3% IIHHIHHHHNWNilINlHlHHlfIHIIII'HWHHUHJI mrg'. LIBRA 12‘ Y ‘.-. Michigan Stamp University -"’ . . This is to certify that the thesis entitled PALEOMAGNETISM AND SHEAR HISTORY OF PRECAMBRIAN X DIKES presented by MARK ALLEN FORTUNA has been accepted towards fulfillment of the requirements for Masters degree in Geology J. A). (MW V Major professor Date 8/1/79 0-7639 OVERDUE FINES ARE 25¢ pER DAY PER ITEM Return to book drop to remove this checkout from your record. PALEOMAGNETISM AND SHEAR HISTORY OF PRECAMBRIAN X DIKES BY Mark Allen Fortuna A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1979 ABSTRACT PALEOMAGNETISM AND SHEAR HISTORY OF PRECAMBRIAN X DIKES BY Mark Allen Fortuna A paleomagnetic study was conducted on the Archean gneisses and Lower Proterozoic metadiabase dikes intruded into the Archean Granite-Greenstone terrain north of the Marquette Trough, Upper Michigan. From the standard thermal and A.C. (alternating current) demagnetization techniques used to date the rocks, four different thermal events were determined to have effected this area. The Compeau Creek Gneiss thermal event yielded poles which fell on Irving's Apparent Polar Wander Path at approximately the 2.5 G.a. position i .06 G.a., closely corresponding to dates given for the Algoman Orogeny. Metadiabase intruded the gneiss in what appears to be a period of tension that effected the area about 2.18 G.a. i .04 G.a. Following this extensional phase came the compression and metamorphism of the Penokean Orogeny which partially reset the paleopoles in the gneiss and metadiabase and was dated by such at 1.88 G.a. i .03 G.a. Mark Allen Fortuna Lastly, another series of diabase dikes, the Keweenawan Series, intruded the region at about 1.11 G.a. + .02 G.a. in what is thought to be a failed attempt at continental rifting. ACKNOWLEDGMENTS To God goes my greatest thanks for the personal strength given me to pursue this goal to its conclusion and for a creation in which this study could succeed. The author wishes to express appreciation to Dr. F. W. Cambray, Chairman of the Department of Geology, Michigan State University, under whose counseling this study was undertaken, for his assistance and many suggestions. Gratitude is expressed to Dr. James Trow, Dr. John Wilband, and Dr. Hugh Bennett, other members of my committee, for their time and help at critical times. Special thanks are offered to Dr. Rob VanDerVoo of the University of Michigan for the use of his paleomagnetic laboratory and equipment. Many thanks are given to Dan Jensen, Tom Urban, Rick Hamrick, Al Trippel, Dave Shanabrook, and Doyle Watts, good friends who offered their time and assistance when it was needed. Fondest thanks are expressed to my parents, Mr. and Mrs. H. S. Fortuna and my sister, Susan, for their encourage- ment and help, not only during this study but for every- thing. ii TABLE OF CONTENTS Page LIST OF FIGURES O O O O O O O O O O O O 0 iv INTRODUCTION 0 O O O O O O O O O O O O O 1 General Geology . . . . . . . . . . . . 3 Location and Topography. . . . . . . . . . 5 Field and Laboratory Methods . . . . . . . . 8 GEOLOGY AND MINERALOGY. . . . . . . . . . . 9 Precambrian W--Gneiss . . . . . . . . . . 9 Precambrian X--Metadiabase. . . . . . . . . 11 Precambrian Y--Diabase . . . . . . . . . . 12 Structure--Foliation Study. . . . . . . . 13 GEOPHYSICS. . . . . . . . . . . . . . . 15 Paleomagnetic Theory. . . . . . . . . . . 15 Techniques . . . . . . . . . . . . . . 17 Sampling Techniques and Sample Preparation . . . 44 AnaIYSiS I O O O O O O C I O O O O O O 45 Results and Discussion . . . . . . . . . . 51 CONCLUSIONS . . . . . . . . . . . . . . 63 BIBLIOGRAPHY . . . . . . . . . . . . . . 64 APPENDIX A O O O O O O O O O O O O O O O 67 iii LIST OF FIGURES Figure Page 1. Precambrian Chronology and Sequence of Events . 2 2. Location of Study Areas, Sites 1, 2, 3, A, B, C O O O O I I O O O O O O O O 7 3. Thermal Intensity Spectra for Gneiss Samples . 20 4. Thermal Intensity for Metadiabase Samples . . 22 5. Typical Zijderveld Diagram for A.C. Demagnetized Metabiabase (Sample 2-10) . . . . . . . 24 6. Typical Zijderveld Diagram for A.C. Demagnetized Gneiss (Sample A-SB). . . . . . . . . 26 6a. Resultant Vector Subtraction Diagram for A-SB . 28 7. Typical Thermal Demagnetization Diagram for Gneiss (Sample A-SA). . . . . . . . . 30 7a. Resultant Vector Subtraction Diagram for A-SA . 32 8. Typical Diagram for Thermally Demagnetized Metadiabase (Sample 2-1B) . . . . . . . 34 9. Typical Zijderveld Diagram for Baked Gneiss (Sample C-BB) . . . . . . . . . . . 