d e n b e r g i t eF h F e r r o h e d e n b e r g i t eAu AugiteF a F e r r o a u g i t e

Mp Magn es ium Pigoen i t eI p I n t e r m e d i a t e P i g e o n i t eFp Fe r r i f e rous P igeon i t e

Figure 2. Pyroxene CaSi0 3/FeSi03/MgSi0 3 discriminant plot. Circles = Group B, Group A2 not analyzed. Triangles = Group A l; filled triangles = dykes D4 and D7. Shaded field covers analyses ofupper series and D5 and D6.regimes outside of arc volcanism. The two sites in the NE Atlantic where the mineral association has been identified are c omp arable, each suffering two separate phases of superimposed riftingand sedimentation. These unusual l i thologies found within therifts may have formed as a result of melting of upper crustal

sediment adjacent to the rising magma source which eventuallysupplied the material for both the dipping reflector sequenceand the new ocean floor. The sediments appear to have accumulated during or after an early failed rifting attempt (and in theexamples discussed here the development of sites of Mesozoic

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L. PARSON, ET AL.Table 4. Representative microprobe analyses of lower series samples,phenocryst phases, and glass mesostases.

S i 0 2A 1 2 0 3F e OM gOC a ON a 2 0K 2 0T i 0 2M nOp 2 o 5C r 2 0 3TotalA nA bOr

S iO sA 1 2 0 3F e OM gOC aON a 2 0K 2 0T i 0 2M nOp 2 o 5C r 2 0 3TotalWoEnFs

F112642E 99-3-13-15

Plag(core)49.0733.410.360.0016.442.010.190.000.070.000.00

101.5581181D4

642E 101-2-138-140O px

49.633.2426.8017.740.500.290.000.830.210.000.4199.65

15445

F117642E 106-2-94-96

Plag47.4532.470.520.0016.891.800.120.000.000.100.1399.4883161F116

642E 102-2-1023O px50.843.5924.01317.460.680.720.130.570.300.040.4698.80

25642

F109642E 98-2-62-64

Glass66.0711.427.381.232.361.121.251.300.090.000.0292.24

F117

642E 106-2-94-96O px

49.002.7725.0118.050.480.190.000.700.460.000.3997.05

15643

F117642E 106-2-94-96

Glass70.0513.951.950.002.873.161.190.960.240.030.0894.48

D4

642E 101-2-138-140Cord50.3333.417.179.350.020.280.130.000.360.000.00

101.04

F117642E 106-2-94-96

Glass68.2713.084.790.542.913.091.151.280.120.150.0595.43

F116

642E 102-2-102-103Cord49.3033.227.189.110.000.190.150.070.120.000.0799.41

F117642E 106-2-94-96

Cord49.9832.778.009.050.110.020.220.000.090.100.00

100.34

basins) and provided a cover for the subsequent melting phase.The migration of the magmatic activity westward to the finallysuccessful Tertiary separation generated progressively less silicicand trace-element-enriched magmas.The geological setting of Site 642 at the Wring Plateau is onthe upper f lanks of a basement high, the surface of which hasnow been identif ied as volcanics with a continental component(the Group B lavas). The tholeiitic f low basalts which act as theimmediate "cover" to this high are recognized as a sequence ofsubaerially extruded lavas immediately preceding or contemporaneous with the inception of seafloor spreading. The peralumi-nous nature of these lower series lavas, although not unusualfor volcanic arc basalts (VAB) and documented for sites elsewhere (Clemens and Wall, 1984; Ambruster, 1985), is almostunique in marginal r if ting environments. The only other locality at a passive continen tal ma rgin was recently identified in thenorthern Rockall Trough by Morton et al. (1988). The geological framework is similar. A failed rift of Mesozoic age (Robertset al. , 1984; Talwani and Eldholm, 1977) was invaded by volcanics during a resurgence of volcanism associated with a laterattempt at r if ting (in both cases, again, a Tertiary event) . In theRockall example, the failed rift is obvious as the Rockall Trough,but a t the Wring Plateau the magnitude of the basement highhas perhaps complicated the setting. The Wring Basin to theeast is a major intracontinental r if t feature and the intermediate

volcanism could have been associated with the development of amagma source within or on the flanks of this basin. Anotherpossibility is that the volcanism developed during the intervalbetween anomalies 24/25 and 23, and marked a temporary hiatus in the formation of the Norwegian Sea Basin. The successfulseparation finally took place to the west of both of these sites,but the rif ted, thinned, and weakened continental basement ofthe developing margin was subject to intrusion and volcanism.The volcanism took place following a buildup of a differentiating magma source, which melted and assimilated adjacent continental crust.The intermediate composition of the lower series volcanicswould be one likely to cool and solidify under subaerial conditions close to the source, and therefore unlikely to be responsible for the major par t of the Wrin g Plateau b asement high. Evidence of structural control of the Wring Plateau at depth is notresolved on existing geophysical data. A model favoring attenuated and probably faulted basement blocks beneath site 642 cutby intermediate volcanics requires testing with further deep seismic reflection profiling.