36 9a. Resultant Vector Subtraction Diagram for C—BB . 38 10. Intensity Spectra for A.C. Demagnetized Gneiss. 41 11. Intensity Spectra for A.C. Demagnetized Metabiabase O O O O O O O O O O O O 43 12. A.C. Intensity Spectra for Superparamagnetic Samples . . . . . . . . . . . . . 48 iv Figure 13. Thermal Intensity Spectra for Superparamagnetic Samples . . . . . . . . 14. Apparent Polar Wander Path and Poles for Sites 2 and F . . . . . . 15. Apparent Polar Wander Path and Poles for Site 3 . . . . . . . . l6. Apparent Polar Wander Path and Poles for Sites A and C . . . . . . 17. Apparent Polar Wander Path and Poles for Site B . . . . . . Page 50 53 54 55 56 INTRODUCTION This study was undertaken to ascertain whether, in the absence of isotope dates, the paleomagentically deter- mined age and magnetic history of certain Precambrian meta- diabase dikes could be determined through the analysis of their paleomagnetic pole positions. In order to do this it must be demonstrated that the magnetism held within the rocks has remained stable since the acquisition of remanence. This requirement can be satisfied in a number of ways but the method of choice when working with igneous rocks is the "baked contact test" (Irving, 1964). This test utilizes the field relationships at contacts between igneous rocks which were intruded and cooled at different times. In this study it must be shown that unintruded country rock, the Compeau Creek Gneiss, carries an Archean palopole (Figure l), and that in areas reheated by intrusion of metadiabase the gneiss has acquired the same younger pole as the metadia- base. A third pole should also be discernable due to the metamorphism of the Penokean Orogeny. Age Events Orogenies 570 M.a. . . Cambrian Precambrian Z Upper Jacobsville Proterozoic Post 800 M.a. . . Keweenawan Tilting Intrusion Precambrian Y Middle Keweenawan Proterozoic Diabase 1.6 G.a. . . 1.85-l.9 G.a. . Penokean Huronian, Animikie, Precambrian X Lower Marquette Proterozoic Supergroup, Mafic Dikes and Sills 2.5 G.a. . . 2.5 G.a. Algoman = (Kenoran) Algoman Granite Precambrian W Figure 1. Precambrian Chronology and Sequence of Events. General Geology The major elements of the Superior Structural Pro- vince can be readily observed in and about the region of the Marquette Trough, Michigan. The province is divided into two contrasting terrains, an older Archean gneiss sequence south of the Trough and extending into Wisconsin that may be greater than 3.1 billion years old (Sims, 1976) and a granite-greenstone terrain north of the Trough, also of Archean age but younger than the gneiss (2.7-2.6 G.a.). Besides differing in age and geographic location these two terrains reflect differences in rock assemblage, structural style and metamorphic grade (Morey and Sims, 1976). The greenstone sequences were originally deposited primarily underwater along with shales, cherts, and volcanoclastics possibly offshore of the protocratonic gneisses to which they are now joined (Morey and Sims, 1976). This sequence was then folded and metamorphosed concurrently with the emplacement of the granitic plutons (Compeau Creek Gneiss) during the Algoman Orogeny at the end of the Archean. The span of time following the Algoman Orogeny and extending through the next major metamorphic event, the Penokean Orogeny, is known as the Lower Proterozoic (Precambrian X). The approximately 800 million years between these two events saw the formation, subsidence of, and sedimentation in a series of basins, including the Marquette Trough, situated at the juncture of the two Archean terrains. Sedimentation of the Marquette Supergroup within the basin is cyclic and records a complete transition from stable shelf to deep water eugeosynclinal environments (the Chocolay, Menominee, and Baraga groups respectively). Emplacement of the series of mafic dikes and sills in the Marquette Trough area (the objective of this study) is thought to have occurred prior to, or synchronously with the sedimentation of the Menominee Group (Sims, 1976). The trough sequence, but apparently not the basement rocks, were then folded and metamorphosed during the Penokean (Canon, 1973). Although a correlation with other mafic dikes, such as the Nipissing Diabase, has been suggested (Sims, 1976), the lack of isotOpic dates for these rocks makes correlation at present just Speculation. With the entire timing of subsidence and sedimentation in the Marquette Trough dependent on dates determined to an accuracy no better than 800 million years, the need to date these rocks paleomagnetically becomes apparent. The methods of dating formations by their paleomagnetic pole positions are not new, nor is the separation of several poles from one site (due to igneous cooling, metamorphism, viscous effects, and others). However, these techniques have never been tried in this area on rocks as old or as metamorphosed as these. During the Middle Proterozoic (Precambrian Y—- about 1.1 G.a.) another series of diabase dikes and lavas were emplaced in the Superior Province. Known as the Keween- awan Series, these rocks are believed to be a failed attempt at continental rifting (Chase and Gilmer, 1973). Minor uplift and sedimentation then followed during the Upper Proterozoic (Precambrian Z). The study area itself con- sisted of seven collecting sites spaced over some 20 square miles north and northwest of Marquette, Michigan. Sites chosen reflect the varying Archean and Proterozoic rock types and crosscutting relationships that occur in the Granite-Greenstone terrain north of the Marquette Trough. Location and Topography Originally the study was planned to investigate the relationships between the X age metadiabase and the W age gneiss at 5 to 10 locations. Seven sites were finally chosen and collected at, six of them shown in Figure 2. Outcrop terrain varied, but as sites were chosen mainly for easy road accessability, gneiss and metadiabase generally appeared as low rounded domes or low steep sided ridges rising above a forest can0py rooted in Pleistocene outwash and till. The following is a list of site locations and lithologies sampled. Site 1 T48N R25W sec 3. 765'N.L. 765'E.L. Metadiabase Site 2 T48N R25W sec 4 3960'N.L. 1795'E.L. Metadiabase & Gneiss Site 3 T49N R25W sec 30 3696'N.L. 396'E.L. Metadiabase & Gneiss Site A T49N R26W sec 24 3009'N.L. 122'E.L. Gneiss Site B T49N R25W sec 29 3764'N.L. 2587'E.L. Diabase & Gneiss Site C T49N R25W sec 32 3748'N.L. 2100'E.L. Metadiabase & Gneiss Site F T48N R27W sec 15 l478'N.L. 792'E.L. Metadiabase .0 .m .4 .m .m .H mmuflm .mmmum wussm mo coflpmooq .m musmflm 5.2.9. 02439 _ mqoozz 963m. 395%.. m0. awn 3w mx<._ . N 350 Hz / a. 0mm OZOQ DO mOZhwz’ 5 0mm 0 m Ba 6257:: $025; n utw Om 0mm Emma _ ~3on Field and Laboratory Methods The geologic maps used in this investigation were prepared by the U.S.G.S. and appeared in Professional Papers #788 (Puffet, 1974) and #397 (Glair and Thaden, 1968). At each site chosen an attempt was made to collect at least five oriented blocks for each lithology observed (Irving, 1964) i.e., Site C--five blocks of gneiss, five blocks of metadiabase. In the laboratory the blocks were then reoriented, cut and cored to produce samples that could be analyzed at the University of Michigan's Paleomagnetic Laboratory. Standard A.C. and thermal demagnetization techniques were employed during the work. GEOLOGY AND MINERALOGY Four different igneous rock types are recognized in the literature as outcrOpping in the study area. These are the Mona Schist, the Compeau Creek Gneiss, the Lower Proterozoic Metadiabase bodies, and the Middle Proterozoic Keweenawan Diabases (Glair and Thaden, 1968). Of these, the only rocks not sampled or studied were those of the Mona Schist. Thin sections were used for mineralogic identifi- cation and compositions were determined optically. Precambrian W--Gneiss The gneiss is best exposed in clean patches on the shores of Lake Superior and Harlow Lake but typically out- crOps throughout the area is massive, poorly jointed, domes or low rounded hills rising above intervening forested valleys. Although often encrusted with lichen and moss the gneiss is easily identifiable when well exposed by its characteristic light grey to pale salmon weathering colors and by its predominant west-northwest to east-southeast foliation. Based on the classification scheme found in Moorhouse (1959), the composition of the gneiss ranged from some rare 10 granitic forms (Site A) to granodiorite (Sites 1 and 2), quartz monzonite (Site 3), and quartz diorite (Sites B and C). This variability is almost never discernable in the field due to weathering and the compositional description almost always given is "granitic" (Glair and Thaden, 1968). The dominant minerals, quartz, microcline, and plagioclase account for 90% to over 95% of the rock. De- pending on the variety of gneiss examined, compositions ranged from 20-40% quartz, 2-40% microcline, 30-50% plagio- clase, l-l4% hornblende, chlorite, and chloritoid, and 1-5% Opaques and accessory minerals. The Opaques and common accessory minerals consisted of