C O N C L U S I O N S1. The lower series volcanics can be subdivided into threechemically distinct Grou ps, A l , A2 , and B. Two of the threeGroups, Al and A2, are related to the third by varying degrees

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PETROLOGY OF LOWER SERIES VOLCANICS

T iQUpper Series All da ta > 2cro O ne or more data < 2< j

L O W E R S E R I E SA F121 D5x 642D

WP A

V A B - W P T - W P A

MnOFigure 3. Minor element geochemistry of clinopyroxenes from the basalts of both upper and kower volcanic series, plotted on the discriminant diagram formulated by Nisbett and Pearce (1976). Elementalabund ances determined by electron micro probe . OFB = ocean floor basalt; WPA = within-plate alkali;VAB = volcanic arc basalt; WPT = within-plate tholeiite.

o f mi x i n g o f a M O R B ma n t l e s o u r c e w i t h a me l t o f i n t e r me d i a t e c o m p o s i t i o n , wh i c h wa s p r o b a b l y g e n e r a t e d f r o m a c o mb i n a t i o n o f p r i ma r y me l t a n d c o n t i n e n t a l r o c k s a d j a c e n t t o t h i ss o u r c e .

2 . This pa rage nes i s i s s imi la r to tha t invoke d fo r a l i tho log i -c a l a s s o c i a t i o n i d e n t i f i e d f r o m d e e p d r i l l in g in t h e n o r t h R o c k a l lT rou gh . Th i s f a i led r if t t o the wes t o f the Un i t ed K ing do m has as imi la r s t ruc tu ra l and geo log ica l f r amework to tha t o f the Vr i r ingP l a t e a u , b u t i s w i t h o u t a p r o m i n e n t b a s e m e n t s t r u c t u r a l h i g hsuch a s fo rms the co re o f the P la teau .

ACKNOWLEDGMENTSWe thank a number of people for discussions an d critical reviews ofthis paper, especially Joe Cann and Peter Oakley at Newcastle University, U.K., Doug Masson and other colleagues at IOSDL, K. Seifert,and J. Fitton are thanked for reviews of an early draft. Emma Woodward and Julie Cobley are acknowledged for their drafting work on thefigures and Margaret Campbell and Annie Williams for typing the textunder difficult conditions.

REFERENCESAmbruster, T. 1985. Fe-rich cordierites from acid volcanic rocks, an optical and x-ray single crystal structure study. Contrib. Mineral. Pet-rol., 91:180-187.Clemens, I. D., and Wall, V. J., 1984. Origin and evolution of a pera-lumino us silicic ignimbrite suite; the Violet Town volcanics. Contrib.Mineral. Petrol., 88:354-371.Eldholm, O., Thiede, J., Taylor, E., et al., 1987. Proc. ODP, Init.Repts., 104: College Station, TX (Ocean Drilling Program).Hinz , K., 1981. A hypothesis on terrestial catastrop hies. Wedges of verythick ocean and dipping layers beneath passive continental mare-

ginstheir origin and palaeoenvironment significance. Geol. Jarb.,E22:3 -28 .Joron, J . L., Bougault , H., Maury, R. C , Bohn, M ., and Desprair ies ,A., 1984. Strongly depleted tholeiites from the Rockall Plateau margin, North Atlantic: geochemistry and Mineralogy. In Roberts , D.G., Schnitker, D., et al., Init. Repts. DSDP, 81: Washington (U.S.Govt. Printing Office), 783-794.Larsen, H. C , and Jakobsdo ttir, S., 1988. Distrib ution, crustal properties and significance of seawards-dipping sub-basement reflectorsoff east Greenland. In Mo rton, A. C , and Parson, L. M. (Eds.) ,Early Tertiary volcanism an d the opening of the NE Atlantic: Spec.Publ. Geol. Soc. London, 39:95-114.Mo rton, A . C , a nd Parson , L. M . (Eds.) 1988. Early Tertiary volcanism and the opening of the NE Atlantic. Spec. Publ., Geol. Soc.London: 39 .Morton , A. C , Dixon, J . E., Fi t ton, J . G., Maclntyre , R., Smythe, D.K., and Taylor, P. M., 1988. Early Tertiary volcanic rocks in well163/6-1A, Rockall Trough. In Morton , A. C , and Pa r son , L . M.(Eds), Early Tertiary volcanism and the opening of the NE Atlantic.Spec. Publ., Geol. Soc. London, 39: 293-308.Nielsen, T. F. D ., and B rooks, C. K ., 1981. The east Greenland riftedcontinental margin: an examination of the coastal flexure. J. Geol.Soc. London, 138:559-568.Nisbett, E. G., and Pearce, J. 1976. Clinopyroxene composition inmafic lavas from different tectonic settings. Contrib. Mineral. Pet-rol., 63: 149-160.Parson, L. M., and ODP Leg 104 Scientific Party, 1988. Dipping reflector styles in the NE Atlantic Ocean. In Morton , A. C , and Pa r son ,L. M. (Eds.), 1988. Early Tertiary volcanism and the opening of theNE Atlantic. Spec. Publ., Geol. Soc. London, 39:57-68.Parson, L. M., Masson, D. G., Miles , P . R., and Pel ton, C. D., 1986.Structure and evolution of the Rockall and East Greenland continen-