hematite, magnetite, pyrite, pyrrhotite, leucoxene, illmenite, sphene, epidote, and zircon. Euhedral to anhedral plagioclase grains, although extensively saussuritized, were determined to be in the compositional range Anlo-An3O by the Michel Levy method. In the granite, clear unaltered microcline occurred mostly as anhedral grains whereas up to 20% have quite pronounced vein micrOperthite. Minor amounts of granophyre micropeg- matite were found in samples of quartz diorite grouped about plagioclases. Effects of strain could be readily seen in aggregates of quartz grains showing strain lamellae. Hornblende occasionally retained euhedral twinned crystals but was most often found as ragged pleochroic fragments difficult to distinguish from chlorite. In all cases any original igneous texture has been replaced during metamorphism by a granoblastic arrangement of lobed decussate grains. 11 Although of uncertain origins and with often unclear field relationships, it is suggested that the gneiss probably acquired its foliation when it was deformed during the Algoman Orogeny, perhaps being intruded contemporaneously with the deformation (Sims, 1976). Precambrian X--Metadiabase X age metadiabase dikes and sills intrude all earlier Precambrian units in the study area and some of the meta- sediments within the Marquette Trough. Weathering dark grey to grey-green, often lichen and moss covered,the poorly jointed, massive metadiabase can be found to outcrop individually or within the gneiss. In the former case the dikes appear as low steep sided elongate hills surrounded by valleys filled with Pleistocene glacial deposits. In the latter case their appearance is best seen in cleared gneiss domes on the shores of the lakes or ex- posed in railroad and highway cuts. While some of the thinner dikes are distinctly chloritized and display an internal foliation, the larger metamorphosed bodies still retain recognizable traces of igneous fabric. Textures are fine grained intragranular and diabasic although these have been altered by metamorphic recrystalization. Dominant minerals are chlorite and amphibole, plagioclase, and Opaques. Amphibole with com- positions in the actinolite-tremolite group varies in appearance from relatively well preserved crystals, often 12 pseudomorphic after pyroxene, to shred-like pleochroic fragments. Except for extinction angle, these fragments are often difficult to distinguish from chlorite which also makes up a significant portion of the rock. Amphibole and chlorite together account for between 30 and 40% of the identifiable minerals. Sodic plagioclase (An ) is the 10-20 common feldspar making up about 35 to 45% of the rock. It occurs as twinned and untwinned laths pseudomorphic after calcic plagioclase which is commonly sericitized and/or extensively saussuritized. Opaques and accessories consti- tute the remaining fraction of minerals. The opaques identi- fied under reflected light, include in decreasing amounts hematite, pyrite, magnetite, leuxocene, pyrrhotite, and illmenite. These can be found disseminated throughout the sample or isolated in skeletal bodies about relic pyroxenes and amphiboles. Common accessory minerals encountered are epidote, sphene, and zircon. These mineral assemblages along with the alignment of chlorite and amphibole are taken as indicating metamorphism in the chlorite zone or Greenschist facies. Precambrian Y--Diabase The predominantly east-west trending unmetamorphosed Keweenawan dikes are the youngest rocks in the area and are seen to cut all previous units except the X age metadiabase. The larger X age dikes are unfoliated and are often mistaken for these diabases due to their similar fresh appearance and 13 grey-green color. The only certain methods Of distinguish- ing between the two are the existence Of sheared margins on the X dikes, the mineralogy, and the differences in re- manent magnetism. Composition Of the diabase is dominated by plagio- clase feldspar, 40-60%, Of the andesine-labradorite family. Among the remaining minerals, pyroxenes comprise between 30 and 40%, followed by hornblende and chlorite 5 to 15%, Opaques 5 to 10%, and minor amounts Of serpentine and accessories. Over 95% Of the Opaques are magnetite, occur- ring as disseminated grains and irregularly shaped bodies near pyroxenes and serpentine, the remaining 5% being hematite and pyrite. Dewatering of the diabase probably accounts for the remaining alteration minerals chlorite and serpentine. Textures among the plagioclase and pyroxene grains are intergranular and subOphitic, typical Of diabases. Structure--Foliation Study Besides the primary paleomagnetic Objective of this study, a secondary structural Objective was to Obtain a statistical number Of internal shear foliation orientation within the X age metadiabase dikes. It was hoped that this foliation could be used to define a direction Of final- applied stress during the Penokean Orogeny (Berger, 1971). Unfortunately during the study the lack Of fresh exposures plus the need to visit larger dikes to acquire the oriented blocks meant that fewer than a statistical sampling Of dike l4 orientations were recorded. This Objective therefore had tO be abandoned. GEOPHYSICS Paleomagnetic Theory Physicists have described several states Of magnetism, only one Of which is of importance in carrying natural remanence in rocks, that being ferrimagnetism. There are very few ferromagnetic rock forming minerals, therefore those few have been studied in great depth; they are magnetite-- maghemite--ulvospinel solid solution, hematite-illmenite solid solution, and pyrrhotite-pyrite solid solution. The first end-member Of each solid solution is ferrimagnetic while the other is either paramagnetic or antiferromagnetic. Fortunately there is great difficulty in forming and/or maintaining a solution between any two end members except at high temperatures. This means that the magnetism measured in normal rocks is essentially pure ferrimagnetism (McElhinny, 1973). Two magnetic mineral properties that are especially important in paleomagnetic considerations are the Curie temperature and grain size. Normally the Curie temperature can vary with the percent solid solution but since the minerals are almost pure end members the values are essenti- ally constant. The Curie point, the temperature above which 15 16 ferromagnetic materials become paramagnetic, is distinctive for each mineral: magnetite 578°C, hematite 680°C, and pyrrhotite 320°C. The grain size, on the other hand, varies considerably and controls the number and size Of the magnetic domains. Since each domain carries its own individually oriented magnetic vector, the number Of domains per magnetic grain controls the orientation Of the total remanence vector and the ease with which it can be broken into its original components. The situation Of one domain per grain being the least complex both physically and mathematically to handle. In hematite the grain size threshold, above which grains are multidomain, is about .15 centimeters while in magnetite the threshold is 3 tO 30 microns (depending on the grain shape). From these sizes Stacy (1967) has concluded that virtually all rocks Of interest paleomagnetically contain enough mag- netic minerals below this threshold size for single domains to dominate the remanence. Paleomagnetism in igneous and metamorphic rocks is founded on the important assumption that the domains in rock forming magnetic minerals are aligned in a direction parallel to the direction Of the Earth's magnetic field at the time the rock cools through its Curie point(s) (Irving, 1964). Although this assumption can never be proven, its constant use and production Of coherent results appears tO make it valid. If these domains were aligned parallel to the field lines Of some ancient pole then the position Of this ancient pole can be located by measuring the direction and intensity 17 Of the magnetism left in the minerals. Studies which com- pare the isotopic ages and associated paleOpole position give consistent groupings Of poles for each geologic period; connecting these groupings creates the so-called "Apparent Polar Wander (A.P.W.)" paths (Irving, 1964), which are thought to be more a reflection Of plate motions than Of the wanderings of the North Pole (Irving and McGlyn, 1976). Therefore, if the age Of a rock is known only to the accuracy Of an era but its paleOpole is well defined, then it can be compared to the appropriate A.P.W. path and its age approxi- mated. Moreover, if the rocks are metamorphosed then Often more than one pole can be determined, one for the metamorphism and one for the original igneous thermal event (Buchan and DunlOp, 1976). Techniques Two methods that are commonly applied in paleomag- netic investigations of igneous rocks are stepped thermal and A.C. (alternating current) cleaning techniques (McElhinny, 1973), both Of which were used in this study. As the word cleaning implies, the original magnetism Of the rock is cleaned out in successively larger amounts by each increase in thermal or A.C. intensity. The aim in removing the mag- netism by steps is to