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tal margins. Report of work undertaken by IOS in the period up to1985. Institute of Ocean ograph ic Sciences Rept. 233.Roberts , D. G., Backman, J . , M orton, A . C , Murray, J. W., and Keene,J. B., 1984. Evolution of volcanic rifted m argins: synthesis of Leg 81results on the west margin of Rockall Plateau. In Roberts , D. G.,Schnitker, D., et al., Init. Rept. DSDP, 81: Washington (U .S. Govt.Printing Office), 883-912.Roberts, D. G., Schnitker, D., et al., 1984. Init. Rept. DSDP, 81: Washington (U.S. Govt. Printing Office).Skogseid, J., and Eldholm, O., 1987. Early Cenozoic crust at the Norwegian continental margin and the conjugate Jan Mayen Ridge. J.Geophys. Res., 92: 11471-11491.

Talwani, M., and Eldholm, O., 1977. Evolution of the Norwegian-Greenland Sea. Geol. Soc. Am. Bull., 88:969-999.Viereck, L. G., Taylor, P. N., Parson, L. M., Morton, A. C., Hertogen,J., Gibson, I. L., and the ODP Leg 104 Scientific Party, 1988. Origin of the Palaeogene Wring Plateau volcanic sequence. In Morton ,A. C , and Parson , L. M. (Eds.), Early Tertiary volcanism and theopening of the NE Atlant ic . Spec. Publ., Geol. Soc. London, 39 :69-83 .Date of initial receipt: 19 October 1987Date of acceptance: 20 June 1988Ms 104B-134

Table 5. Ranges and means of all analyses completed on lower series volcanics, Site 642. Figures inparentheses are only single analyses, not means.

0. 5

S i0 2T i 0 2A 1 20 3MnOMgOCa ON a 2 0K 2 0p 2 o 5LOIVCrCoNiCuZnRbSrYZrNbScBaLaCeNdSmEuGdTbHoYbLoHfTaTh

Group BF106-F117(27)range48.00-67.201.12-1.4413.90-19.260.01-0.410.11-2.761.19-7.661.68-3.130.19-8.590.09-0.610.25-13.80116.00-203.0058.00-177.003.00-34.003.00-25.0010.00-31.0057.00-165.0016.00-120.00102.00-230.0028.00-118.00251.00-363.0016.00-24.0026.00-34.00189.00-530.0023.00-53.6042.54-121.9516.56-56.552.58-12.480.92-1.843.36-11.210.43-1.701.43-5.181.35-9.060.17-1.277.23-9.261.11-1.8211.58-15.80

1 0 0 -^ _ _ T i 0 2 > 2 . o

mean58.611.2615.200.111.373.472.37

2.030.216.07149.7680.7813.8712.9416.71113.2869.55163.7056.89297.6720.6930.00338.9740.0094.7340.898.911.486.731.253.315.090.727.761.3613.86

-V-H> 40Q0 3 0 0

mean53.601.4215.420.123.045.622.90

2.150.19(2.17)201.0091.5030.0010.5021.00140.5079.50178.00(44.00)(208.50)(13.00)(44.00)(382.00)(30.70)(75.80)(36.10)(7.67)(1.62)(1.26)(2.26)(5.29)(0.74)(7.90)(1.01)(8.47)

D4mean62.301.2715.620.091.623.882.63

0.720.192.39141.0074.7012.7015.3016.70101.7049.70209.7052.70296.7020.3024.00431.5044.30107.0046.609.111.481.072.274.230.655.701.3414.50

4 0 00 3(

Figure 4. Chemical stratigraphy of lower series flows illustrated using Ti0 2 , V, Cr, Zr, Nb, and presenting a subdivisioninto Groups Al, A2, B, and dykes. For discussion, see text. Vertical scale is in meters below seismic reflector K (MBK).Concentration is in % for Ti0 2 , ppm for V, Cr, Zr, and Nb.

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A 4 0 0 1 B 4 0 0 1 / /F 1 0 6 T O F 1 2 1 A F 1 0 6 T O 1 2 1 J? /

A A A /A y /^L ^^ ^ A3 0 0 - t 3 0 0 - * AA * / / ^

Zr ( ppm) A A Gr oup B z r ( ppm) /? yS / ^2 0 0 - * f 200- Gr oup ^*S L+/ / /X G r o u p A 1 /


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L. PARSON, ET AL.H f / 3

B F106-117A2 F118-120A1 F121D4 + D7

Figure 6. Hf-Ta-Th discrimnant plot for lower series volcanics. Shaded area is field of upper series flows.F119 F121

Hf Zr Eu Ti Tb Yb NiFigure 7. N-MORB normalized composite spidergrams for the lower series flows. Stippled area indicates field of Group B analyses.Addi t ional spidergrams for Group Al (F121), Group A2 (F119), and daci tes from the Rockal l Trough (in boldface).

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Documents

THE PETROLOGY OF THE LOWER SERIES VOLCANICS, ODP SITE 642