sample, in a short period Of time, the Spectrum Of variation in strength and orientation of the magnetic vectors contained within the domains Of the magnetic minerals. The pattern of variation can then be used to l8 determine the stability Of the magnetism and whether the sample contained one or several superimposed magnetizations. In A.C. demagnetization, the specimen is placed within a shielded container and is subjected to a peak A.C. field which then decays exponentially. This has the effect Of randomizing domains by imposing an exponentially decreas- ing hysteresis loop on those whose coercive force is less than the peak field generated. By increasing the field by discrete steps, one samples the pole contribution Of domains with increasingly high coercive energies (Figures 3 and 4). Note this is not always equivalent to the highest temperature T.R.M. (thermal remanent magnetism) pole (McElhinny, 1973). The magnetic intensity values for each step were eventually normalized to a ratio Of the original N.R.M. (natural re— manent magnetism) value, JO, for presentation in the figures. The amount by which the step is incremented reflects a com- promise between the time available for study and the amount Of data needed for a pattern Of variation to develop. In this study the step increments varied anywhere between 50 0e and 150 oe (oersteds) and 75° and 200°C. At this point the data could then be analysed and paleopoles determined, either by the use Of sophisticated computer programs (Stupavsky and Symons, 1978) or through the traditional method Of vector subtraction by the use Of Zijderveld diagrams (Zijderveld, 1967) Figures 5, 6, 7, 8, and 9). Computer programs, as Opposed to graphical methods, have the advantages Of being quick, accurate, and relatively l9 .mmamfimm mmflmcw MOM muuommm muflmcmucH anemone .m musmflm 20 O o o “OovmmDF4mwQ2wh 21 .wOHOEmm mmmnmflpmuoz mom ADHmGODcH anemone .v musmflm 22 Se mmnEamazfl coo one cow .4 550?“ «L. fit— «I. c .o.. ..O.N ®.N 23 Figure 5. Typical Zijderveld Diagram for A.C. Demagnetized Metabiabase (Sample 2-10). Open circles are in vertical plane; solid circles are in the horizontal plane; units are 10'"5 emu/sample. 24 FIGURE 5 . 8... DOWN JS 25 Figure 6. Typical Zijderveld Diagram for A.C. Demagnetized Gneiss (Sample A-5B). Solid circles are in the horizontal plane; Open circles are in the vertical plane; units are times 10'5 emu/sample. 26 FIGURE 6 . NRM DOWN —-()- m1 27 Figure 6a. Resultant Vector Subtraction Diagram for A-SB. Vectors to Open circles are in the vertical plane; vectors to solid circles are in the horizontal plane; units are times 10'5 emu/ sample. 28 «3 ”R" UP N * FIGURE 6a. “a +1 -I l w DOWN s 33...... 29 Figure 7. Typical Thermal Demagnetization Diagram for Gneiss (Sample A-5A). Open circles are in the vertical plane; solid circles are in the horizontal plane; units are 10'5 emu/sample. 220° 300° 900 ~ka 50m! 550 U P 400' DOWN 17, 72 -i s FIGURE 7. h 31 Figure 7a. Resultant Vector Subtraction Diagram for A-SA. Vectors to Open circles are in the vertical plane; vectors to solid circles are in the horizontal plane; units are times 10'5 emu/ sample. 32 NRM %UP : J DOWN -3 -: é Ileana 7a. 33 .OHmEmm\OEw mica mOEHD mum mafic: “madam HODCOquon map :a mum mOHOHHO OHHOm umcmam HOOHDHO> map CH mum mmHOHHO ammo .Amalm mamfimmv mmmnmwomumz OONHDOOOOEOQ >HHOEHO£B MOM Emummwo HOOHQSB .m musmflm 34 <>¢ . m 55on 230 o 5&2 35 Figure 9. Typical Zijderveld Diagram for Baked Gneiss (Sample C-8B). Open circles are in the vertical planes; solid circles are in the horizontal plane; units are times 10"5 emu/sample. 36 UP FIGURE 9. DOWN 37 Figure 9a. Resultant Vector Subtraction Diagram for C—8B. Vectors to Open circles are in the vertical plane; vectors to solid circles are in the horizontal plane; units are times 10‘5 emu/ sample. 38 UP FIGURE 93. 39 unbiased in their treatment Of data (inevitably some bias must occur when the programmer creates the program). Graph- ical methods, on the other hand, are typically slower, more approximate, and more likely to be biased; while bias is normally something to be avoided, in this case it can be valuable since it allows the experimenter the ability to vary parameters and incorporate information in ways not readily programable. Ideally use Of computers and Zijderveld diagrams should compliment each other in yielding similar results, that occurs in this study with more use being made Of Zider- veld diagrams due tO their simplicity Of use and clearity Of results. During thermal demagnetization, at any given temperature the vector measured is the sum Of the T.R.M. and any secondary components formed at temperatures higher than the one used. If all goes well, as the step temperature is increased, more and more domains will be randomized till all secondary components are destroyed and only the original T.R.M. is left. Finally the Curie temperature is reached and all remanent magnetization is destroyed. This method in particular lends itself to analysis on Zijderveld diagrams where, working from the origin out, one encounters and can identify the poles due tO intrusion, metamorphis, and the viscous effects Of the Earth's field (Figures 10 and 11). 40 .mwflmco OONHDOCOOEOQ .U.¢ HON muuommm wuflmcmucH .OH museum 41 com qL . op $5on Amomkmmwcv 0.5.... .0.< x 3 as 69 k = (N-1/N-R). The probability of any one direction being Observed to make a angle 8 with the true mean can be given for the various probabilities as follows: 1) P = .5 850 = 67.5/ k degrees 2) P = .37 650 = 81//k degrees 3) P = .05 695 = 140//E degrees These are analogous to l) the quartile distance, 2) the standard deviation, and 3) the 95% deviation, for normal distributions. The last represents the angle from the mean beyond which only 5% of the directions lie. Fisher (1953) has shown that the true mean direction of the pOpulation of N directions lies within a circular cone about the resultant vector R with a semiangle Of a. When a is small the approxi- mate relations standard error Of the mean A63 = 81/JEN circle of 95% confidence A95 = 140//kN may be used. In order to determine whether a paleomagneti- cally determined direction differs significantly from some known direction, such as the A.P.W. path or the present Earth's field at the sampling site, A95 may be used directly. The two directions are significantly different at the 95% 70 confidence level if the angle between them is greater than A95. The question then arises as to whether or not these directions could arise from the sampling Of a random pOpula- tion, in that case the mean direction would have no signi- fiance. For a truly random population K is zero, in practice however, k, the best estimate of K, is never zero, this therefore requires the following test. For a sample Of size N, the length of the resultant vector R will be large if a preferred direction exists or small if it does not. Assuming no preferred direction exists, a value Ro may be calculated which will be exceeded by R with any stated probability. Irving (1964) has tabulated R0 for sample sizes up to N = 100 for P = .05. To carry out the test one merely enters the table at the row corresponding to the sample size N in order to find the value of RO which will be exceeded with the given probability. The V.P.G. paleomagnetic pole (1', ¢') can then be calculated from the site mean direction Of magnetization (D Im) according to: ml 1 = station latitude ¢ = station longitude p = arctan (2/tan Im) 1' = arcsin (sin 1 cos p + cos 1 sin p cos Dm) B = arcsin (sin p sin Dm/cos 1') if cos p 3 sin 1 sin 1' ¢' = ¢ + B ¢ + 180 - B if cos p < sin 1 sin 1' O' 71 Since the mean direction has its associated circle of con- fidence, A95, corresponding to errors in inclination and declination, the error in the inclination will correspond to an error dp in the ancient colatitude, p, given by: dp = %A95(1+3coszp). The error dp lies along the great circle passing through the sampling site S and the V.P.G. pole P and is the error in determining the distance from S to P. The error in declina- tion corresponds to a displacement dm from P in the direc- tion perpendicular to the great circle SP where: dm = A95(s1n p/cos Im). The polar error (dp, dm) is termed the 95% confidence ellipse about the pole. The following represents a summary Of the statisti- cal tests just outlined when applied to the paleOpole data of this study. 72 0.00 0.00 0.000: 5.vHI 0.00 0.00 0.00: 0.0 055.0 OH 0 mmmnmwwmuwz 0 0.00 0.00 5.000I 0.5: 0.00 H.000 0.00 0.0 000.5 00 < OmMQMfivmuoz m 0.00 0.00 0.00: 0.50 0.vH 0.000 0.H5I 0.00 000.0 0 0 000000 U 0.00 0.00 H.00HI 0.00 0.00 0.000 5.00 0.0a 000.5 0 0 000000 D 0.00 5.0 0.00: 0.00: 0.0 0.00M 0.00 0.00 050.0 m m mmeMAGMDmS 0 0.00 0.0 H.050: 0.50 0.0 0.000 0.00: 0.05 050.0 m 0 mmMQMvaumz 0 0.00 0.0 H.050: 0.00 0.0 0.00H 0.00: 0.00 000.0 5 0 mmflmcw 0 0.5 0.0 H.000: 0.00 0.v 5.000 0.00: 0.000 000.5 0 0 mmmnmwa m 0.00 0.0 5.00 0.50 0.00 0.050 0.0: 0.0 000.0 00 < mmflocw 0 0.00 0.0a 0.000: 0.0: m.vH 0.00 0.00: 0.00 v00.0 5 U 000000 m 0.00 0.00 H.00HI 0.00 5.0 0.000 0.05 0.0a 000.0H 0H m mmwmcw m 5.00 0.00 0.050 0.00 0.00 H.000 H.0H 0.0 050.0 0 d 000000 m 5.0 5.5 0.00: 0.05 0.0 0.000 0.05 0.000 000.0 0 U mmMQchmumz m 5.05 0.00 H.00I 0.0 0.00 0.050 5.00 0.0 mv0.0 0 0 unmanacmuwz m 5.00 5.00 0.00HI 5.00 0.0 0.000 0.00: 0.00 000.0 0 0 omnfl0000u0= m 0.00 0.00 0.5 0.05 «.00 0.00 0.00 0.00 000.0 m U mmflmcw 0 0.00 0.00 0.50: 0.00 0.00 5.0vH 0.00 0.00 000.0 m m 000000 0 0.50 0.00 5.500: 0.50 0.50 5.500 0.00 0.0 000.0 5 < 000000 0 0.00 0.00 0.000: 5.0 0.00 0.000 0.00 0.0 000.v 0 m mmmnmwomumz 0 0.0 0.5. 0.00a: 0.00 0.0 0.000 0.00 5.00 500.5 0 a mmmanvmumz 0 20 00 .e .4 000 so Ba x m z umnssz mmoHonuflq @000 0000 FIN 0.60933 00/09/79 .15023oQU PAGE 73 OP1=1 73/73 PROGRAM SYATPOL L C A .V .v A O In ? O U ‘l O h. 7 I Q. 0 I A 5 NC 0 U . D. o o \o .1 t S o c Ch.) I . )A 9. .5 3 A S S H L V V \o 1.4 r. . 3.6 F f I. o I C .L I FL C L S ) LIA RD S . .0 O 9 38 s S 0 cl E .L 0% DR DS C ‘1 DC.” AA NC . HR 2 : 0F S .L r. A N .UR R r. .b E N 1 N u L L..— IL .)FP h o C N a R R A R5 5 0 .LL 0.? CA A D YNL U R o . OH V cl r... N I A 0 cl ‘7. DC 5 Lo R RI. N 4 3 10L C 55 o . :8 .L N I I. S U I N S CU DD CC C A3 AD I. S E 7C0 If. .LL . o 9. .u A :G 5 0 0 C 0 N N A 0L E L 06 DA 0 3 .I A P DL .3 In“. . .ru 0 I K O C S r. I. 0 AN ON RR 1C N NE. .K C A LL LO 1. A .751 D L [0 C v cl C .II NI. 95 CM. A A D O L OLD [D O N I o .09 cl U UL C .L C s I CC L IS IN US .3 U CCN p 0 10 . .LI. 5 5 U0 N H H .l C N N 70. 0 PD CC. SI. CN 1 C FA 8.! 6 .uN - O r. TC 0 cl 1. N R 0 R0 SO .L 0.1 A A A L N1 [A o R .NB R R I I A I. 1 Ava I. LN F R... RN NI. L A 1C." P A 05 .0 o A H CF 1: or F Y- O 5 Lol XI 81. SN ‘0 AA 00 o C o I. OH .18 L I ND C 0 0 L I. UA A A UC LA LC 1A C PT... [L N CA .TF U I O f. U N C C1. 6|. 6‘ 1C UR UH TR ( L 0A.! 0C 7 1 .A 0 G 9 LR R H F S A C IV 9V 0A 0 C G A .4 L RCA RP. F LO .ol: N I O I U U r. C R I}; 0C RC Ans.nh NF IN I 0 RR! RI. LA :3 A A [R D S S R H P CD 10 PH I9 A1 A0 S! S P [56 LC AI .HC H 0 LR o .QN 7)..) Dr. N .1) .II I} 0 PM I n.) . O O .8 H7 0 Al. .XH IMF 61 G L: . QA IF 0 l o N (D ) .IE 10.. 0X 0 39 o . 0H 0: A.) L LA 9. ~3H. 3 N Q1 9 0 CL 0 00 OCA 1.. 1| C (1| 0 8|. ECC) '3 A I: 6 OS \1 F O Fun-.31. o L 0 ‘. lit \I Q, .C 0 I? I 2 o )C Y :9 :DFPBF 2 all: I I. vl’ A 1- 1.0“ O )1 luv: 0 n. A) A L cl 0 VF 00: In. PP N : L) Y O G A.) CVV: : UI. n o C v 0:4 L C N L 910°C v. P I CC I C CD .t l. ) C a... CDLLA 1.1. [C 0 (I A. S t L .3 NF .0 TD... UR DD ) ) SN If D l 9 9 GOBTFDAUD OP 1‘ o o L1 L1 C ( Y A I:FSCYrYC 0" SN 2 .0 C5 h? ( S A o T 0 5.11 10A 0!. 0 C. )-o )0. o O L A l. AC:RL TIC ULG , CS 0 91 J))) :11 o C c H RF TC ALRSAO POJ o no I 1L ) 3N?! RM" 3 / G 6 PL SDNLCACL u” 0 0 Pa,” .L O o .l K oIAA 0.99. O \- 1 1v 1.. 1L AURL R IAP 9 N1... 4 HI. I .l i .1 ICLL F : : o I : ODS ND .1 HNCCIU-JLD ‘LJ 0 I IPPP O or. Iv 'l N K \J .\‘0 All: I L I Ft). P. ,- G‘MRCGULN 90 2 ) one. ) :a 1 R a o P (ARC LFR I L 11 O JABKOT o 1C 1 N102 LCD 0 o CCCC o .1 ID I G K a. 7 I K114 (DP 9 0 DP 0 096 QIA)Uun OHCCSASPC ONJ 0 1 CNN" 2 C ) ON 1 G S I ( N T A XISN N11 5 C I // OuARRLall 9HHHUNIHH PI 0 o - DI] IA! 0 a L) I. .l s I I 'I .I 9. I S 0‘?- AU d 9 a! O II] J!!! "CAC.1 ‘nLVlhrI-n PIE... cl 0” U l‘ 94“ LH‘ O A E? \I o c I o 9. 9 i. a: An 60.45159 2. A .‘ I 3.4 Q QCAMLQ CA Q1. 1 UZCAA ANJ I. NLUSSN Cr}! L U II o o u. o 0 C .i. .l c 61 ((NCY. o I A PP l I. NHHYAC 07.! 09.? 0A 0R0. 7.0). IOIOOI 000 C 0 FM. 03 I. I l. S 5 S / 26 (.NILAICI) .3 S KIN! 7. IP JDILLOD .- o nUoRoPo o SNLLU SIICCS LIN ( S [C o o I 8 I. I I 0 A 3.: FASNLMSS . o LOAN I PI 9.009.160 00 DHUHDU 104° ‘19.. .. .. LLC F... .K (LILY—l .1 .. = K. a C V II. IILle'ICA o o .. .3 1/ L3 1 II. 9.0.3.50 urusulrl . a 0.1.0 0U. 00 "9010 ("All =:: UL I. VNPPLC o 3 5 u U A 5 l o PP.A:SCL.L.I T 9 SA: .. 7 "H 3Q567BLL‘.440(2|2((I AKUAAOOUS [LIV LHN N. I Ara/I o o I b O. 7 Q .. .1 F1. 0 a o ..IA((CL A A :11! AP IIIIIIIITI .IY‘I Ool IT I 10) o ooBURCCC [CC 'IA R ISHHLL . A A n. 1. A re 0. HI?!R..<. - o L : AERR L: IITYIIAAA.AARAAAIAAC CLDLLUOOAS .IU G AADICC N T T .. : t. : :: Ul..ADAOlCC L .r I..PP :G NNNANNNHHH .HICHZHHRHYP 0AAFC..::: NO 5 ..:::(( | E C D 5 G A I] ::LL%TCS(( L L ,uA-.v.. IN IIIIIIRHR.R.K .HIRIRR OD HIKIZPLI ~10 OS .. NHMI'F .. H P .3 9 .1 1| Ln. KN‘JCAKIIJPS. .l .t ITAH y L Aru at! n.u.~RQCOO—UC.LOOU ICCOIV PRRNICCCPD CR K 0.10le K. .I T A L c. r. LP DlPLL.CCr.lI ....L rt SCLP 5 LL FPFPPPPFFF .FFLFSFIOFFCSL 9 1 2 3 Q U U D T 333 33 D 0 00 U A 2 3 A «11-? t C :4 6 70; AL VI 1111: IA A 1A ‘A U (L l C C C C C C CC (C CC CC CC CC CC C (CC CC CC . v. A Iv ,U C R (P l 15 it 2 2 ‘- IL 6 Q L I b b. T 7 y p C Q 10- M'TlTa'ITILflfilLfijfliLUfirEnliujiflfififlflfmflflimflm'ES 6 6008